Separated by billions of miles and shaped by very different environments, Saturn's smoggy moon Titan and the dwarf planet Pluto have, on paper, little in common.

Titan is a geologically active world cloaked in an atmosphere denser than Earth's, its landscape shaped by seas of liquid methane and vast dunes built from organic particles that rain out of its skies. Pluto, by contrast, is a frigid outpost with a tenuous atmosphere hovering above an icy, frozen surface.

Yet, observations from the James Webb Space Telescope (JWST) have revealed these two disparate worlds share an identical, unexplained infrared signature on their surfaces — a sign that similar chemistry may be unfolding on both worlds despite their stark differences, scientists say.

Detailed in a new study, the feature appears at precisely the same infrared wavelength on both Titan and Pluto, indicating that it absorbs light identically on both worlds. The signal points to the presence of an unknown compound, or a family of related compounds, that scientists have yet to identify.

"It's rather mysterious," Bruno Bézard, a planetary scientist at the Paris Observatory who led the discovery, told Space.com. "We cannot say what it is."

Chasing the signal

Bézard first spotted the unexplained feature while analyzing Titan observations from November 2022, when JWST gathered infrared light leaking through the haze in very specific, narrow wavelength bands, allowing scientists to peer directly through the otherwise opaque atmosphere.

To make sense of this light, researchers rely on spectroscopy, a technique used to identify compounds by the specific wavelengths of light they absorb. Because every molecule leaves behind a unique spectral fingerprint that telescopes can detect, astronomers can routinely identify substances across the solar system and beyond, ranging from simple water ice to complex molecules.

But the newly detected feature neither matches any of the standard library spectra cataloged so far, nor does it appear to be a fluke of the hardware, the study notes. The exact same absorption feature was detected by two different JWST instruments — the Near-Infrared Spectrograph (NIRSpec) and the Mid-Infrared Instrument (MIRI) — ruling out the possibility of an instrumental artifact.

After failing to find a match with the cataloged spectral fingerprints of the simplest and most common planetary ices, Bézard wondered whether another world with similar atmospheric chemistry might hold a clue. He turned to his colleague Emmanuel Lellouch of the Paris Observatory, who had led a JWST program that had observed Pluto in May 2023.

"I asked him, 'Do you also have a signature [on Pluto] at this exact wavelength?'" Bézard recalled in the interview with Space.com. "So we looked, and we found that it is present there."

The ground truth

There is no evidence that the mystery molecule is a biosignature, Bézard said. Instead, it likely reflects the kind of prebiotic chemistry that has been operating in Titan's oxygen-free environment for more than 4 billion years.

One compelling possibility is that it belongs to a family of hydrocarbons known as allenes, which the study notes are essentially the only organic compounds known to exhibit strong absorption bands within the same 5-micron infrared range as the mystery signal.

But the allene family contains many complex variations for which complete spectra do not yet exist, Bézard said. Alternatively, the signature could belong to a known molecule whose spectral fingerprint has shifted, potentially because it is mixed with other planetary ices that have not yet been fully cataloged.

"We only have spectra for the simplest of them," Bézard said. "But it is possible that within this family you could find one or several compounds which are present together and make such an absorption feature."

Whatever the compound ultimately proves to be, he and his team believe it forms in the atmosphere through photochemical reactions before eventually "snowing" down onto the surface.

Despite their stark differences, both Titan and Pluto possess atmospheres dominated by nitrogen and methane. Scientists know from spacecraft observations and lab simulations that in the upper reaches of the planets' atmospheres, ultraviolet sunlight breaks these molecules apart, triggering a cascade of highly reactive chemical fragments. These fragments then reassemble into complex organic compounds, some of which condense and snow back down onto the surface below, which is where the JWST detected them.

A couple of different lines of evidence support this atmosphere-to-surface pipeline. First, the team analyzed how the signal behaved across Titan's disk. As the JWST scanned Titan from the center of its circular face toward its outer edge, or limb, the mysterious signal steadily weakened, the study notes.

"This is what you expect if it comes from the surface," said Bézard.

Because of the three-dimensional geometry of looking at a sphere, a signal coming from a flat surface will appear strongest at the center and grow fainter near the edge, where the angle of view through the atmosphere becomes longer and more obscured. This specific dimming trait was not shared by atmospheric carbon monoxide the team checked, which remained essentially unchanged from center to limb, Bézard said, which helped the team infer that this mystery molecule likely resides on the ground.

Pluto offered an independent line of evidence to back up this surface theory. Its near-vacuum atmosphere is physically too thin and sparse to generate an absorption feature of the observed depth, Bézard said, leaving the dwarf planet's solid surface as the only plausible source for the deep signature.

To further test their atmospheric-origin hypothesis, the team studied JWST observations of Jupiter's moon Ganymede. Because Ganymede lacks a nitrogen-methane atmosphere, it cannot run the same photochemical engine like Titan and Pluto do. Sure enough, the researchers found no trace of the mysterious feature, lending additional support to the theory that the compound is uniquely tied to the chemistry of nitrogen and methane skies.

Mapping Titan's surface

More clarity may arrive in the coming months. Researchers are currently analyzing newer JWST observations that provide a more complete view of Titan as it rotates. The data will allow the team to map the mysterious infrared signature across the moon's surface and determine whether it is associated with specific geologic features, such as Titan's vast organic-rich dune fields, which are continually replenished by a slow drizzle of complex particles produced in the atmosphere, Bézard said.

Further clues may have to wait for NASA's Dragonfly mission, a car-sized rotorcraft expected to launch in 2028 and arrive at Titan in the mid-2030s.

Although Dragonfly does not carry an infrared spectrometer to replicate JWST's light measurements, its onboard mass spectrometer will allow it to "taste" organic molecules across the various geologically interesting areas the spacecraft will fly to. By revealing which compounds are actually present on Titan's surface, the mission could give scientists a shortlist of candidates to replicate in laboratories back on Earth to see if they match the traits of the mystery molecule, Bézard said.

"It's always exciting when you discover something that was not seen before," Bézard said. "It's really the nicest part of our job."

The new study was accepted for publication in the journal Astronomy & Astrophysics.

The aftermath of landslides have been found in images of Pluto's surface taken when the New Horizons mission flew past the dwarf planet in 2015. The landslides are evidence that the icy world is still active, albeit on geological timescales.

A team led by geologist Marco Emanuele Discenza pored over images taken by New Horizons' LORRI (Long-Range Reconnaissance Imager) instrument, which was capable of detecting surface features as small as 984 feet (300 meters). They found convincing evidence for six landslides in total that have taken place down the inner walls of three craters on the western edge of Sputnik Planitia, the heart-shaped feature that characterizes Pluto's appearance.

Previously, geological features left behind by landslides have been found on a host of bodies in the solar system, including Mars, Ceres in the asteroid belt, some of the icy moons of the gas giants, and even Pluto's companion, Charon. However, these are the first to be found on Pluto.

A landslide that fell 1.4 miles (2.2 kilometers) was identified in Pluto's Coughlin crater, close to a secondary crater on Coughlin's rim that may have triggered the landslide in the first place. Two further landslide features were seen in Giclas crater, and another three were spotted in a third, unnamed, crater.

The landslides were identifiable by their large debris aprons that spilled out onto the crater floors, the distance the landslide material travelled ranging being between 6.3 and 9 miles (10.1 and 14.5 km). Some of these debris aprons appeared bumpy, as if they contain large boulders of solid ice, while the areas around the source of the landslides feature well-defined, concave-shaped cliffs where material has broken away and tumbled down the craters' steep walls.

The great lengths that the debris rolled means that Pluto's landslides are among the most mobile in the solar system, a product of low gravity and low-friction icy rubble. The largest of the debris aprons covers 50 square miles (130 square kilometers), which would be large enough to bury a small city or a large town.

Landslides are important processes in shaping planetary surfaces, enabling the transport of material across great distances. The trigger for Pluto's landslides, however, is not yet clear. While the landslide in Coughlin appears to have been caused by a smaller impact nearby, the other five have less certain origins. One possibility is thermal stresses in the surface ice caused by the slight temperature changes that cause Pluto's volatile materials – among them molecular nitrogen, carbon monoxide and methane – to periodically sublimate and then condense again. These temperature changes are the result of Pluto subtly heating up and cooling down as its elliptical orbit brings it slightly closer to the sun, crossing inside Neptune's orbit, and then moving farther away again.

There is also evidence for more landslides in other craters, but New Horizons' coverage of Pluto's surface was limited as it hurriedly flew past on July 4, 2015, and the imagery required to confirm these other landslides is lacking.

The findings were published in the journal Icarus.

NASA's New Horizons probe has woken up in good health nearly 6 billion miles away beyond Pluto after spending nearly a year in hibernation.

Traveling such vast distances between our solar system's most remote objects means New Horizons often cruises for months at a time with little to do other than passively collect data. During these periods, the probe goes into a hibernation mode in which its instruments still collect data, but most other systems power down.

New Horizons entered just such a hibernation period last August, and has now woken up in "good health", according to a NASA statement. The spacecraft is 5.9 billion miles (9.5 billion kilometers) from Earth, so far away that it takes around 9 hours for its radio signals to reach us. Now that it's awake, New Horizons will begin transmitting the data it has collected over the last 321 days and letting its controllers on the ground know how its systems are faring in the cold, dark reaches of deep space.

So far, the probe appears to be in perfect health. "Every status report through this hibernation period was 'green,' meaning all was well aboard New Horizons each and every week," said Alice Bowman, the New Horizons mission operations manager at the Johns Hopkins Applied Physics Laboratory (APL) in the NASA statement.

New Horizons is the first and only flyby spacecraft to conduct a flyby of the Pluto system, which it did in 2015. Four years later, the plucky probe studied the most distant object ever explored in our solar system, the snowman-shaped planetesimal Arrokoth, while it was one billion miles (1.6 billion kilometers) past Pluto.

Since then, the long-distance voyager has been probing the edge of our sun's influence and studying objects in the Kuiper Belt, the cold, donut-shaped ring of icy objects that circles the outer solar system beyond Neptune.

New Horizons is currently speeding away from Earth at a rate of 300 million miles (483 million km) per year, according to NASA.

Three weeks from now, New Horizons will begin conducting a study of hydrogen in the outer heliosphere, the region of space influenced by the stream of charged particles blowing outward from the sun, known as the solar wind.

The data the probe is collecting at the farthest reaches of our solar system is the first of its kind. It could help scientists understand what happens at the boundary between the sun's region of influence and interstellar space, known as the "termination shock."

Only two spacecraft have crossed this boundary before, NASA's twin Voyager probes. However, those far-flung explorers weren't equipped with the same scientific instruments as New Horizons, which enable it to conduct more sensitive measurements of this distant region of the solar system.

"The data from the termination shock encounter will be a treasure trove for space physicists worldwide who are eager to understand how this vast boundary works," Pontus Brandt, New Horizons project scientist at APL, previously told Space.com. "All these discoveries from pioneering missions like Voyager and New Horizons teach us how little we know about what lies beyond."

Over the past 250 years, the number of "planets" in our solar system has ranged from six to nine — and, briefly, even 11 — depending on what astronomers knew at the time and how they defined a planet. As the United States prepares to celebrate its 250th anniversary, that changing tally offers a unique lens on humanity's evolving understanding of the cosmos since 1776.

Throughout history, astronomers have discovered new worlds, identified entirely new classes of celestial objects and repeatedly revised the very definition of a planet. The result is a surprisingly complicated answer to one of astronomy's most basic questions: How many planets are there in our solar system?

"The change in the number of recognized planets well represents how science is done," Kevin Schindler, historian and public information officer at Lowell Observatory, told Space.com. "Scientists discover something — a planet, dinosaur fossil, or beetle, for example — study it and classify it. With further study, and by studying more examples, scientists learn more and update their understanding of the thing, sometimes reclassifying it."

When the United States declared independence in 1776, astronomers recognized just six planets: Mercury, Venus, Earth, Mars, Jupiter and Saturn. That changed only five years later when William Herschel discovered Uranus in 1781, expanding the known solar system and increasing the planet count to seven.

The tally grew again in 1801 with the discovery of Ceres, a world orbiting between Mars and Jupiter. Three similar objects were found soon afterward, and for a time astronomers considered all of them planets, briefly bringing the total to 11. However, as additional objects were discovered, scientists realized they represented a distinct population and those objects were reclassified as asteroids, reducing the planet count down to seven, Schindler explained.

Neptune's discovery in 1846 raised the total to eight, while Clyde Tombaugh's discovery of Pluto at Lowell Observatory in 1930 established the familiar nine-planet solar system. This changing planet count underscores a broader pattern in humanity's journey of exploration and discovery over the past 250 years.

"I think it shows that exploration and the thirst to understand the universe around us is ingrained in us, it's part of our DNA and we continue to want to learn," Schindler said. "Our founding fathers established the United States based on scientific principles such as reason and logic, observation, evidence-based analysis, and openness to debate different points of view and be open to change."

For much of the 20th century, the answer seemed settled: The solar system had nine planets. However, beginning in the late 1950s, advancing spacecraft technology allowed scientists to study planets, moons and smaller bodies up close, revealing worlds far more diverse and dynamic than could be seen through telescopes alone. Then, in the early 1990s, astronomers began discovering a growing population of icy worlds beyond Neptune.

"The discovery of Trans-Neptunian Objects (TNOs) — numerous objects similar to Pluto but with orbits extending much farther away from the sun — tells us a lot about our 'local' environment," astronomer Kyler Kuehn, director of science, technology and operations at Lowell Observatory, told Space.com in an email, emphasizing the impact discovering TNOs has had on the classification of Pluto.

"While Pluto was the first TNO to be discovered, the fact that there could be millions of similar objects populating the outskirts of the solar system naturally leads to the question 'Why should we treat Pluto differently than any of the others?'" Kuehn said.

The growing number of TNO discoveries ultimately established that Pluto was part of the Kuiper Belt, a vast reservoir of icy bodies extending beyond Neptune's orbit. As a result, Pluto was no longer viewed as a unique outlier, but rather one member of a much larger population. This dramatically reshaped astronomers' understanding of the solar system, exposing a far more complex outer frontier than anyone in 1776 could have imagined.

"It doesn't fit into the relatively tidy structure of the solar system as understood in 1776," Schindler said. For 18th-century astronomers, "it likely wouldn't be a surprise to find new planets or a new class of planet-like bodies (asteroids), but the discovery of an entire new zone of bodies, which was more [chaotic], probably would have been a surprise, showing that the solar system is not nearly as tidy and stable as believed."

The debate surrounding Pluto's classification culminated in 2006, when the International Astronomical Union adopted a formal definition of a planet. Under that definition, a planet must orbit the sun, be massive enough to become nearly round under its own gravity and have "cleared the neighborhood" around its orbit. While Pluto met the first two requirements, it did not meet the third and was reclassified as a dwarf planet, dropping the official planet count from nine back down to eight.

Controversy over Pluto's classification continues today. Schindler said scientists who oppose Pluto's planethood typically favor a dynamical definition focused on how a body interacts with and dominates its orbital environment. Meanwhile, those who favor Pluto's return to planetary status generally support a geophysical definition based on an object's physical properties. In fact, Pluto's 2015 flyby by NASA's New Horizons spacecraft further fueled the debate by revealing a surprisingly complex world with mountains, glaciers and active geology.

"There has not been any resolution between the two sides," Schindler said, noting that public interest has also helped keep the debate alive.

Most recently, NASA Administrator Jared Isaacman added his voice to the conversation by saying he believes Pluto should once again be considered a planet and that the scientific community should revisit its classification.

"Arguing about the technical definition of 'planet' doesn't actually change anything about the (dwarf) planet itself, but how we classify things can be extremely important to the kinds of questions we even think to ask, " Kuehn added. "Scientific definitions have changed and will continue to change over the course of decades and centuries as we learn more."

The changing planet count demonstrates the foundations of how science works. Schindler compared Pluto's reclassification to the dinosaur Brontosaurus, which was renamed Apatosaurus after further study before later regaining its original classification as scientists refined their understanding.

Therefore, future discoveries, both within our solar system and beyond it, could further reshape scientists' understanding of planetary systems and how planets are classified.

"That might lead us to a more useful definition of planet than anything we are even considering now," Kuehn said. Looking forward, "I think we will learn a lot more about the extreme edges of our solar system — we have barely scratched the surface."

Two hundred and fifty years ago, astronomers knew of only six planets. Today, the official count in our solar system stands at eight, as the debate continues over whether that number tells the whole story.

As the United States marks its 250th anniversary, the changing planet count serves as a reminder that discovery is never finished. Every new observation has the potential to reshape our understanding of the cosmos — just as it has since America's founding — and perhaps even change the answer to one of astronomy's oldest questions: How many planets are there in our solar system?

A very small body, far from the sun in an icy outer realm of the solar system, has mysteriously grown an atmosphere — and scientists are stumped as to how and why this happened.

One possibility is that the atmosphere was produced through cryovolcanism, or basically ice volcanoes Another idea is the atmosphere could be related to an impact, but in that case, the impact must have happened relatively recently since such a thin atmosphere would leak into space and vanish within a thousand years.

The object with an atmosphere, designated (612533) 2002 XV_93, is a trans-Neptunian object, or TNO, meaning it exists in the Kuiper Belt beyond the orbit of the outermost planet, Neptune. More specifically, it is a "plutino," meaning that, like Pluto, it is in a 2:3 orbital resonance with Neptune. This means it orbits the sun twice for every three orbits that Neptune makes.

Furthermore, (612533) 2002 XV₉₃ is significantly smaller than Pluto, being about 310 miles (500 kilometers) across compared to Pluto's diameter of 1,477 miles (2,377 kilometers).

Pluto is massive enough to retain a thin atmosphere, known as an exosphere, when it is near perihelion (the closest point in its elliptical orbit to the sun). It also retains some ices, such as molecular nitrogen, methane and carbon monoxide, that can sublimate into gases. As Pluto begins to move away from the sun over the course of its 248-year-long orbit, those gases gradually freeze out again back onto the surface.

However, no other object in the Kuiper Belt or beyond has been found to possess an exosphere, although methane outgassing has been detected on Pluto's fellow dwarf planet, Makemake. The Japanese astronomers, both professional and amateur, who took part in a series of observations of (612533) 2002 XV₉₃ as it occulted — or passed in front of — a star on January 10, 2024, didn't expect to find an atmosphere either.

If an object such as (612533) 2002 XV₉₃ were truly airless, then the star would instantly wink out as the object passed in front of it from our point of view. However, that's not what happened.

From Earth, stellar occultations of this sort can be seen along very narrow strips on the surface of our planet. By positioning observers along the edges of that strip, it is possible to determine the size and shape of the occulting body.

To that end, a team made of professional and amateur astronomers, led by Ko Arimatsu of the Ishigakijima Astronomical Observatory of the National Astronomical Observatory of Japan (NAOJ), observed (612533) 2002 XV₉₃'s occultation of a 15 magnitude star on January 10, 2024 from four different observing sites in Japan. (The magnitude of a star explains how bright it is; the moon's magnitude, for instance, is about -12 so a magnitude 15 star is quite faint.)

The telescopes the team used included the 3.4-foot (1.05-meter) professional telescope at Kiso Observatory run by the University of Tokyo, and amateur-grade 200mm and 250mm (approximately 8- and 10-inch) telescopes armed with CMOS cameras, which are sensitive enough to detect the the light of the star fading gradually before disappearing behind the limb of the occulting object. This would only happen if there were an atmosphere to attenuate or refract the starlight.

The exosphere is pretty thin, amounting to a surface pressure of between 100 and 200 nanobars, which is 5 million to 10 million times thinner than Earth's atmosphere. To place the exosphere on (612533) 2002 XV₉₃ into greater context, Pluto's thin atmosphere has an average surface pressure of 10 millibars.

What the atmosphere of (612533) 2002 XV₉₃ is made from is uncertain. As is the case with Pluto, a nitrogen-rich atmosphere with traces of methane and carbon monoxide would be expected, but previous observations by the James Webb Space Telescope found no evidence of these particular ices on the surface that could sublimate to form an atmosphere. And at between 40 and 50 degrees above absolute zero, (612533) 2002 XV₉₃ is far too cold for water-ice and carbon-dioxide ice to sublimate into vapor.

With none of these ices available, where has the atmosphere come from? Arimatsu and their team have two possible explanations, but neither is without their problems.

One scenario is that a cometary body has impacted (612533) 2002 XV₉₃. The gases in the atmosphere could then have come from the impactor, but given the rate of loss to space in the low gravity, such an atmosphere would be temporary and would dissipate within a thousand years. If true then this would mean that we are exceptionally fortunate to have observed (612533) 2002 XV₉₃ at around the same time that this presumably rare impact happened.

A huge coincidence, or just good luck?

An alternative idea is that the ices that have sublimated are below the surface, and some form of cryovolcanic activity has released them. However, what could be driving this activity is unknown.

Either way, the existence of the exosphere around (612533) 2002 XV₉₃ changes what we thought we knew about which kinds of objects can support an atmosphere.

"This discovery suggests that the traditional idea that global dense atmospheres form only around larger planets must be revised," said Arimatsu's team in their research paper.

The next step is to try and figure out what the composition of the exosphere is, a task ideally suited to the James Webb Space Telescope. Monitoring the density of the exosphere will also offer clues. If the density decreases over the next few years, then the source of the exosphere is likely to be due to an impact as the gases will be leaking into space. If the density remains constant, then the atmosphere is probably being replenished via outgassing.

The findings were published on May 4 in Nature Astronomy.

NASA chief Jared Isaacman wants to restore Pluto to its former glory.

In 2006, the International Astronomical Union (IAU) stripped Pluto of its planethood, reclassifying the icy world as a "dwarf planet." The decision was controversial, and not just because it forced schoolchildren around the world to learn a new mnemonic for our solar system's major denizens.

Little Pluto was beloved and remains so, especially in the United States. After all, it's the only planet discovered by an American, Clyde Tombaugh, who made the historic find in 1930 using imagery captured by Lowell Observatory in Arizona. Twenty years on, many Pluto lovers are still fighting the IAU's decision, claiming it was unscientific and inconsistently applied.

The IAU defined a planet according to three newly pronounced criteria: It has to orbit the sun, be massive enough to be spherical, and clear its orbit of debris. Pluto fell short on the third count, according to the IAU, as it shares space in the distant Kuiper Belt with many other dwarf planets. But Earth shares orbital space with lots of asteroids, as does Jupiter, Pluto-planet advocates note. So why was Pluto singled out?

We now know that such Pluto defenders include Isaacman, a billionaire private astronaut and tech entrepreneur who became NASA chief this past December.

Isaacman testified about the White House's 2027 NASA budget request today (April 28) before the U.S. Senate Committee on Appropriations. At the very end of the hearing, Republican Sen. Jerry Moran asked the NASA administrator his thoughts on Pluto, noting that Tombaugh hailed from Moran's home state of Kansas.

"Senator, I am very much in the camp of 'make Pluto a planet again,'" Isaacman replied.

"And I would say, we are doing some papers right now on, I think, a position that we would love to escalate through the scientific community to revisit this discussion and ensure that Clyde Tombaugh gets the credit he received once and rightfully deserves to receive again," the NASA chief added.

As those words indicate, all NASA (or any Pluto advocates) can do on the matter is escalate the discussion. The ultimate decision on Pluto's status lies with the IAU, a global society of professional astronomers that defines celestial objects and assigns official names to them and their surface features.

A significant escalation occurred in July 2015, when NASA's New Horizons spacecraft returned the first-ever up-close imagery of Pluto. Those photos revealed a stunningly diverse world with towering mountains, vast nitrogen-ice glaciers and other jaw-dropping features, including a now-famous heart-shaped landform that mission scientists dubbed Tombaugh Regio.

New Horizons' historic flyby wasn't enough to get Pluto its planethood back. Will things be different now that NASA's chief is pulling so openly for the farflung world? We'll have to wait and see.

Dwarf planets may be small, but they're some of the most intriguing objects in our cosmic neighborhood.

These icy bodies orbit the sun just like the major planets, yet they follow their own rules, often traveling along elongated paths far beyond Neptune. Their distant homes in the Kuiper Belt and scattered disk make them difficult to study, but every new mission reveals something surprising.

As you dive into this word search, you'll encounter the names of these fascinating dwarf planets.

Think of this quiz as your own mini‑expedition to the outer solar system —no spacecraft required.

See how well you score below!

Scientists have detected methane gas on the dwarf planet Makemake, indicating that the distant body is a dynamic icy world.

The discovery was made by a team led by the Southwest Research Institute (SwRI) using NASA's James Webb Space Telescope (JWST).

Makemake is one of the largest and brightest worlds beyond Neptune, and becomes only the second trans-Neptunian object, after Pluto, found to have a confirmed presence of gas.

"The Webb telescope has now revealed that methane is also present in the gas phase above the surface, a finding that makes Makemake even more fascinating," said SwRI's Silvia Protopapa, lead author of a new paper soon to be published in The Astrophysical Journal Letters, in a statement.

"It shows that Makemake is not an inactive remnant of the outer solar system, but a dynamic body where methane ice is still evolving," Protopapa added.

Makemake is about 890 miles (1,430 kilometers) in diameter, meaning it is two-thirds the size of Pluto. JWST made the methane find by picking up a telltale spectral signature of solar light re-emmitted by methane molecules.

The finding of methane in gas form could be due to either the presence of a tenuous atmosphere on Makemake, or a more transient activity such as that experienced by comets when its volatiles sublimate, or from cryovolcanic plumes, according to the study authors.

The detected atmosphere, if it is indeed a permanent feature of Makemake, is very tenuous. Its surface pressure is only about 10 picobars, or 100 billion times below Earth's atmospheric pressure, according to the authors' models.

The authors produced models for a scenario in which methane is released in plume-like outbursts. These simulations suggest that methane could be released at a rate of a few hundred kilograms per second, making the level of activity comparable to the water plumes on Saturn's ocean moon Enceladus.

"Future Webb observations at higher spectral resolution will help determine whether the methane arises from a thin bound atmosphere or from plume-like outgassing," said Ian Wong, staff scientist at the Space Telescope Science Institute and co-author of the paper.

Earlier observations of Makemake made when the icy world passed in front of a star had shown that it did not harbor a significant atmosphere, but did not rule out a much thinner one.

The paper, titled "JWST Detection of Hydrocarbon Ices and Methane Gas on Makemake," is available on the preprint repository arXiv.

Anyone hungry for the icy crunch of a Pluto-like body? No? Well, one nearby white dwarf is going all in on a Pluto-esque snack.

According to NASA, the Hubble Space Telescope has spotted a "burned-out star" snacking on a "fragment of a Pluto-like object" that's not too far from Earth — only about 260 light-years away, which is very close when considering the vastness of the cosmos. However, it's not actually the snacking itself that's fascinating, but rather the icy composition of the exo-Pluto. The object appears to be filled with volatiles like carbon, sulphur, nitrogen and oxygen, which indicate the presence of water. According to Hubble data, the exo-Pluto comprises 64% water ice.

"We were surprised. We did not expect to find water or other icy content," Snehalata Sahu of the University of Warwick, who led the analysis of a Hubble survey of white dwarfs, said in a statement. "This is because the comets and Kuiper Belt–like objects are thrown out of their planetary systems early, as their stars evolve into white dwarfs. But here, we are detecting this very volatile-rich material."

This white dwarf was once a sun-like star, but after it burned up its fuel, it collapsed into an extremely dense stellar remnant. Given the dwarf's intense gravitational pull, scientists believe it drew in a Pluto-like planetesimal from its own version of the Kuiper Belt — a region in our solar system filled with icy bodies, from comets to dwarf planets — and tore it apart. What Hubble has captured is just the remaining fragments of the exo-Pluto, something it's uniquely qualified to do with its Cosmic Origins Spectrograph, which analyzes both near- and far-ultraviolet wavelengths.

Studying the white dwarf and the exo-Pluto fragments is like looking into the future of our own solar system. Eventually, the sun will also turn into a white dwarf, pulling in not only planets, but also icy Kuiper Belt objects. "If an alien observer looks into our solar system in the far future, they might see the same kind of remains we see today around this white dwarf," says Sahu.

Next up, the team hopes to use NASA's James Webb Space Telescope to study the white dwarf and the exo-Pluto in infrared light.

The team's research was published in the journal Monthly Notices of the Royal Astronomical Society on Sept. 18.

The first interstellar object to visit the solar system may have been a fragment of an icy exoplanet, research suggests.

When 1I/'Oumuamua was first spotted in 2017, astronomers quickly determined that it came from outside the solar system. But although it was initially classified as a comet from another star system, it may actually be the skin of an "exo-Pluto," a completely unexpected class of Pluto-like objects anticipated to visit the sun.

"Everything about this object is consistent with it being a slab of nitrogen ice like you see on the surface of Pluto," said Steve Desch, an exoplanet researcher at Arizona State University. Desch presented his findings in July at the Progress in Understanding the Pluto Mission: 10 Years after Flyby conference in Laurel, Maryland.

Instead of being a mix of water ice, rock and carbon-rich material left over from the formation of the solar system, 'Oumuamua appears to be almost pure nitrogen ice. And rather than being a compact ball, the visitor is more elongated than any known body in the solar system and starkly different from the interstellar Comets 2I/Borisov and 3I/ATLAS, the only other known interstellar visitors.

"'Oumuamua is in a different category of object," Desch told Space.com by email. "It's much harder to find, but there are a lot more of them."

"We weren't expecting objects like this"

Planets arise from the cloud of gas and dust left over after a star is born. The first few million years are chaotic as the growing worlds jostle for their place around the young star.

In the solar system, the dance of the giant planets cast out a wealth of material. Most of the icy stuff was ejected; scientists think the icy bodies in the Kuiper Belt beyond Neptune today make up only a small portion of the original ejecta. Early on, there may have been enough material to create as many as 2,000 Pluto-like objects, along with 6,000 other, larger dwarf planets, according to Desch.

"Each Pluto would have been pummeled with a Vesta-mass of material," Desch said at the conference, referring to the second-largest object in the asteroid belt. (The largest, Ceres, is also classified as a dwarf planet.)

These collisions would have carved out some of the outermost layer of the wannabe planets. Observations made by NASA's New Horizons spacecraft during its 2015 flyby suggest that most of Pluto's surface is made of nitrogen ice, with some water ice acting as "bedrock." Although some of this base layer was likely ejected as well, Desch and his colleague Alan Jackson, also of Arizona State University, used simulations to determine that most of the material scraped from the baby Plutos was nitrogen.

During the shake-up of the solar system, these objects would have been redistributed. Passing by the sun too often would have caused many of them to evaporate quickly. Some would have been hurled inward, toward the sun. Others would have been tossed outward by Jupiter’s gravity. A handful of that group may have been captured in the Oort cloud at the very edge of the solar system, but most would have ended up adrift in interstellar space.

If planetary dances are common around other stars — and a growing number of observations suggest that they may be — then fragments of exo-Plutos may be ejected alongside comets and full-size planets.

There are hints that some objects classified as comets may actually be chunks of Pluto. In 2018, a separate research team reported that the unusual chemistry of Comet C/2016 R2 hints that it might be a collisional fragment from a Kuiper Belt object. Two other comets, C/1908 R1 Morehouse and C/1961 R1 Humason, have similar nitrogen-rich compositions that could classify them as scrapings from a proto-Pluto.

In a pair of papers published in 2018 and 2021 in the Journal of Geophysical Research: Planets, Desch and Jackson more fully explored how the unusual properties of 'Oumuamua would be better explained by a fragment of a Pluto-like object than by a comet.

"Because we had hardly seen such objects in the solar system, we weren't expecting objects like this," Desch told Space.com. "But we should have. Fragments of icy surfaces from Pluto-like dwarf planets were almost certainly ejected from our solar system, and 'Oumuamua made us come to grips with how much material must have been ejected."

An unlikely comet

When astronomers first spotted 'Oumuamua, it didn't quite meet their expectations of an exocomet. Although its rapid speed was one of the first signs of its extrasolar origin, it was moving much more slowly than anticipated. Solar system comets are made of water ice, silicates and carbon-rich material, while 'Oumuamua was nitrogen-rich. At about 330 feet (100 meters) in diameter before the sun began to melt its ice, 'Oumuamua was also far smaller than most comets, which typically range from about a few kilometers to tens of kilometers in diameter.

Finally, the object had an unusual shape that puzzled astronomers. Eventually, they determined that 'Oumuamua didn't have the roughly spherical core typically seen in comets; instead, it was elongated, or "pancake-shaped," Desch said.

'Oumuamua's low speed could be explained by its ejection from a young star. As stars age, gravitational interactions with their neighbors provide an occasional speed boost. If a fragment from an icy world was ejected early, the star would be traveling relatively slowly, imparting that reduced speed to its expelled material.

The nitrogen-rich material also suggested a youthful lifetime. Exposure to cosmic rays erodes the nitrogen ice, leaving behind water-ice objects that are likely more plentiful. Desch and Jackson estimate that 'Oumuamua is less than 2 billion years old, and perhaps as young as 500 million years old. They suspect it came from a young system, perhaps in the Perseus arm, the closest spiral in the Milky Way to the sun's location in the Orion arm.

The short-lived nitrogen is what made 'Oumuamua so easy to detect. While the water-ice leftovers may be more plentiful, nitrogen ice shines more brightly. But it also evaporates easily; Desch and Jackson estimate that by the time 'Oumuamua was observed, it had lost more than 90% of the mass it had brought into the solar system.

Altogether, it looks as though chunks of exoplanets may be quite plentiful.

"I think these objects are strong support for the idea that fragments of Pluto surfaces are part of the population of things ejected from the solar system," Desch said.

Indeed, the prompt discovery of 'Oumuamua suggests that interstellar objects may be an order of magnitude more abundant than formerly thought. Desch said he expects astronomers will find many more interstellar visitors using the Pan-STARRS and ATLAS surveys that found 'Oumuamua and ATLAS, as well as the newly operational Vera Rubin Observatory. By studying objects from beyond the solar system, researchers may be able to understand more about the outermost dwarf planets.

"More observations of 'Oumuamua-like objects … would tell us a lot about the composition of Plutos," Desch said.

He pointed to observations from New Horizons that suggest our own Pluto may have had a thick coat of nitrogen ice that was lost by impacts and other processes over the 4.5 billion-year life of the solar system.

Two of Pluto's midsize moons may be made of the guts of its largest moon, Charon, new research suggests.

New observations with the James Webb Space Telescope (JWST) show that the two moons, Nix and Hydra, are more similar to the interior of Charon than to other objects in the surrounding Kuiper Belt. The findings hint that the midsize satellites may be composed of the bits of Charon's interior that were ejected during the rough collision that formed Pluto and Charon.

According to previous simulations, Pluto and Charon formed by a "kiss-and-capture" process. The pair's progenitors engaged in a rendezvous that stripped off the outer layers of proto-Charon to create a disk of icy debris. That debris later came together to form at least four smaller moons: Nix, Hydra, Kerberos and Styx, according to this hypothesis.

JWST has provided the opportunity to investigate that idea. Earlier this year, researchers used the space telescope to classify the colors of trans-Neptunian objects (TNOs) — objects beyond the orbit of the eighth planet — in the outer solar system. But the new research shows that Nix and Hydra are not well matched to any of those classifications.

According to Brian Holler, a planetary scientist at the Space Telescope Science Institute, the mismatch is caused primarily by reddish material on the surface of the moons, likely something containing carbon. Holler presented his new findings at the "Progress in Understanding Pluto: 10 Years After Flyby" conference in July in Laurel, Maryland.

"This surface type appears to be largely unique in the trans-Neptunian region," he told Space.com by email.

In fact, Charon's ancient interior may be falling back onto the surface of Charon today. As Nix and Hydra are hit by small meteorites, debris from their surfaces may be flung into space due to their small mass and low gravity. The ejecta can then be captured by Charon's gravitational pull and then fall to its surface.

"We may be looking at the original material of the proto-Charon," Holler said in his presentation in July. Ejecta from Nix and Hydra may fall back onto the surface to create a centimeters-thick layer of dust over time. Because Charon is a geologically dead world, that dust remains on the surface, creating a layer similar to the crust and upper mantle that were stripped off in the collision that birthed it.

"We might be seeing Charon today as it was [before the collision]," Holler said.

A well-timed glimpse

When NASA's New Horizons spacecraft flew by the Pluto system in 2015, it caught a tantalizing glimpse of Nix and Hydra. That included compositional data for the pair. Enhanced color images of Nix revealed a red-tinted bull's-eye pattern. Both moons are irregularly shaped.

JWST's observations didn't necessarily reveal significant detail about the satellites, Holler said. Instead, they provided the ability to directly compare them with other TNOs. The space telescope was focused on studying Pluto and Charon. "The detections of Nix and Hydra are completely serendipitous," he said.

The previous study that used JWST to classify TNOs filled a gap between the visible and near-infrared wavelengths captured by New Horizons. This allowed a direct comparison between Pluto's midsize moons and other outer solar system objects "that would otherwise not have been possible," Holler said.

Nix and Hydra themselves are likely to be relatively unaltered since their birth. They are too small to host active geologic processes, so the only changes in their surface likely would have come from impacts and space weathering.

"Overall, I believe the surfaces of Nix and Hydra are as close to unaltered as you can get," Holler said.

Holler hopes to use JWST to make more in-depth observations of the pair.

"The obvious next step after an imaging appetizer is to order a spectroscopic entree," Holler said. Such observations would pry into the composition of the moons and may answer questions about the ammonia-rich material on the surface. Ammonia is efficiently destroyed by solar radiation and cosmic rays, so either something is replenishing the molecule on Nix or there is some sort of balance in its destruction and re-formation, Holler said.

By looking at similar systems, researchers may get a better understanding of what's going on in the two minor moons. If other TNOs have undergone similar impacts that created their own minor satellites and those compositions could be studied, it could provide even more insight into Pluto's small-but-mysterious moons.

Once hailed as the ninth planet, Pluto has long captured the imagination of stargazers and scientists alike. Though it was reclassified as a dwarf planet, its story is far from small.

Pluto is a land of extremes — icy mountains, vast plains of frozen nitrogen, and a thin atmosphere that changes with its seasons. Its surface is marked by heart-shaped regions and mysterious dark patches, revealing a surprisingly dynamic world.

In this quiz, you'll explore its secrets, history, and the discoveries that continue to reshape our view of the solar system.

Whether you still think of Pluto as a planet or embrace its dwarf status, this quiz will take you on a journey through its icy terrain, curious moons, and scientific milestones.

Try it out below and see how well you score!

Beyond the familiar planets lies a fascinating frontier filled with icy worlds, mysterious orbits, and cosmic curiosities. These are the dwarf planets—small but mighty members of our solar system that don’t quite fit the mold of the eight major planets.

From the icy reaches of the Kuiper Belt to the mysterious depths of the asteroid belt, dwarf planets come in all shapes and sizes.

This quiz will test your ability to name them all—and maybe even learn a few fun facts along the way.

Whether you're an astronomy buff or just curious about space, this quiz will challenge your knowledge and spark your imagination. So grab your telescope (or just your brain), and let's see how many dwarf planets you can name!

Try it out below and see how well you score!

When NASA's New Horizons spacecraft sped by Pluto in 2015, it revealed an incredible world of ice and haze carved by various geological processes — hinting that an ocean may have played a role in the dwarf planet's recent history. The bounty of scientific riches has left researchers working to solve some of the tiny world's mysteries a decade after the spacecraft's flyby.

"There's still a lot of questions that are open," Carly Howett, a planetary scientist at the University of Oxford and a New Horizons team member, said last month at the Progress in Understanding the Pluto Mission: 10 Years after Flyby conference in Laurel, Maryland. With such questions lingering, Howett and her colleagues designed a follow-up mission in hopes of finally solving some of Pluto's mysteries.

Such a mission, sent to investigate the outskirts of the solar system, would span several decades. But it's far from being approved. "This mission could operate for over 50 years, challenging engineering, mission operations, and data analysis in ways that have never been done before," Howett wrote in a 2021 study published in the Planetary Science Journal detailing the mission concept.

A subsurface ocean?

In Roman mythology, Pluto is the god and ruler of the dead. For a return mission to the dwarf planet, Howett and her colleagues opted for the name Persephone, after Pluto's wife and "queen of the underworld" in Greek mythology.

"Given that Pluto is named after the Roman god of the underworld, and that we wanted a female name to reflect our diverse team with many women in leadership roles, this name seemed apt," Howett wrote.

Persephone would carry 11 instruments, all based on tools flown on previous missions but with some alterations. The primary question it would seek to answer would be whether Pluto has a subsurface ocean today.

If that question had been asked before New Horizons sped by, most scientists would have said it was unlikely. While many icy worlds may start off with a watery layer, it freezes over time. To remain liquid for the 4.5 billion-year life of the solar system, that ocean must stay warm.

Related: Supervolcano eruption on Pluto hints at hidden ocean beneath the surface

Some moons constantly flex as they gravitationally interact with their host planet and neighboring moons, keeping their ocean from freezing. Charon, Pluto's largest moon, is nearly as massive as Pluto; they are often called a "double planet" (though neither meets the criteria for a planet). Charon could potentially keep Pluto warm, if the ocean had a high enough nonwater component to lower its freezing point, scientists think.

It wasn't until New Horizons revealed the remarkably young, lightly cratered surface of Pluto that most scientists began to consider the possibility of an ocean (although some did before the spacecraft's arrival). New Horizons studied Pluto in depth for only a few hours, although it continued to observe the dwarf planet for months before and after its closest approach.

Persephone, by contrast, would aim to enter orbit around the tiny world for just over three years, allowing much longer close-up views. "There's no substitute for proximity," New Horizons principal investigator Alan Stern said at the same conference.

What Persephone would study

Persephone would study the shape of Pluto to hunt for signs of a telltale fossil bulge — a "beer belly" of sorts caused when gravity pulls on the mass of a world. Bulges form more easily when the layers are liquid, and they can be frozen into place. New Horizons didn't observe such a pileup, but Persephone would send a more sensitive instrument that could make a more detailed examination.

"This mission should be able to image the whole of Pluto," Howett said at the conference. "It should be phenomenal."

Persephone would also seek to determine the composition of Pluto and Charon, using gravity and topography measurements similar to those taken of Saturn's moon Enceladus, and potentially calculate the thickness of the internal ice layers.

Pluto suffers from a century-long winter, and Persephone would arrive in the thick of it. Much of the dwarf planet would be shielded in darkness, so the mission would require instruments capable of peering through the veil. Cameras would map the surface of the entire world in greater depth and in varying wavelengths, including the half that was shrouded when New Horizons sailed past. They would search for hotspots, signs of ongoing activity and eruptions that might indicate a warm interior, as well as look for indications that the surface has changed since the 2015 observations. They would also provide a more detailed crater count on both Pluto and Charon, which would help scientists better understand how active the Kuiper Belt has been over the years.

Although icy worlds abound in the solar system, both Pluto and Charon have unusual surface features. In Pluto's Tartarus Dorsa region, blades of methane ice cover the surface. Scientists suspect that these spikes formed by sublimation as methane skipped instantly from solid to gas, but that's not definitive. And Charon has a strange ice mountain submerged in a frozen moat — a unique feature among icy peaks. Both surfaces are covered with exotic ices, and an understanding of their properties at the frigid temperatures on Pluto would be a key component of the mission.

Related: James Webb Space Telescope deciphers the origins of Pluto's icy moon Charon

In 1988, astronomers spotted a wispy atmosphere around Pluto, but its composition eluded them. New Horizons answered questions even as it raised more. Curiously, it appears that Pluto is slowly shedding part of its atmosphere onto Charon, creating a distinctive red pole that may shift hemispheres over time. One of the key objectives of Persephone would be to perform a direct detection of the atmosphere's composition through mass spectrometry.

Persephone would also study the space around the tiny world. Pluto is so far away that light from the sun takes just over 5.5 hours to reach it. From Pluto, the sun is a point-like spot in the sky.

Charon wouldn't be the only moon targeted by the mission. Persephone's launch time would determine how many of the smaller moons it could observe, but it would likely get a good look at the four other satellites as well.

New Horizons revealed bands of water ice on Nix, Hydra and Kerberos, as well as hints of ammonia on Nix and Hydra. Persephone would take a more detailed spectra for these three, as well as Styx, and try to determine how much of their surfaces, along with Charon's, is littered with debris from the collision that likely formed them. Although the best observations of the smaller satellites would most likely be of the close-orbiting moon Styx and the worst of Pluto's outermost moon, Hydra, a flyby of either Kerberos or Hydra might be possible, depending on when the mission launched and arrived at Pluto.

Persephone would remain at Pluto for just over three Earth years. During that time, the spacecraft could use orbits around the binary system to ultimately fling it from the pair.

Extending the mission by one year would allow the spacecraft to visit another Kuiper Belt object, much as New Horizons visited Arrokoth after the Pluto flyby. Such an extension would depend on when the mission launched and arrived, but it could provide significant insight into the debris left over from the solar system's formation. This would provide a big scientific return, because the distance of Kuiper Belt objects makes them challenging to study from Earth.

Pluto: The next generation

A return to Pluto would not be a casual undertaking. While it took New Horizons less than a decade to make the original trip, the changes in planetary alignment would make the next visit's flight alone take just over 27.5 years and would require five Next-Generation Radioisotope Thermoelectric Generators (NGRTGs), nuclear batteries that use the decay of radioactive material to power the spacecraft. The trip's extensive lifetime would require several NGRTGs to keep things warm in the freezing temperatures of space. That's a big ask at a time when plutonium for spaceflight is still at a premium. Currently, NASA's goal is to create 1.5 kilograms of plutonium per year; current RTGs use 4.8 kilograms.

That would require a significant investment in plutonium for outer-planet exploration. It would also take a financial investment. The estimated price tag for the Persephone mission is $3 billion, Howett said in her presentation.

With the potential of a half-century mission, the spacecraft carries several backup systems. But Stern pointed out that, although the timescale is long, it's not unheard of. NASA's Hubble Space Telescope is still functioning after 35 years, and so are NASA's Voyager 1 and 2 spacecraft, which launched 48 years ago.

Another challenge for the Persephone mission relates to the people working on it. With the potential for a 50-year mission, the researchers estimate that Persephone would be a three-generation mission that would cycle through three sets of scientists over their career lifetimes. Information and training would need to be passed from one generation to the next. In fact, Stern said delayed engagement was one of the hardest parts of planning the New Horizons mission.

"We know how to do these things," Stern said. "You have to be patient, and you have to plan for it."

Launch opportunities for the spacecraft are available every year from 2029 to 2032. After that, Jupiter's orbit prevents subsequent opportunities for a full decade.

Persephone was part of NASA's Planetary Mission Concept Study, which funds a range of projects to permit the decadal study to make informed decisions about potential future missions.

"While I think any mission needing more than one RTG is going to struggle to get selected at the moment, I do think the process of doing the mission proposal was useful," Howett said. Not only did the proposal show that such a mission is valid, parts of it, such as the orbital tour, could be used by other missions.

But don't look for Persephone to fly in the near future: the power request alone may keep it off the official books. Howett's mission proposal was one of several requested to help inform NASA's decadal survey about priorities and viabilities of future missions.

But the space agency is continuing to work to improve RTGs and their supporting technology. Howett is hopeful that ongoing technology developments will improve the mission's odds.

"One of the things that Persephone showed was that returning to Pluto to orbit was possible, but it wasn't cheap," she said. "It would have to be a Flagship-level mission."

A landmark on Pluto that was previously designated as an impact crater may actually be the caldera of a supervolcano that has exploded in the past few million years, new research suggests.

When NASA's New Horizons mission flew by Pluto in 2015, it revealed a geologically rich world, rather than the cold, dark landscape many had anticipated. Almost immediately, researchers identified two features, called Wright Mons and Piccard Mons, that were strongly suspected to be icy volcanoes, and further study confirmed their identity.

But not every cryovolcano was easy to spot. The suspected supervolcano, Kiladze, was initially classified as an impact crater. However, now scientists suspect it's something else.

"We evaluated the possibility of the depression as a cryovolcanic caldera versus having an impact crater origin," said Al Emran, a planetary scientist at the Jet Propulsion Laboratory in California. Emran presented his team's results in July at the Progress in Understanding the Pluto System: 10 Years After Flyby conference in Laurel, Maryland.

"We think it's more like Yellowstone Caldera in Wyoming," Emran said. At least two of Yellowstone's eruptions, millions of years ago, reached supervolcano status.

Impact crater or caldera?

Kiladze remains listed as a crater. But the rich supply of water ice surrounding the bowl-shaped feature sparked Emran's curiosity, and he wondered if it might be a cryovolcano instead.

At first glance, the elongated oval bears a strong similarity to an impact crater. It's large, with an average diameter of 2.5 miles (4 kilometers). Its walls are irregularly shaped, and the complex features it would require could easily have been eroded by Pluto's active surface processes.

The landscape itself is marked by pits and other geological features, many of which have collapsed. If an incoming impactor broke through the surface and exposed veins of frozen lava beneath, it could have created the explosive distribution of water ice seen on the surface.

But when Emran dug into the topography maps of Pluto created by the New Horizons team, he realized there was a problem: The crater was too deep. Across the solar system, crater depth scales with crater diameter in a predictable way, and the same law appeared to hold true for other craters on Pluto — but for not Kiladze.

At best, estimates put an impact crater of its size at 1.7 miles (2.74 km) wide. But with the activity flowing across Pluto, material would have been more likely to fill in the crater over time, making it even shallower. Haze particles would have piled up, and melting or slumping ices would have fallen inward.

However, Kiladze isn't shallower than projected; it's deeper. Parts of the basin reach 2.5 deep, and the entire site averages nearly 2 miles (3 km) in depth.

For these reasons, Emran and his colleagues suspect that Kiladze is a caldera, a massive depression created by the eruption and subsequent collapse of a volcano. Magma — or cryomagma — spewing from the surface rapidly over a short period of time can weaken the supporting material, causing it to collapse inward on itself.

Despite the supervolcano's collapse, the eruptive power of Kiladze would have been impressive. Emran and his colleagues calculated that the explosion could have ejected as much as 240 cubic miles (1,000 cubic kilometers) of icy cryomagma across the surrounding region, achieving the definition of a supervolcano. Although Yellowstone has erupted more than 80 times over its lifetime of more than 2 million years, only two explosions have been classified as supervolcanic.

Kiladze may have blasted out its cryomagma in a single explosive event, or it may have spread its eruptions over time. Either way, its most recent event spewed water ice at least 60 miles (100 km). Emran suspects that estimate is low, however, as more water ice is likely visible at resolutions smaller than New Horizons could reach.

"One or more cataclysmic explosive eruptions that resulted in the excavation and collapse of what is seen as the Kiladze caldera would be expected to scatter the water-ice cryomagma widely for a thousand or more kilometers, leaving exposures too small to be seen in the data at hand," the authors wrote in a paper recently published in The Planetary Science Journal.

Clues in the ice

Kiladze sits just north of Sputnik Planetia, the icy heart to Pluto. Although much of the dwarf planet's surface is covered with a variety of ices, very little of the surface matches what you might find in your freezer at home. Temperatures on Pluto are so cold that water ice serves as the bedrock for the dwarf planet, while other ices pile on top.

But in the neighborhood surrounding Kiladze, water ice stretches across the surface. The ice has traces of an unidentified ammoniated compound. "It's difficult to determine the exact composition," Emran said. In fact, that particular signature of ammonia is not seen anywhere else on Pluto.

Ammonia may be what allows the frigid ice to flow. Its addition lowers the freezing point of water, allowing it to remain liquid for longer periods. Beneath the surface, pockets of water and ammonia could have avoided freezing as Pluto's bedrock solidified. Eventually, tectonic pressure could have driven the icy magma to the surface, spewing it across the landscape around Kiladze, Emran explained.

The mysterious traces of ammonia could also help to date the cryovolcano. Pure ammonia is quickly obliterated by the solar wind, ultraviolet particles and cosmic rays. Its faint presence suggests that the latest eruption occurred fairly recently in Pluto's history, Emran said.

Pluto's haze can help to nail down geological activity. The light shroud covers the dwarf planet — a result of methane and other gases jumping from solid to gas. As the particles grow, they eventually fall back to the surface and spread all over the dwarf planet. Grounded haze particles would cover the water ice like a blanket, obscuring signs of the cryolava, Emran explained. If such a layer had formed, New Horizons would have seen nitrogen-rich ice instead of signs of water-rich volcanism.

Burying the water ice requires at least 0.4 inches (10 millimeters) of haze particles falling to Pluto. That process takes at least 3 million years, Emran said. That could mean Pluto isn't as frozen as previously thought.

"If Kiladze erupted as recently as 3 million years ago, it would indeed suggest that Pluto's interior may still retain some residual warmth today," Emran said. "This aligns with the idea that cryovolcanism on Pluto could be ongoing or episodic."

Astronomers have discovered a massive new solar system body located beyond the orbit of Pluto. The weird elongated orbit of the object suggests that if "Planet Nine" exists, it is much further from the sun than thought, or it has been ejected from our planetary system altogether.

The strange orbit of the object, designated 2023 KQ_{14 a}nd nicknamed "Ammonite," classifies it as a "sednoid." Sednoids are bodies beyond the orbit of the ice giant Neptune, known as trans-Neptunian objects (TNOs), characterized by a highly eccentric (non-circular) orbit and a distant closest approach to the sun or "perihelion."

The closest distance that 2023 KQ₁₄ ever comes to our star is equivalent to 71 times the distance between Earth and the sun. The sednoid is estimated to be between 136 and 236 miles (220 and 380 kilometers) wide. That makes it 45 times wider than the height of Mount Everest.

This is just the fourth known sednoid, and its orbit is currently different from that of its siblings, though it seems to have been stable for 4.5 billion years. However, the team behind the discovery, made using Subaru Telescope as part of the Formation of the Outer Solar System: An Icy Legacy (FOSSIL) survey, thinks that all four sednoids were on similar orbits around 4.2 billion years ago. That implies something dramatic happened out at the edge of the solar system around 400 million years after its birth.

Not only does the fact that 2023 KQ₁₄ now follows a unique orbit suggest that the outer solar system is more complex and varied than previously thought, but it also places limits on a hypothetical "Planet Nine" theorized to lurk at the edge of the solar system.

"The fact that 2023 KQ₁₄'s current orbit does not align with those of the other three sednoids lowers the likelihood of the Planet Nine hypothesis," team leader Yukun Huang of the National Astronomical Observatory of Japan said in a statement. "It is possible that a planet once existed in the solar system but was later ejected, causing the unusual orbits we see today."

Hello 2023 KQ14. Goodbye Planet Nine?

2023 KQ₁₄ was first spotted in the wide field of view of the Subaru Telescope, located on Hawaii's Mauna Kea volcano, in observations collected during March, May, and August 2023.

The sednoid was confirmed using the Canada-France-Hawaii Telescope during follow-up observations performed in July 2024.

This data was combined with archival data from other observatories, allowing astronomers to reconstruct the orbit of 2023 KQ₁₄ over the past 19 years.

But this is a celestial body that likely formed as the planets of the solar system were taking shape around the infant sun around 4.6 billion years ago. Thus, astronomers were keen to retell the story of its orbit for much longer than two decades.

To do this, Huang and their FOSSIL team colleagues turned to the computer cluster operated by the National Astronomical Observatory of Japan to perform complex numerical simulations. This revealed the orbital stability of 2023 KQ₁₄ for 4.5 billion years and the implications of that steady orbit.

"2023 KQ₁₄ was found in a region far away where Neptune's gravity has little influence," team member and planetary scientist Fumi Yoshida said. "The presence of objects with elongated orbits and large perihelion distances in this area implies that something extraordinary occurred during the ancient era when 2023 KQ₁₄ formed.

"Understanding the orbital evolution and physical properties of these unique, distant objects is crucial for comprehending the full history of the solar system."

Yoshida added that, at present, the Subaru Telescope is one of the only telescopes on Earth capable of making a discovery like that of 2023 KQ₁₄.

"I would be happy if the FOSSIL team could make many more discoveries like this one and help draw a complete picture of the history of the solar system," Yoshida concluded.

The team's research was published on Monday (July 14), in the journal Nature Astronomy.

The James Webb Space Telescope (JWST) has discovered that a hazy sky over frozen Pluto is helping to cool the dwarf planet's atmosphere, while at the same time giving methane and other organic molecules a kick out of Pluto's atmosphere, where some are subsequently being gathered up by Pluto's close companion, Charon.

The discovery of the haze was predicted back in 2017 by planetary scientist Xi Zhang of the University of California, Santa Cruz, to explain why Pluto's thin atmosphere is so leaky. Based on measurements from NASA's New Horizons spacecraft, which hurtled past Pluto and Charon in 2015, planetary scientist Will Grundy at the Lowell Observatory in Arizona calculated that Pluto's atmosphere is losing 1.3 kilograms (2.9 pounds) of methane to space every second, and about 2.5% of this methane is being intercepted by Charon, staining its poles red with organic chemistry. Nowhere else in the solar system do we see an atmosphere leaking onto a neighboring body.

The cause of this atmospheric escape was unknown, but Zhang reasoned that if Pluto's atmosphere contained a layer of haze, then this haze would absorb what little extreme ultraviolet light from the distant sun reaches Pluto, providing the energy to give molecules the nudge they need to escape into space.

Besides the haze heating the atmospheric molecules so that they can escape, Zhang also realized that the haze could have a cooling effect on Pluto's atmosphere — an effect that had previously been detected in Pluto's mesosphere, which is the third layer of the atmosphere above the virtually non-existent troposphere and the denser stratosphere.

Pluto's mesosphere is found between 20 kilometers and 40 kilometers (12.4 to 24.9 miles) high and reaches a maximum temperature of minus 163 degrees Celsius (110 Kelvin/minus 262 degrees Fahrenheit) before cooling at a rate of 0.2 degrees Celsius per kilometer, to a minimum of minus 203 degrees C (70 Kelvin/minus 334 degrees F).

The problem was that, until now, no haze had been detected on Pluto. Then along came the JWST.

Zhang had predicted that any atmospheric cooling spurred on by a layer of haze would result in thermal emission at mid-infrared wavelengths. Mid-infrared emission had been detected coming from the Pluto-Charon system before, going all the way back to Europe's Infrared Space Observatory in 1997, NASA's Spitzer Space Telescope in 2004, and Europe's Herschel Space Observatory in 2012. However, on each occasion, the telescope lacked the resolution to distinguish between Pluto and Charon and determine where the emission was coming from. But JWST, with its 6.5-meter (21.4 feet) primary mirror and Mid-Infrared Instrument (MIRI), is able to distinguish between Pluto and Charon. So Zhang, as part of a team led by Tanguy Bertrand of the Observatoire de Paris, was able to use JWST to detect the thermal mid-infrared emission from the long-elusive haze.

"We use the term 'haze' to describe layers of solid aerosols suspended high in an atmosphere," Bertrand told Space.com. "These aerosols scatter light and reduce visibility, forming a diffuse and semi-transparent layer."

Pluto's atmosphere is mostly nitrogen, with a smidgen of carbon dioxide and hydrocarbons such as methane, benzene, diacetylene and hydrogen cyanide. This atmosphere is exceptionally thin; the surface pressure is just 13 microbars, in comparison to Earth's surface pressure of about 1 bar. (One bar is equivalent to one million microbars.) And because of Pluto's low gravity, the upper atmosphere extends quite a long way from the surface, by several Pluto radii (the radius of Pluto is 1,188.3 kilometers, or 737 miles). All molecules need is a slight nudge to send them spinning out of the atmosphere, and the energy to give them that nudge comes from the sun.

"A significant fraction of the incoming solar extreme ultraviolet radiation is absorbed by the upper atmosphere, leading to heating that powers atmospheric mass loss," said Bertrand. "Atmospheric gases such as nitrogen and methane are responsible for absorbing radiation at these wavelengths."

But how can the haze alternatively cause both atmospheric heating and cooling?

"Cooling or heating depends on the haze properties, such as particle size, shape and composition — i.e., icy with hydrocarbon ice, or non-icy — which are not very well known," said Bertrand. "We are currently investigating this with state-of-the-art microphysical [i.e., on the scale of atoms and molecules] models."

The ability of the haze to cool or heat the atmosphere means that it therefore controls the balance of energy in Pluto's atmosphere, affecting global temperatures, atmospheric circulation and what passes for climate on the frigid dwarf planet. This climate system is dominated by cycles of sublimation and freezing out of molecular nitrogen, methane, and carbon monoxide, much of which hails from the deep glacier in Sputnik Planitia, which is the heart-shaped feature on the dwarf planet's surface.

Zhang described for Space.com this energy balance in detail. "Based on New Horizon's temperature observations from 2015, we found that gas heating significantly exceeds gas cooling," he said. "So there is a net radiative heating of the atmosphere. To maintain energy balance under these conditions, the haze must provide the necessary net radiative cooling. But it remains unclear whether haze has a net cooling effect during other seasons, as Pluto's seasons vary dramatically!"

Those "seasons" are so drastically different because of Pluto's elongated orbit, which takes it from closer to the sun than Neptune to almost twice as far out. Even out here, in the depths of the solar system, this difference in distance markedly affects the amount of heating Pluto receives.

Pluto's haze is similar to the hydrocarbon-rich haze found on Saturn's moon Titan. Both hazes result from the photochemistry of solar extreme ultraviolet light reacting with molecules such as nitrogen and methane. Even the early Earth, prior to the rise of an oxygen-enriched atmosphere over 2.4 billion years ago, may have harbored a haze of hydrocarbons in its atmosphere similar to Pluto, albeit much more dense. Understanding Pluto's atmosphere could therefore potentially teach us something about our own planet's beginnings.

The new study was published in the journal Nature Astronomy on June 2

A potential new dwarf planet has been discovered in the outer reaches of the solar system, and its existence poses the greatest challenge yet to the hypothesis that a ninth planet lurks far from the sun.

"We were very excited to discover 2017 OF₂₀₁ because it was not expected at all," study leader Sihao Cheng of the Institute for Advanced Study in Princeton, told Space.com. "It's very rare to discover an object both large and with an exotic orbit."

"The object's aphelion — the farthest point on the orbit from the sun – is more than 1,600 times that of the Earth's orbit," Cheng explained in a statement. "Meanwhile its perihelion — the closest point on its orbit to the sun — is 44.5 times that of the Earth's orbit, similar to Pluto's orbit."

The outer solar system

We're learning more and more about the outer solar system. Beyond Neptune is the Kuiper Belt; a ring of icy cometary nuclei and planetesimals dominated by Pluto and Charon. The Kuiper Belt begins about 30 astronomical units (AU) from the sun (one AU is the distance of Earth from the sun), its inner edge guarded by Neptune, and extends out to 50 AU. NASA's New Horizons spacecraft is currently exploring the Kuiper Belt.

Meanwhile, the twin Voyager spacecraft have already sped through the Kuiper Belt and have entered a realm called the Scattered Disk, which is thought to go all through way out to more than 1,000 AU and is home to icy bodies on highly elongated and highly inclined orbits. These objects were literally scattered in the region through gravitational interactions with Neptune, and have had their orbits further modified via torques induced by the gravity of passing stars, or the "galactic tide" (the overall gravitational field of the Milky Way galaxy). Beyond the Scattered Disk is the Oort Cloud, which is an immense volume of space that possibly stretches up to a light-year from the sun and is the source of long-period comets.

However, much about the Scattered Disk is still unknown, and besides those long-period comets that venture this way every now and then, no Oort Cloud object has ever been seen — they are too far away and too small. This is why every discovery of a trans-Neptunian object (TNO) on a greatly elongated orbit is vital for piecing together the mystery of the outer solar system.

A new dwarf planet

Around 5,000 TNOs have been discovered until now, but the latest discovery may be one of the most important. Known as 2017 OF₂₀₁, it is currently 90.5 AU away from the sun, but its orbit brings it as close as 4.14 billion miles (6.66 billion kilometers) from our star and as far away as a whopping 157 billion miles (244 billion kilometers). from the sun. For the vast majority of its 24,256-year orbit, 2017 OF₂₀₁ is too far away to be seen with current telescopes; it could only be discovered because its last perihelion came in 1930, and that it's still relatively close.

The object's last perihelion also came, coincidentally, during the same year that Clyde Tombaugh discovered Pluto with a 13-inch (330mm) telescope at Lowell Observatory in Arizona. Would it have been possible for Tombaugh to have also found 2017 OF₂₀₁? Probably not — at magnitude +20.1, this object would have been four magnitudes fainter than Pluto, and it is even fainter today.

Fortunately, telescopic technology has come a long way in the past 95 years, with deep surveys that can capture the passage of a faint object. For example, the Dark Energy Survey (DES) has identified about 800 TNOs — and that's even though DES is ostensibly a cosmological survey. In the same vein, Cheng, along with Jiaxuan Li and Eritas Yang of Princeton University, have been scrutinizing observations made by the Dark Energy Camera Legacy Survey (DECaLS) on the Victor M. Blanco 13-foot (4-meter) telescope at the Cerro Tololo Inter-American Observatory in Chile. They discovered 2017 OF₂₀₁ in archive data going back to 2017 from DECaLS, and also spotted it in old data dating from 2011–12 captured by the 11.7-foot) (3.58-meter) Canada–France–Hawaii Telescope on Mauna Kea.

Based on its brightness and its expected albedo of 0.15 (meaning it would reflect just 15% of the sunlight incident upon it), Cheng's team calculated that 2017 OF₂₀₁ is probably about 435 miles (700 kilometers)) across. This would make it the second largest object found on such an elongated orbit. Although it is substantially smaller than Pluto, which is 1,477 miles (2,377 km) across, 2017 OF₂₀₁ is nevertheless large enough to be classified as a dwarf planet.

Problems for Planet Nine

However, 2017 OF₂₀₁'s existence contradicts the Planet Nine hypothesis, based on our best guess as to Planet Nine's orbit. Planet Nine is a concept that was introduced in 2016 by Caltech astronomers Michael Brown and Konstantin Batygin to explain a perceived clustering of the orbits of many extreme TNOs. The gravity of Planet Nine, which is speculated to be a super-Earth or modest ice giant, would be influencing the orbits of extreme TNOs — or so the hypothesis goes. Yet, the orbit of 2017 OF₂₀₁ is not clustered with the others.

"Many extreme TNOs have orbits that appear to cluster in specific orientations, but 2017 OF₂₀₁ deviates from this," Jiaxuan Li said in the statement.

In our e-mail interview with Cheng, he laid out the repercussions that this could have for the existence of Planet Nine's orbit.

"Planet Nine does allow for extreme TNOs to have unclustered orbits, but those orbits are not stable," he said.

The timescale in which Planet Nine would render 2017 OF₂₀₁'s orbit unstable, and kick it out of the solar system, is in the region of 100 million years. However, the process of placing 2017 OF₂₀₁ in its current orbit, through gravitational interactions with Neptune that pushed 2017 OF₂₀₁ out of the Kuiper Belt — followed by nudges from the galactic tide — takes billions of years. It's possible that 2017 OF₂₀₁ has only recently arrived in its current orbit, which would mean Planet Nine might not have had time to disrupt its orbit yet.

"One important thing is to see if the orbit of our object is stable," Cheng said. "I think, based on analytical criteria, our object is at the boundary between stable and unstable, so further investigation with more comprehensive simulations is needed to definitively rule out the Planet Nine hypothesis."

Intriguingly, 2017 OF₂₀₁ is probably not alone in the outer solar system. It's purely by chance that it happens to be close enough to be detectable — for 99% of its 24,256-year orbit it is too far away to be seen.

"2017 OF₂₀₁ spends only 1% of its orbital time close enough to us to be detectable," said Cheng. "The presence of this single object suggests that there could be another hundred or so other objects with similar orbits and size; they are just too far away to be detectable now.

Just think of that: There could be hundreds of dwarf planets in the outermost reaches of the solar system.

"Even though advances in telescopes have enabled us to explore distant parts of the universe, there is still a great deal to discover about our own solar system," said Cheng.

A pre-print of a paper describing the discovery is available on arXiv.

A surprising chemical difference between Pluto and Sedna, another dwarf planet in the distant Kuiper Belt, is helping scientists nail down their respective masses, a new study reports.

The Kuiper Belt is a region in space beyond the orbit of Neptune that's home to Pluto and most of the known dwarf planets, as well as some comets that are thought to be relics of the solar system's planet-formation era.

"Kuiper Belt objects are icy worlds [that] can tell us what conditions were like billions of years ago," explained study lead author Amelia Bettati, a researcher at Elon University in North Carolina. "Studying them helps scientists understand how planets formed and evolved."

Recent near-infrared spectroscopic studies carried out by the James Webb Space Telescope (JWST) found that Pluto contains both methane and ethane on its surface, key volatile molecules often found in the outer solar system and thought to be remnants from the time when the planets were forming. Sedna, which is less than half as wide as Pluto, was found to have only methane.

Related: What is the Kuiper Belt?

"We hypothesized that the reason for this difference is that Sedna is much smaller than Pluto, so its gravity is weaker," Bettati told Space.com. "This weaker gravity allows methane to escape into space over billions of years, while ethane, which is a heavier compound, stays behind."

While previous studies have identified a general boundary between objects that can hold onto these volatiles and those that cannot, the difference between Pluto and Sedna offers a new clue about how specific escape processes might shape the surface compositions of these distant objects. Sedna, being close to the mass threshold at which volatiles are lost, highlights the importance of understanding how certain chemicals are retained or lost, especially when comparing different Kuiper Belt objects.

"By studying how methane and ethane escape from Sedna, we calculated how massive Sedna must be to explain its current surface composition," said Bettati. "To explain the lack of methane but presence of ethane on Sedna, we must raise the minimum mass estimate for Sedna. This is important because it helps refine our understanding of Sedna's structure and history."

In their study, Bettati and co-author Jonathan Lunine, of NASA's Jet Propulsion Laboratory and the California Institute of Technology, modelled the levels of methane and ethane on Sedna. They verified their model's accuracy using two analogues: Comet 67P/Churyumov-Gerasimenko and Saturn's moon Enceladus.

Europe's Rosetta probe studied Comet 67P up close, and NASA's Cassini spacecraft gathered a wealth of data about Enceladus during the probe's time in the Saturn system.

"Both objects have well-defined measurements and are outer solar system objects, which justifies considering them analogues," said Bettati.

To figure out if enough methane and ethane escaped from these objects to no longer show up in their surface spectra, the scientists needed to estimate how much of these chemicals were originally trapped inside.

They did this under two different scenarios. One assumed that the ratio of methane and ethane to water ice is similar to what was measured on Enceladus, while another looked at the ratio found on Comet 67P during the winter. These comparisons helped them understand how much of these compounds may have been lost over time.

"[We used the] Jeans escape model, [which] is a kind of thermal escape driven by the atmospheric temperature, in which the fastest-moving molecules exceed the escape velocity, but the bulk of the molecules do not," Bettati said.

They also used another model known as hydrodynamic escape, which occurs when the bulk of the molecules are able to escape, rather than just those at the high-velocity end of the distribution. "Much of the atmosphere is in motion, escaping to space," said Bettati.

The models demonstrated that methane has remained stable on Pluto but escaped from Sedna due to its lower mass. Ethane, however, has stayed stable on both objects, even when using two different outgassing rates — 100% (indicating full release of volatiles) and 10% (a smaller release).

This result aligns with the observed surface spectra and provides a more accurate mass estimate for Sedna. The model also explains the absence of methane on another Kuiper Belt object known as Gonggong.

Related: Our solar system map may need an update — the Kuiper belt could be way bigger

"Like Sedna, Gonggong also lacks surface methane," said Bettati. "Since Gonggong is similar in size to Sedna, we believe methane must have escaped from it in a similar fashion. This suggests that smaller Kuiper Belt objects lose methane over time, while larger ones, such as Pluto, can hold onto it.

"If scientists know which gases are likely to be present on different Kuiper Belt objects, their loss rates, and their past compositions, they can better plan future missions."

These findings, in conjunction with JWST observations, will help scientists understand how atmospheres and surface compositions change in the Kuiper Belt and beyond.

"It highlights how JWST is revolutionizing our understanding of the most distant solar system bodies," Bettati said.

The new study was published in February in the journal Icarus.

NASA's Hubble Space Telescope has discovered that a system of asteroids in the distant Kuiper Belt may be triplets, not twins as previously suspected.

If so, the stable trio of icy space rocks would be just the second example of three gravitationally bound space rocks found in the Kuiper Belt, the doughnut-shaped region of icy bodies that lurks out beyond the orbit of Neptune.

The discovery could also challenge our understanding of how Kuiper Belt objects (KBOs) form.

If it is confirmed as a triplet, the system — designated 148780 Altjira — could offer scientists a chance to improve their models of how three gravitationally bound bodies move through space together.

This puzzle, known as the "three-body problem," has been a challenge since Isaac Newton published his work "Principia" in 1687.

"The universe is filled with a range of three-body systems, including the closest stars to Earth, the Alpha Centauri star system, and we're finding that the Kuiper Belt may be no exception," team leader Maia Nelsen, a physics and astronomy graduate of Brigham Young University in Provo, Utah, said in a statement.

Related: What is the Kuiper Belt?

Three's company in the Kuiper Belt

Astronomers Dave Jewitt and Jane Luu discovered the first icy body in the Kuiper Belt, known as 1992 QB1, in 1992. Since then, a further 3,000 KBOs have been cataloged.

Astronomers estimate that several hundred thousand more KBOs measuring over 10 miles (16 kilometers) in diameter could lurk in this icy donut, which begins around 2.8 billion miles (4.5 billion km) from the sun.

The Kuiper Belt is thought to stretch as far as 4.6 billion miles (7.4 billion km) from the sun, which is around 50 times the distance between Earth and our star. The Altjira system sits in the middle of the Kuiper Belt, at around 3.7 billion miles (6.0 billion km) from the sun, or about 40 times the distance between Earth and the sun.

The Hubble Space Telescope images initially seemed to show that the Altjira system was composed of two KBOs located about 4,700 miles (7,600 km) apart.

When the team conducted repeated observations of the Altjira system object's unique co-orbital motion, however, they found the inner object is actually two bodies. These KBOs are so close together that they can't be distinguished individually from such a great distance away.

"With objects this small and far away, the separation between the two inner members of the system is a fraction of a pixel on Hubble's camera, so you have to use non-imaging methods to discover that it's a triple," Nelsen said.

It took 17 years of data from Hubble and the Keck Observatory in Hawaii to observe orbital changes in the Altjira system and make this determination. The data was added to various modeling scenarios, with the most likely explanation being a triple-body system.

"Other possibilities are that the inner object is a contact binary, where two separate bodies become so close they touch each other, or something that actually is oddly flat, like a pancake," Nelsen added.

Of 40 multiple-body systems identified in the Kuiper Belt, this is just the second identified composed of more than two objects. The researchers think these aren't outliers and there are more multi-asteroid systems out there in the outer reaches of the solar system, waiting to be discovered,

The Hubble observations of the Altjira system, which suggest it has a third occupant, support a theory of KBO creation that involves the direct gravitational collapse of matter in the disk of material surrounding the infant sun around 4.5 billion years ago.

This direct collapse pathway is similar to the formation process of stars, albeit it on a vastly smaller scale. Star formation from dense patches of gas and dust can also result in two and three-body systems.

The alternative KBO creation theory, which sees these icy space rocks created from collisions between larger bodies, would not create a three-body arrangement as the Altjira system appears to be.

The Altjira system joins the dwarf planet Pluto and the "space snowman" Arrokoth, a contact binary composed of two touching space rocks, as the most heavily studied bodies in the Kuiper Belt.

NASA's New Horizons probe flew by Pluto in 2015 and Arrokoth in 2019. There's no visit to Altjira in the works, but the researchers behind the new study hope that detailed remote observations of the system will be possible in the future.

Particularly exciting will be forthcoming observations of Altjira conducted by the James Webb Space Telescope during its third year of operations.

"Altjira has entered an eclipsing season, where the outer body passes in front of the central body," Nelsen said. "This will last for the next 10 years, giving scientists a great opportunity to learn more about it." \

The new study was published on Tuesday (March 4) in The Planetary Science Journal.

NASA's New Horizons spacecraft conducted the first and only flyby of the Pluto system, culminating at the closest approach of that distant world in July 2015.

Sailing onward, the probe carried out a Jan. 1, 2019 flyby of Arrokoth, a Kuiper Belt Object, or KBO, located in a region of space beyond Neptune called the Kuiper Belt. There are scads of other icy worlds residing in the Kuiper Belt, celestial leftovers from the formation of our solar system.

For New Horizons, the gathering of more exploration science is, pun intended, on the horizon.

Invaluable observations

Late last year, a study by the U.S. National Academies titled "The Next Decade of Discovery in Solar and Space Physics: Exploring and Safeguarding Humanity's Home in Space" observed that "key challenges are to keep receiving the invaluable observations from the New Horizons and Voyager spacecraft, which are the only means to gain firsthand knowledge of the environment in the outer heliosphere and outside the heliospheric bubble."

Related: New Horizons: Exploring Pluto and beyond

That report also noted that "moving outward, the boundary of the solar system where the sun's influence wanes and is replaced by the interstellar environment, there is much to be discovered."

The heliospheric decadal report is important for several reasons, said New Horizons Principal Investigator Alan Stern of the Southwest Research Institute in Boulder, Colorado."It's a completely independent validation by the community about how important and unique the New Horizons science is to that field," he told Space.com.

New Horizons is gearing up to cross the sun's "termination shock," Stern said, where the subsonic solar wind slows down and becomes subsonic as it rams into the interstellar medium.

Related: New Horizons Pluto probe notches 3 new discoveries in outer solar system

Guessing game

Though New Horizons is now in hibernation mode, the spacecraft is still collecting heliophysics data around the clock, Stern said, squirreling that data into onboard solid state memory — basically a big flash drive.

"We went into hibernation mode on October 3 of last year. We exit that mode on April 2 of this year. When we wake up, we'll transmit the backlogged New Horizons data down to NASA's Deep Space Network," said Stern.

"But actual crossing of the termination shock, the timing is a guessing game. No one can fully predict that, but it could potentially be as early as 2027 … and we want to be on guard then so we don't miss it," Stern noted.

Meanwhile, New Horizons is in perfect health. "There's nothing broken on the spacecraft and the seven instruments that it's carrying," he added. "They are working super-well, as well as when they were launched."

Related: New Horizons Pluto probe notches 3 new discoveries in outer solar system

Fuel gauge reads low

But New Horizons is low on propellant. "And that just means we have to be miserly with that fuel. Any fuel we spend is not going to get us to a new flyby of a KBO, so it reduces the odds of a flyby," said Stern.

Being tight on fuel means that Stern has adopted a new title to go along with principal investigator: "Fuel hoarder in chief."

Regarding the spacecraft's power and data transmission, the long-distance craft is good to go. Its nuclear power generating system will perhaps last into 2050, Stern advised.

So, could New Horizons perform a flyby of another far-flung KBO?

Possibly, if the mission gets some help from Earth-based observatories, particularly the soon-to-come online Vera C. Rubin Observatory. Rubin's detection of KBOs along an attainable New Horizons flight path would "significantly raise the odds of getting a flyby," Stern added. "But it's a needle in the haystack search," he added, "even using the world's best tools."

Related: Just how dark is the universe? NASA's New Horizons probe gives us best estimate yet

Unanswered questions

Meanwhile, a New Horizons heliophysics team consisting of about a dozen scientists and engineers are intently focused on spacecraft measurements taken in the outer heliosphere, said Andrew Poppe of the Space Sciences Laboratory at the University of California, Berkeley. He is a co-investigator and heliophysics science lead on the New Horizons mission.

That team is gearing up for New Horizons' crossing of the termination shock, one of the key outer boundaries between our heliosphere and interstellar space, Poppe told Space.com.

"Both Voyager spacecraft crossed this boundary and revealed a wealth of new physics," he said. "However, due to certain limitations in the Voyager instrumentation, key questions regarding a population of ions known as 'pickup ions' were left unanswered."

Poppe added that, since the Voyager measurements, it has become increasingly clear that these pickup ions may in fact dominate the transfer of energy and momentum across the termination shock.

First-ever measurements

Fortunately, New Horizons carries key instrumentation — the Solar Wind Around Pluto (SWAP) and Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) — that will conduct the first-ever measurements of these critical pickup ions in the outer heliosphere and across the termination shock.

"With this in mind, the New Horizons heliophysics team has been planning out specific instrument observing modes, planning data downlink budgets (no easy feat from 60-plus astronomical units!), and engaging the broader outer heliospheric theoretical community to prepare for the groundbreaking measurements that New Horizons will return in the near future," Poppe said.

Overall, the New Horizons team is humbled to follow in the footsteps of Voyager, said Poppe, "but extraordinarily excited to contribute first-of-a-kind measurements of the outer boundaries of the heliosphere we call 'home.'"

Historic encounter

The crossing of the termination shock itself could be as short as 10 minutes, said Pontus Brandt, the New Horizons project scientist at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland.

"But there will likely be multiple crossings as the shock moves back and forth over the spacecraft for multiple days and surely will be another historic encounter for New Horizons," said Brandt.

"The data from the termination shock encounter will be a treasure trove for space physicists worldwide who are eager to understand how this vast boundary works," Brandt told Space.com. "All these discoveries from pioneering missions like Voyager and New Horizons teach us how little we know about what lies beyond, and pave the way for a future dedicated Interstellar Probe mission," he said.

Essence of exploration

Brandt underscored another possible New Horizons exploration bonus.

"I think we may have only seen the tip of the iceberg of the Kuiper Belt, which could be much more extended than we ever could imagine," Brandt said.

"Dust hits measured by the spacecraft just keep being elevated, defying all our expectations of a 'Kuiper Cliff.' One must always give oneself the opportunity of discovery," Brandt added, "the essence of exploration."

In a few years, New Horizons could very well find itself in the midst of a new region of the Kuiper Belt, Brandt suggested. That would be "a historic opportunity for planetary science with important implications for understanding exoplanetary systems."

New surprises

As a "first responder" and record title holder of a spacecraft, New Horizons has already chalked up history-making observations as the first spacecraft to explore Pluto and its moons up close. Also, after a nine-year journey, the probe passed its second major science target, zipping by the KBO Arrokoth in 2019, the most distant object ever inspected up close.

New Horizons' findings are taking center stage at an upcoming 10th anniversary of the Pluto flyby science meeting now being planned for this July at APL, which designed, built, and operates the New Horizons spacecraft and manages the mission for NASA.

"We're pulling together everything that has been learned since the flyby of New Horizons. And not just from the spacecraft, but also from the Hubble Space Telescope, the James Webb Space Telescope, and from Earth-based observations, too," Stern said. "So stand by. I'm betting on some new surprises!"

Clyde Tombaugh didn't set out to discover Pluto when he sent his sketches of the night sky to Lowell Observatory in Flagstaff, Arizona in 1929. More than anything, he just wanted to get off the farm in Kansas where he spent his days working the earth.

At just 23 years old, Tombaugh sent his drawings, unsolicited, to several institutions and observatories around the United States, hoping someone — anyone — would give him some feedback on what he'd produced.

The response from Lowell Observatory, then, must have been a shock when it hit his mailbox. It was more than a critique of his drawings from professional astronomers. Instead, it was a job offer (from the head of the Observatory, no less), and one that within two years would put Tombaugh in an elite and narrow pantheon of stargazers who could claim what only the ancients could boast: the discovery of a new planet.

The search for Planet X

The search for Pluto did not begin with Clyde Tombaugh, to be fair. That distinction belongs to the visionary astronomer Percival Lowell.

In 1894, Lowell founded Lowell Observatory in the Arizona territory (Arizona wouldn't become a state until 1912). Originally a scion of an elite Boston family, Lowell became fascinated with the study of Mars and its purported canals. With family wealth to back him, he founded an observatory in the western dark-sky desert of the U.S. and hoped to unlock the mystery of the Red Planet.

He soon became fascinated by another cosmic mystery, however, and one that would be far more consequential. Uranus and Neptune, the most recently discovered planets in the solar system at the time, seemed out of sorts. Their orbits didn't add up mathematically, and Lowell was convinced there was an undiscovered planet in the far reaches of the solar system that was knocking them off-kilter.

"Lowell wanted to learn more about the planets and started coming under the belief that there might be a ninth planet out there,” Kevin Schindler, Lowell Observatory’s Public Information Officer, told Space.com. "That was based on some irregular motions, or apparent irregular motions, of [Uranus and Neptune]."

He called this hypothetical world "Planet X."

Lowell dedicated years to predicting the location of this elusive planet. He and his team at the observatory came up with calculations for the world's path and location, but by the time of his death in 1916, the planet (like most other hypothetical worlds), failed to materialize.

With many in the scientific community convinced that there were no more planets to discover, the search for Planet X was abandoned soon after Lowell's death.

The self-taught astronomer

Clyde Tombaugh's journey to planetary discovery is one of the most remarkable in the history of astronomy. Born in 1906 in Illinois and raised on a farm in Burnett, Kansas, Tombaugh had a deep fascination with the night sky. With little formal training, he built his own telescopes and meticulously sketched astronomical observations out in Kansas's dark-sky country.

"Tombaugh sent some of his drawings off to different places, including Lowell Observatory, and he got this letter back from the director saying, 'We're just recommencing the search for a ninth planet, and we need somebody to help with it. It looks like you know what you're talking about, so why don’t you come work for us?'" Schindler said. "He got a one-way bus ticket, hoping that he wouldn't have to go back home in a short time, and started working at the observatory."

Tombaugh arrived in 1929 hoping to prove himself, but his job was painstaking compared to sketching the night sky in Kansas. Using a technique called "blink comparison," Tombaugh searched through countless, and nearly identical, photographic plates for the elusive planet.

Blink comparison involves looking at photographs of the same portion of the sky on different nights and then rapidly switching between the two images to spot any moving objects. Because far-off stars would be static over successive nights, nearer, moving objects would stand out against the backdrop — but, for an object as distant as Pluto, the motion would be barely perceptible.

It took months of tedious work, flipping back and forth between effectively identical photographic plates, but on Feb. 18, 1930, Tombaugh finally found what he was looking for: a tiny moving speck against the starry background, roughly where Lowell had predicted it would be.

A ninth planet, Pluto, had been discovered.

An ironic discovery

One of the most fascinating aspects of Pluto's discovery is that it was, in part, a cosmic coincidence.

"Pluto was found really close to where Lowell thought a planet should be, but it's also a great example of serendipity in science," Schindler said. "When Lowell was doing his work, there was an estimate of what Uranus' and Neptune's masses were, but they weren't very accurate. Today, we have much more accurate estimates and we know that the irregular motions [that led to the prediction of a Planet X] actually don't exist. If you know the true value of the masses of Uranus and Neptune, everything's accounted for.”

In other words, Pluto showing up in Tombaugh’s photos where it did wasn’t the result of careful calculation, but rather a good bit of dumb luck.

"Clyde Tombaugh was looking for a phantom," Schindler said, "but he found a planet. It was just coincidence that it was right where Lowell thought it should be."

Pluto's cultural impact

Lucky find or not, the discovery of Pluto came at a crucial moment in history, particularly for the U.S. By 1930, the Great Depression had left much of the world in economic ruin, and scientific breakthroughs provided a much-needed source of inspiration.

"It had such a cultural impact because we were in the Depression, and there wasn't much good news in the newspaper," Schindler said. "It was a good news story in a time of otherwise pretty lousy news."

Pluto's discovery was widely celebrated, especially in the U.S. After all, it was the first planet discovered in the new world, and America needed the win in 1930.

"I think because it was discovered by the United States and also discovered in the 20th century with mass media, it has a lot of fascination," Schindler said. "For decades, Pluto was seen as the ninth planet of the solar system, an underdog among its much larger planetary siblings of the inner planets and the gas giants beyond the asteroid belt."

However, as scientific understanding evolved, so did Pluto's classification — though not without controversy.

The reclassification of Pluto

In 2006, Pluto's planetary status was re-evaluated by the International Astronomical Union (IAU), leading to its reclassification as a "dwarf planet." The change was based on new criteria for defining planets. According to the International Astronomical Union (IAU), a planet must:

1. Orbit the sun

2. Be massive enough to achieve hydrostatic equilibrium (i.e., be a spheroid under the force of its own gravity)

3. Clear its orbit of other debris

Pluto fails the third criterion because its orbit overlaps with objects in the Kuiper Belt, a region of icy bodies beyond Neptune. So, a new class of celestial object — the dwarf planet — came into being, with Pluto at its head.

This decision sparked widespread debate, both among the public and among many astronomers.

"There's this perception that Pluto got dumped," Schindler said. "So many people took that personally, and we would have guests come to the [Lowell] Observatory saying things like, 'Are you guys okay?'"

Part of the problem with the reclassification, pioneered by Caltech astronomer Michael Brown and supported by other prominent astronomers like the Hayden Planetarium’s Neil deGrasse Tyson, is that, until the reclassification of Pluto as a dwarf planet, no one had really bothered to define what a planet actually was.

"I think [the 2006 reclassification] was a starting point," Schindler said. "It's not a bad thing to have a definition of what a planet is, but it's not really the best definition. It has problems. I think one of the issues [fueling the controversy] is, if scientists are confused about the definition, what's the rest of the public supposed to do? How are we supposed to understand what a planet is, if scientists can't even agree on it?”

With Pluto's "demotion" to dwarf planet status, it’s understandable that the IAU faced criticism from the public for redefining a "planet" in such a way that seemed almost tailored to exclude Pluto, along with bodies like Eris, Ceres and Pallas.

"I don't want to cast aspersions on the IAU," Schindler said. "It may be a legitimate definition, but it's interesting to think about the fact we didn't have one beforehand. We don't really have a clear definition of what a planet is; I guess you can say we do now, but it still needs help."

After all, what does it mean for a planet to "clear its orbit of other debris," which is the single criterion that Pluto fails? Do Jupiter's Trojan asteroids qualify as cleared? Earth has plenty of asteroids and other debris in its orbital vicinity, as well. Why do Earth and Jupiter meet this criteria, but Pluto doesn't?

"At some point, the International Astronomical Union will probably revisit it," Schindler said, "but I think in some ways, nobody wants to touch it because it's become sort of an embarrassment. In trying to get to an agreement, not only on what to call a planet but how to decide on what qualifies as a planet, well, the first step was kind of embarrassing. So nobody is in a hurry to revisit it right now."

Seeing Pluto up close revived the debate

Before Pluto was demoted to dwarf planet status, it was really nothing more than a tiny dot in telescopes; barely a few blurry pixels in an image, and that's if you were lucky. Even the Hubble Space Telescope, despite all its might, revealed little about the enigmatic object lurking beyond Neptune — but in 2015, NASA's New Horizons probe gave us our first detailed look at the distant world.

What most people were arguing about in 2006 through 2014 was mostly academic and sentimental. If you grew up learning that Pluto was a planet, chances are you were sticking to that, no matter what the IAU said.

After all, it's not like anyone had truly seen Pluto before New Horizons performed its 2015 flyby. And sure enough, afterward, there was definitely a shift — in Pluto's favor. The images New Horizons sent back were breathtaking, revealing an active, complex planet with mountains, valleys and a now-iconic heart-shaped feature known as Tombaugh Regio.

"New Horizons essentially turned Pluto from a dot to a world where you can see mountains and craters and valleys, all this stuff up close," Schindler said. "We never saw anything close to that on Pluto before."

The mission confirmed that Pluto is far from a lifeless rock — it has a dynamic surface, an atmosphere, and potential geological activity, and this only reinforced opinions that Pluto was indeed a planet and had gotten a bad deal from the IAU.

"In some ways, Pluto's more of a planet than some of the other established planets, like Mercury," Schindler said. "Pluto has several moons orbiting around it and it has an atmosphere. Mercury doesn't have an atmosphere and it has no moons. Venus doesn't have a moon, either. Sometimes, if we think traditionally about what a planet is, people will say 'Well, it's round, maybe it has some moons, maybe it has an atmosphere.'

"Well, Pluto has them all."

Celebrating the legacy of discovery

Despite its "demotion," humanity's fascination with Pluto hasn't waned. Every year, the Lowell Observatory hosts the I Heart Pluto Festival to celebrate the discovery and significance of the King of the Kuiper Belt. This event brings together scientists, space enthusiasts and even members of Clyde Tombaugh's family to honor Pluto's place in history.

"We started this festival as a way to celebrate Pluto's discovery and the cultural connections it has," Schindler said. "It's about Pluto, but it's also about the inspiration of space and science."

This year's festival theme, "Boldly Go Beyond New Horizons," brings together figures from science and pop culture, including astronomers, Star Trek actor Leonard Nimoy's son Adam Nimoy, and others, emphasizing Pluto's role in both scientific discovery and human imagination.

"I heart Pluto because of its connections," Schindler said. "I've worked at Lowell for almost 30 years, and through that time, I've gotten to know so many scientists and others that have a connection to Pluto. I never met Clyde Tombaugh, but I know his children very well, and the scientists at Lowell's laboratories are good friends that I work with, like Jim Christie, who discovered [Pluto's moon] Charon in the 1970s.

"For me, it's definitely the community around Pluto. Yes, I work at the place where it was discovered, but every day I walk past the office where Tombaugh made his discovery, and go past the telescope dome where he huddled himself in the cold and winter to take these pictures. It's really easy to recreate that day of discovery, and kind of follow his footsteps a little bit, and feel what he was feeling."

In the end, we all heart Pluto

Pluto's story is one of perseverance, curiosity and the evolving nature of scientific knowledge. From Percival Lowell's ambitious search to Clyde Tombaugh’s groundbreaking discovery to the stunning revelations of New Horizons, Pluto continues to captivate us, despite the semantics around its classification.

"I personally think Pluto is a planet and it has nothing to do with the fact that it was discovered where I work," Schindler said. "Ultimately, Pluto is the prototype of a third zone of the solar system. You have the inner terrestrial planets, Mercury, Venus, Earth and Mars, then you have the gas giants, Jupiter, Saturn, Uranus, and Neptune. And then, you have this third zone of icy dwarf planets, icy small bodies in the Kuiper Belt — and Pluto's the big one, the king of those.

"In some ways, what we call it doesn't matter," Schindler said. "By naming it a planet or not, it trains us to understand what it is and how it was formed, and that's the kind of classification that science tries to achieve by seeing similar patterns or differences in the universe."

Wherever the debate about Pluto’s status goes, one thing is for certain: Pluto's place in history — and in our hearts — is undeniable. Certainly, the more we learn about it, the more this enigmatic body will inspire us to discover more about the outer frontiers of our stellar neighborhood.

The 95th anniversary of Pluto's official discovery by Clyde Tombaugh at the Lowell Observatory in Flagstaff, Arizona is rapidly approaching and the historic location is gearing up to host a grand celebration on Feb. 13-17, 2025 with a five-day series of educational and social activities to mark the occasion.

Lowell's sixth annual "I Heart Pluto Festival" will honor the remote icy world that was reclassified as a dwarf planet in 2006 and originally found at that location on Feb. 18, 1930. This year is also the tenth anniversary of NASA's New Horizons mission and its flyby of Pluto where it captured close-up images of unparalleled detail and beauty.

"To Boldly Go Beyond New Horizons" is this year's theme and will feature a Pluto Pub Crawl, exclusive book signings, and family-friendly food and fun at the new Astronomy Discovery Center. Adam Nimoy (Son of "Star Trek's" Leonard Nimoy), New Horizons Principal Investigator Alan Stern, and comet-hunting superstar David Levy will also join moderator David Eicher, Editor-in-Chief of Astronomy magazine, for an evening panel discussion at the Orpheum Theater on Feb. 15.

"I'm one of the laypeople on the panel with a couple of distinguished astronomers and scientists who have more in depth knowledge about all things space," Nimoy tells Space.com. "I'm coming as a sort of observer, a fan of astronomy and science fiction, to talk a little bit about 'Star Trek' and Leonard, and the connection to the Lowell Observatory. Percival Lowell, the founder of the observatory, and the family that funded the telescope are based in Boston.

"They were one of the first families of Boston and my dad was from the other side of the street. He came from the west end of Boston, the immigrant neighborhood not far from Beacon Hill where the well-to-do were living. So it'll be a little bit about 'Star Trek,' the popularity of science, the public's awareness of science, how 'Star Trek' as one of the premier science fiction television shows has continued to spark the imagination of the American public. How it's continued to resonate with people about science fiction and space travel. The more popular side of science."

Pluto has always been a favorite of school children, scientists, and amateur stargazers and the allure of this distant dwarf planet has never been stronger.

"To credit Percival Lowell, it was Planet X, and he postulated that there was another planet out there at the far reaches of our solar system. It was just one of the mysteries of that time. It was another thirteen or fourteen years before the planet was actually discovered. This idea that it takes like 250 years for Pluto to orbit the sun, I think that's just bizarre. Also the fact that the planet has been demystified by New Horizons.

"We always have assumptions about the size of these things and the orbit of these planets. Whether or not the mathematical formulas are accurate and it's always being modified. I was never a math guy but it's interesting that sometimes we need to do plain old observation to figure out what the size and trajectories are."

Nimoy said that his father owned a telescope and would use it to watch the skies. "He visited the Lowell Observatory and he was at White Sands Missile Base in the '60s and brought back all these models of satellites," Nimoy said. "I sent a picture of him with one of those models, Vanguard 1, to the people at Lowell to promote my appearance there. He had a fascination with it.

"Funny, he'd talk to astrophysicists who would be asking him detailed questions about their discoveries or their theorems and projections and ask his opinion. He always had a stock answer which was, 'You're on the right track.' It was over his head but he definitely had an interest in it."

On Sunday, Feb. 16, KNAU Science Reporter Melissa Sevigny will host a chat with Nimoy in Lowell Observatory’s Astronomy Discovery Center for his book, "The Most Human," which explores his relationship with his legendary father. A book signing will follow, along with a screening of Nimoy's documentary, "For the Love of Spock."

For more information on "I Heart Pluto" schedules, general admission tickets, and a VIP option for "A Night of Discovery" allowing access to a private reception with the presenters before the main program, visit the main ticket site.

New research suggests that billions of years ago, Pluto may have captured its largest moon, Charon, with a very brief icy "kiss." The theory could explain how the dwarf planet (yeah, we wish Pluto was still a planet, too) could snare a moon that is around half its size.

The team behind this research thinks that two frigid worlds located in the Kuiper Belt, a ring of icy bodies located far from the sun at the edge of the solar system, collided together billions of years ago. Rather than mutually obliterating each other, the two bodies were united as a spinning "cosmic snowman." These bodies separated relatively quickly but remained orbitally linked to create the Pluto/Charon system we see today.

This "kiss and capture" process represents a new theory of moon capture and cosmic collision. It could also help scientists better investigate the structural strength of frigid, icy worlds in the Kuiper Belt.

"We've found that if we assume that Pluto and Charon are bodies with material strength, Pluto can indeed capture Charon from a giant impact," team leader and University of Arizona lunar and planetary researcher Adeene Denton told Space.com. "The process of this collisional capture is called 'kiss-and-capture' because Pluto and Charon briefly merge, the 'kiss' element, before separating to form two independent bodies."

Most planetary collision scenarios are classified as "hit and run" or "graze and merge," meaning this "kiss and capture" scenario is something entirely new.

"We were definitely surprised by the 'kiss' part of kiss-and-capture," Denton continued. "There hasn't really been a kind of impact before where the two bodies only temporarily merge before re-separating!"

The team's research was published on Monday (Jan. 6) in the journal Nature Geoscience.

Pluto won Charon over with a 10 hour kiss

The reason Pluto's relationship with Charon has been challenging to scientists is because of the relatively small difference in size and mass between the two icy bodies.

"Charon is HUGE relative to Pluto, to the point where they are actually a binary," Denton explained. "It's half Pluto's size and 12% of its mass, which makes it much more similar to the Earth's moon than any other moon in the solar system."

For comparison, our moon is just a quarter of the size of Earth, while the largest moon in the solar system, Ganymede, is around 1/28 the size of its parent planet, Jupiter.

The University of Arizona researcher, who is also a NASA postdoctoral fellow, added that it's hard to get such a relatively big moon in a "normal" way. ("Normal" being the gravitational capture of moons like Mars' moons Phobos and Deimos and the moons of the giant planets Jupiter and Saturn.)

That means that the prevailing theory of the formation of the Pluto and Charon system is based on the collisional capture idea, similar to how a massive body is believed to have slammed into Earth to launch out material that our planet captured to birth our moon.

"Something big hits Pluto, and you get Charon, but like with the Earth-moon system, we don't fully know how that works and the conditions under which that occurs," Denton said. "It's a pretty big question since a bunch of other large Kuiper Belt Objects also have large moons, so it seems like this is something that happens in the Kuiper Belt with some frequency, but we don't know how or why."

During a standard "collision capture," a massive collision occurs, and the two bodies stretch and deform in a fluid-like way. This process explains the creation of the Earth/moon system well because the intense heat generated in the clash and the greater mass of the bodies involved causes them to act in a fluid way.

When considering Pluto and Charon in a collision capture process, there is an extra factor to consider: the structural strength of the colder icy and rocky bodies. This is something that has been neglected in the past when researchers considered the collisional creation of Charon.

To factor this into simulations, the team turned to the University of Arizona's high-performance computing cluster. When Denton and colleagues accounted for the strength of these materials in their simulation, something completely unexpected emerged.

"Because both bodies have material strength, Charon did not penetrate deep enough into Pluto to merge with it; this isn't true when the bodies are fluids," Denton explained. "For the same impact conditions, if we assume Pluto and Charon to be strengthless, they do merge into one large body, and Charon is absorbed. With strength, however, Pluto and Charon remain structurally intact during their brief merger."

Because Charon couldn't sink into Pluto in this scenario, it remained beyond the so-called "co-rotation radius" of both bodies. As a result, it could not rotate as fast as Pluto, which meant the two bodies could not stay merged. As they separated and this icy kiss ended, the team thinks that Pluto would have torqued Charon into a close, higher circular orbit from which the moon would have migrated outward.

"The 'kiss' in this kiss-and-capture, the merger is very brief, geologically speaking, lasting for 10 to 15 hours before both bodies separate again," Denton said. "Charon then begins its slow outward migration towards its current position."

The team thinks the initial collision happened very early in solar system history, probably tens of millions of years after the solar system formed, which would be billions of years ago.

"Typical large collisions are straightforward mergers, where the bodies combine, or both bodies remain independent," Denton said. "So this was very new to us. It also raised a lot of interesting geological questions that we'd like to test, because whether kiss-and-capture works depends on the thermal state of Pluto, which we can then tie to Pluto's contemporary geology to test.

"I'd really like to determine how the initial Pluto-Charon impact can influence whether and how Pluto and Charon develop oceans."

Denton explained that there are two avenues the team can follow to build upon this development.

"The first is looking at how this applies to the other large Kuiper Belt Objects with large moons, like Eris and Dysnomia, Orcus and Vanth, and the others," Denton explained. "Our initial analysis suggests that kiss-and-capture can also be the source of these other systems, but since they're all different in their compositions and mass, it's critical to learn how kiss-and-capture may have operated across the Kuiper Belt."

The second avenue the team intends to follow involves looking at the long-term tidal evolution of Charon to confirm their formation theory.

"To really be sure that this is the process that formed Pluto and Charon, we need to make sure that Charon migrates to its current location at around 8 times the width of Pluto away," Denton said. "However, that's a process that occurs over much longer timescales than the initial collision, so our models aren't well-suited to track it.

"We're planning to have a much closer look at this in the future to determine which conditions not only reproduce Pluto and Charon as bodies but also put Charon in the right spot, where it is today."

Astronomers say they have figured out the true chemical makeup of our solar system, revealing that it contained much more carbon, oxygen and nitrogen than previously thought.

Chiefly, the finding could help explain why Pluto, the largest known object in the Kuiper Belt — a vast region of icy bodies in the outer solar system — is dominated by rock rather than ice. Despite its location in the frigid reaches of the solar system, Pluto's unexpectedly high density has puzzled scientists for decades.

Current models of the primitive gas and dust that formed the solar system "cannot produce the dense, rock-rich features of Kuiper Belt Objects like Pluto," Ngoc Truong, a postdoctoral researcher at the Southwest Research Institute (SwRI) in Texas, told Space.com. "The majority of KBOs are rock-rich instead of ice-rich."

A new study led by Truong, however, is the one that suggests the chemical inventory of the early solar system contained much more carbon, oxygen and nitrogen than once assumed; in particular, a higher abundance of solar carbon would have reduced the availability of water, helping explain why Pluto and other large Kuiper Belt Objects are surprisingly rich in rock, the researchers say.

Related: 2nd Kuiper Belt? Our solar system may be much larger than thought

"With this research, we think we finally understand the mix of chemical elements that made the solar system," study co-author Christopher Glein, an expert in planetary geochemistry at SwRI, said in a recent news release.

The finding, published last week in The Astrophysical Journal, will help scientists more accurately understand stars and planets beyond our solar system formed and evolved across billions of years. "Even further, they enable a broader perspective of galactic chemical evolution," said Truong.

Researchers say the revised chemical estimates are consistent with recent solar data and could help reconcile the longstanding mismatch between two independent methods for determining the sun's composition: spectroscopy, which primarily probes the outermost layer of the sun’s atmosphere, and helioseismology, which studies sound waves traveling through the sun’s interior.

The researchers came up with the new chemical abundance estimates by taking measurements of organics in the comet 67P/Churyumov-Gerasimenko, which likely birthed in the Kuiper Belt and now makes regular visits to the inner solar system. Data sent home by the European Space Agency's Rosetta spacecraft from its rendezvous with the comet suggests chemically resilient compounds called "refractory organics" carried a significant portion of primordial carbon to the outer regions of our solar system, where the Kuiper Belt Objects formed.

The carbon would have been locked within relatively stable compounds like carbon monoxide (CO) and carbon dioxide (CO₂), reducing the amount of free oxygen available to form water ice, Truong said. This, in turn, would help explain why Pluto and other large Kuiper Belt Objects are rock-rich, said Truong.

Computer models incorporating the new estimated abundances successfully reproduced the compositions of Pluto and its largest moon Charon, as well as those of asteroids Ryugu and Bennu, according to the new study. Moreover, the asteroid samples, which were recently delivered to Earth by JAXA's Hayabusa-2 and NASA's OSIRIS-REx missions, lack sulfates. This implies the asteroids accreted less water ice than previously thought, Truong said.

While the findings hold true for the KBO region, researchers say the increased carbon in the ancient solar nebula suggests ice giants like Uranus and Neptune could be hosting more rock than is estimated currently.

"We have been calling Uranus an ice giant, but we don't really know its nature," Truong said. "There are lots of mysteries in our backyard."

In 2006, the International Astronomical Union (IAU) voted on the definition of a planet. Famously, Pluto no longer met the criteria and was demoted to a dwarf planet. Things have been a bit of a mess since then — so is it time to redefine the planet? 

To be fair, Pluto had it coming. The word "planet" never had an official definition, and astronomers had always played fast and loose with its use. To the ancient Greeks, a planet was any "wandering star," which included the sun and the moon. With the Copernican revolution, the definition changed: Earth was considered a planet in its own right, the moon was demoted to a satellite, and the sun got promoted.

This worked for over 200 years, until William Herschel discovered Uranus and Giuseppe Piazzi discovered Ceres, the largest object in the main asteroid belt between Mars and Jupiter. Initially, both Uranus and Ceres were classified as planets. But once more objects were found to share similar orbits with Ceres, astronomers had to rethink things — surely, planets lived alone, after all. Herschel proposed the term "asteroids" for the small objects between Mars and Jupiter, while Uranus remained a planet (a situation that certainly benefited Herschel's own legacy).

Astronomers were comfortable with those classifications even when Clyde Tombaugh discovered Pluto in 1930. But that new planet was a stretch — it had a really wonky orbit and was far smaller than the other planets, so it didn't take long for astronomers to start wondering if they should start reexamining how objects were classified.

Starting in the 1990s, astronomers began to find more objects that shared similar orbits with Pluto. But the real nail in the planetary coffin came in 2005, when astronomer Mike Brown discovered Eris, an object about the same size as Pluto orbiting beyond Neptune.

So, in 2006, when astronomers gathered at the IAU meeting in Prague, a large contingent moved to have that body define what a planet should be. There were two camps: the geophysicists who argued that planets should be defined by their appearance, and the dynamists who believed that planets should be defined by their properties.

In essence, the geophysicists argued that a planet should be anything that is large enough that its own self-gravity pulls it into a nearly spherical shape. The dynamists countered that a planet should be anything that can dominate and mostly clear its orbit of any debris. The first definition would allow Pluto, along with Ceres and all of Pluto's co-orbital friends, to become planets. The latter definition would exclude all of those smaller bodies.

In the end, the compromise included both definitions, which meant that effectively, the dynamists won. Pluto does not clear its orbit — it simply has too many neighbors — and thus was famously demoted.

Related: What exactly is a planet? Astronomers want to amend the definition

But not everybody agrees with the new rules, and there are plenty of arguments and counterarguments to go around. For one, the definitions of "nearly spherical" and "mostly clear the orbit" aren't very clear. 

But proponents of the rules counter that all of the defined planets, from Mercury to Neptune, are exceptionally round, while the vast majority of non-planets are obviously not. As for clearing the orbit, Mars is over 5,000 times more massive than the next-largest object in its orbit. In contrast, Pluto captures only about 7% of all the mass in its orbit. So there is a clear and wide discrepancy between those worlds.

But the biggest confusion comes with the subtypes of the non-planets. If an object is large enough to make itself round but not clear its orbit, like Pluto or Ceres, it gets called a dwarf planet. Astronomers often use the term "dwarf" to refer to smaller versions of the same type, like dwarf stars and dwarf galaxies. But dwarf stars are still stars, and dwarf galaxies are still galaxies; they're just smaller. This is not the case for dwarf planets — they are emphatically not planets.

If an object is too small to make itself round, it is classified as a "small solar system body." Asteroids are not officially defined as such, but astronomers generally take the word to mean small solar system bodies that live in the inner solar system. Comets are also considered small solar system bodies.

Then there are "minor planets," which include the dwarf planets and all of the small solar system bodies that aren't comets. And don't forget the plutoids, — dwarf planets beyond the orbit of Pluto — and the trans-Neptunian objects, which include the plutoids and other small solar system bodies that orbit beyond Neptune.

It's a bit of a mess, but it's an attempt to match the complicated nature of our unfolding knowledge of the solar system. We live in an active, dynamic system, where many kinds of objects interact and interplay. We're struggling to come up with clean definitions because the universe itself refuses to be neatly organized.

For now, we should probably stick with the definitions we have, even if they're imperfect. As we make new discoveries and gain new understanding, we can update our definitions accordingly and, hopefully, streamline things a bit.

Using the James Webb Space Telescope (JWST), astronomers have detected carbon dioxide and hydrogen peroxide on the frozen surface of Pluto's largest moon, Charon. Detecting these molecules could tell scientists how Charon and other icy bodies at the solar system's edge were born.

Since its discovery in 1978, Charon has been extensively studied — but previous research has been limited in terms of what wavelengths of light could be explored during these analyses. That left gaps in our understanding of the surface composition of this moon of Pluto. As a result, though scientists have detected water ice, ammonia-bearing species and organic compounds on Charon, carbon dioxide and hydrogen peroxide have evaded detection. Until now, that is.

The team, led by Silvia Protopapa of the Southwest Research Institute (SwRI), filled in these gaps by studying Charon with the JWST's Near-Infrared Spectrograph (NIRSpec) instrument.

"Our research reveals that Charon’s surface preserves evidence of its
formation through the presence of carbon dioxide, as well as signs of irradiation processes, indicated by the presence of hydrogen peroxide," Protopapa told Space.com. "These discoveries expand Charon's known compositional inventory, which includes water ice, ammonia-bearing species and organic materials responsible for its gray and red coloration."

Related: 'That's weird': James Webb Space Telescope spies a strange galaxy outshining its stars

Charon is a midsized body roughly 750 miles (1,207 kilometers) wide and located in the Kuiper Belt, a ring of icy debris, comets and dwarf planets, also referred to as trans-Neptunian objects (TNOs), at the solar system's edge. 

Unlike many of the larger objects in the Kuiper Belt, Charon's surface is not obscured by volatile ices like methane, meaning it offers scientists valuable insights into the effects of sunlight exposure and cratering on these distant bodies. Additionally, Charon is the only mid-sized TNO for which geologic mapping is available. This is thanks to data gathered by NASA's New Horizons spacecraft, which visited the Pluto system around a decade ago.

"Overall, these factors make Charon an invaluable target from which we can learn extensively," Protopapa said. "Our findings provide valuable insights into how processes such as sunlight exposure and cratering shape the surface of Charon and, by extension, other mid-sized icy bodies beyond Neptune's orbit."

Cool suprises on Charon

The composition of stars, planets and moons can be determined from the light they emit or reflect from their surface. This is possible because elements absorb and emit light at specific wavelengths. Thus, looking at a celestial body's spectra via a technique called "spectroscopy" reveals the "fingerprints" of elements and chemical compounds.

Protopapa and colleagues reached their findings by comparing JWST spectroscopic observations with lab-based measurements and detailed spectral models of the surface of Charon. This led them to conclude that carbon dioxide is primarily present as a surface veneer on a water-ice-rich subsurface.

"The surface of Charon, as revealed by the New Horizons mission, features numerous craters surrounded by bright ejecta blankets that are rich in water ice and ammonia-bearing compounds," Protopapa explained. "These geologic features suggest that materials from beneath the surface have been exposed by impact events, providing a window into the moon's subsurface composition.

"Our preferred interpretation is that the upper layer of carbon dioxide originates from the interior and has been exposed to the surface through cratering events."

She added that carbon dioxide was also expected because the compound is known to be present in regions of the protoplanetary disk from which the Pluto system formed. The fact that carbon dioxide wasn't spotted by NASA's New Horizons spacecraft when it visited Pluto and captured images of Charon in 2015 has been troubling scientists for some time.

"The detection of carbon dioxide was a satisfying confirmation of our expectations," Protopapa continued.

What was not expected by the team was the detection of hydrogen peroxide.

"The detection of hydrogen peroxide on Charon came as a surprise. I
honestly did not expect to find evidence of it on the surface," Protopapa said. "Hydrogen peroxide has been known to be present on Jupiter's moon Europa’s surface since the 2000s. I never imagined I would be writing a paper comparing
these icy satellites, Charon and Europa, given how different their
environments are."

The surprise presence of hydrogen peroxide on Charon suggested to the team that the water-ice-rich surface of Pluto's largest moon is being actively altered by ultraviolet light from the sun, energetic particles from the solar wind, and streams of charged particles from beyond the solar system called "galactic cosmic rays."

"Hydrogen peroxide forms from the combination of neighboring hydroxide ion radicals, which originate from the breakup of water molecules due to incoming ions, electrons, or photons," Protopapa continued. "Our team conducted new laboratory measurements to confirm that it is possible to generate hydrogen peroxide even when carbon dioxide is present."

The team hasn't finished with Pluto's largest moon yet. The JWST will continue to study Charon, and scientists will use the resulting data to better understand icy TNOs as a whole.

"Future JWST observations targeting the spectral gaps, not covered in the current data, could lead to new Charon discoveries and further expand its chemical inventory, possibly revealing other mechanisms at play," Protopapa said.

The team's research was published on Tuesday (Oct. 1) in the journal Nature Communications.

A brand-new group of frozen objects, orbiting the sun out beyond the distant Kuiper Belt, has been spotted by the Subaru telescope, working with NASA's New Horizons mission to find new targets for the spacecraft to investigate.

"If this is confirmed, it would be a major discovery," said Fumi Yoshida, from the University of Occupational and Environmental Health Sciences and the Planetary Exploration Research Center, Chiba Institute of Technology, in a statement.

Subaru, which is an 8-meter (26-foot) telescope atop Mauna Kea in Hawaii, has been working with New Horizons since its 2006 launch toward Pluto, which the spacecraft flew by in 2015. Since then, New Horizons has been blazing a trail through the Kuiper Belt, which is a ring-shaped region of icy cometary bodies beyond the orbit of Neptune at distances between 33 and 55 astronomical units (AU). An astronomical unit is equivalent to Earth's average distance from the sun, which is 149.6 million kilometers (93 million miles). 

When Subaru began searching for Kuiper Belt objects in 2004 as potential targets for New Horizons to either visit up close or to watch with its cameras from a distance after encountering Pluto, the telescope was faced with a problem. At the time, Pluto and the area of the outer solar system to which New Horizons was headed was in the constellation of Sagittarius, which has the dense center of the Milky Way as a backdrop, making it hard to pick out Kuiper Belt objects from all the background stars. At the time, Subaru identified just 24 Kuiper Belt objects, all of which were too far away for New Horizons to reach or view with its cameras after leaving Pluto. (Arrokoth, a Kuiper Belt object that New Horizons visited on New Years' Day 2019, was found by the Hubble Space Telescope.)

Related: What is the Kuiper Belt?

Now, however, Pluto and that part of the solar system has moved away from the backdrop of the Milky Way into a sparser region of the night sky. With its Hyper Suprime-Cam (HSC), Subaru has discovered 239 Kuiper Belt objects since 2020. Most of these are regular Kuiper Belt objects, but a smattering seem to be very special indeed.

"The most exciting part of the HSC was the discovery of 11 objects at distances beyond the known Kuiper Belt," said Yoshida.

This new group of objects isn't a mere extension of the Kuiper Belt. There appears to be a gap between 55 AU and 70 AU where no objects have yet been found, and then a second belt — let's call it "Kuiper Belt 2" — between 70 and 90 AU, which is as far out as 13.5 billion kilometers (8.4 billion miles) from the sun. For comparison, Neptune lies at 30 AU and New Horizons is currently 60 AU from the sun, while NASA's Voyager 1 and Voyager 2 probes are 164.7 and 137.6 AU from the sun, respectively — out in interstellar space.

Related: Solar system planets, order and formation: The ultimate guide

The architecture of the solar system, including the asteroid belt and the Kuiper Belt, was determined by the processes that formed the planets, including how the young Jupiter migrated through the system, scattering smaller bodies far and wide.

"I think the discovery of distant objects and the determination of their orbital distribution are important as a stepping stone to understanding the formation history of the solar system, comparing it with exoplanetary systems, and understanding universal planet formation," said Yoshida.

The discovery of this new population of bodies is not entirely out of the blue. The Student Dust Counter instrument on board New Horizons keeps detecting impacts from dust particles, though the rate of impacts should be decreasing as New Horizons departs the Kuiper Belt. The continued presence of dust suggests that it is being produced by a population of bodies farther out. In addition, New Horizons has witnessed unexplained stellar occultations — when an object passes in front of and briefly blocks the light of a distant star — that could be the product of objects in this newly discovered, more distant 'Kuiper Belt 2.'

Furthermore, observations of protoplanetary disks around other stars, such as those seen by ALMA, the Atacama Large Millimeter/submillimeter Array in Chile, clearly show extended regions and numerous belts and gaps in the region beyond where the Kuiper Belt is in our solar system.

"Our solar system's Kuiper Belt long appeared to be very small in comparison with many other planetary systems, but our results suggest that idea might just have arisen due to an observational bias," said Wes Fraser of the National Research Council of Canada, who led the new findings. "So maybe, if this result is confirmed, our Kuiper Belt isn't all that small and unusual after all compared to those around other stars."

Because the Kuiper Belt had seemed small, one theory had been that the solar nebula that formed our planetary system had also been smaller than normal. The discovery of this Kuiper Belt 2 suggests that was not necessarily the case.

"The primordial solar nebula was much larger than previously thought, and this may have implications for studying the planet formation process in our solar system," said Yoshida.

Astronomers will continue to use Subaru to track the 11 objects in this new belt to better define their orbits. Given they were found in a small region of space, they probably are only the tip of the iceberg and hint at a much larger population. Their discovery is further evidence that there is still much to be discovered in the extreme depths of the outer solar system, including the possibility of more dwarf planets and even the hypothesized Planet Nine.

"This is a groundbreaking discovery revealing something unexpected, new and exciting in the distant reaches of the solar system," said Alan Stern, who is the principal investigator on the New Horizons mission. "This discovery probably would not have been possible without the world-class capabilities of the Subaru Observatory."

The findings are set to be published in the Planetary Science Journal, and are currently available as a preprint.  

Space looks black, but just how dark is it? 

It's a deceptively simple question that has puzzled astronomers since the 1960s. Now, thanks to data from NASA's New Horizons probe, they have arrived at the best-yet estimate of how dark — or rather, bright — deep space is: 100 billion times dimmer than the sunlight we see on Earth.

That's the amount of ambient, universe-permating glow from the births and deaths of trillions of galaxies and their countless stars ever resided in our universe. This vanishingly faint light is called the cosmic optical background (COB), and can be thought of as the visible equivalent of the cosmic microwave background (CMB) radiation, the light left over from the universe's creation. 

"If you hold up your hand in deep space, how much light does the universe shine on it?" study lead author Marc Postman, an astronomer at the Space Telescope Science Institute in Baltimore, said in a statement. "We now have a good idea of just how dark space really is."

Precise measurements of the COB allows astronomers to study how and where galaxies and stars formed across the universe's 13.8-billion-year history. But the remnant glow is so imperceptible that even advanced telescopes struggle to distinguish it from unrelated light sources in the inner solar system, including sunlight scattered by the swarm of debris around Earth and countless specks of interplanetary dust. 

"All attempts to measure the strength of the COB from the inner solar system suffer from large uncertainties," said study co-author Tod Lauer, an astronomer at the National Science Foundation's NOIRLab in Arizona.

New Horizons, however, zoomed past Pluto in July 2015 on its one-way trek out of the solar system and is now 5.5 billion miles (8.7 billion kilometers) from Earth — far enough away to witness the darkest possible "skies" and to collect the most accurate measurements to date of the faint background glow.

In the summer of 2023, the spacecraft scanned its surroundings using its onboard camera, gathering snapshots of two dozen pockets of the universe while pointing at high galactic latitudes, away from nearby bright stars and the Milky Way's blinding core. The probe's main body also shielded the sensitive camera such that even the dimmest sunlight couldn't directly reach it, according to the same statement.

While analyzing the probe's images, Lauer and his colleagues deducted light generated by dust in the halo that the Milky Way sits in, leaving them with a precise estimate of the cosmic optical background: roughly 11 nanowatts per square meter per a width of sky about 130 times the moon's diameter. The estimate is consistent with the number of galaxies formed since the Big Bang, the researchers say.

"Importantly, we also found that there is no evidence for significant levels of light produced by sources not presently known to astronomers," Postman said in the statement.

That's a relief to the researchers, whose initial estimates in 2021 suggested that the COB may be brighter than expected, leaving them to wonder whether there are any exotic, as-yet-undiscovered light sources contributing to the cosmic light.

"In our previous paper, we found there was as much light we couldn't account for as light we could measure," Postman told Astronomy.com. "The real 'gotcha' was that we simply weren't as familiar with the distribution of dust in the Milky Way as we should have been."

This time, the team was able to reference recent maps of galactic dust put together by the European Space Agency's Planck spacecraft and correct for the previously overestimated dust-scattered light.

"Looking outside the galaxies, we find darkness there and nothing more," Lauer said in the statement.

New Horizons, which is currently studying the little-explored Kuiper Belt in an extended mission mode, faced an uncertain future last August after NASA considered disbanding the mission's original science team, but agency officials later decided to continue the mission as-is for at least another five years, until 2028. Mission team members have said the probe has enough fuel to continue flying through at least 2040.

This research is described in a paper published Aug. 28 in The Astrophysical Journal.

Pluto's surface, fitting for a world whose surface shivers at a cryogenic -364 F (-220 C), is frozen solid. 

But beneath that nitrogen ice may lie a subsurface ocean of liquid water. A recent study suggested what that ocean might look like: It might be deeper than Earth's crust and denser than Earth's seawater.

It may seem odd to search for liquid water on a world as frigid and distant as Pluto. But in data from New Horizons, scientists have found a few clues hinting at a liquid water layer beneath Pluto's surface. For one, Pluto lacks a bulge at its equator, a feature that is less likely to form if a body has a liquid interior. Secondly, Pluto's icy surface appears to have fractured due to stretching over time, which could have been caused by liquid water freezing underneath the nitrogen ice on its surface, as water is one of the few substances that expands when it freezes.

Related: Supervolcano eruption on Pluto hints at hidden ocean beneath the surface

Most strikingly, some scientists believe that Pluto holds cryovolcanoes that spew out water vapor or even solid water ice. That water has to come from somewhere — and a layer of liquid water beneath Pluto's crust would fit the description.

Researchers at Washington University in St. Louis and the Lunar and Planetary Institute in Houston set about constructing a model of what that liquid layer might look like. In particular, they wanted to match Sputnik Planitia — a heart-shaped lowland basin on Pluto's surface thought to be the aftermath of an impact. So, the researchers tried different configurations of ocean thickness and water density that would result in the Sputnik Planitia criss-crossed with cracks that New Horizons observed.

"We estimated a sort of Goldilocks zone where the density and shell thickness is just right," said Alex Nguyen, a graduate student at Washington University in St Louis and one of the authors, in a statement.

Their calculations indicated that a Plutonian ocean would most likely be around 25 to 50 miles (40 to 80 kilometers) thick, and about 8 percent denser than Earth seawater. That is about as dense as the Great Salt Lake. 

But the idea of a Plutonian ocean is still controversial. Scientists don't know enough about Pluto to know if the evidence really points to liquid water or if it's just circumstantial. Most recently, a study simulating the origin of Sputnik Planitia suggested that the heart-shaped basin was most likely to have formed if Pluto had a solid interior.

So, until a successor to New Horizons lets us revisit Pluto, what lies beneath the world's surface will remain cloaked in shadow.

Nguyen and co-author Patrick McGovern published their work in the journal Icarus on February 15.

When NASA’s New Horizons mission flew past Pluto in 2015, it gave humanity our first glimpse of a colossal depression on the isolated world. It's named Sputnik Planitia. Comparable in size to the country of Mexico and dominating one of Pluto’s hemispheres, Sputnik Planitia is likely the result of an impact — but few impact craters come in Sputnik Planitia's unique pear-like shape.

How Sputnik Planitia formed remains unknown, but researchers have now painted a possible picture of its origins. It's possible, they say, that a body about the size of Switzerland crashed into Pluto long ago, at a shallow angle. If true, this picture would also hint at what Pluto's interior may look like under its cryogenic surface.

Related: Why is Pluto not a planet?

"Most thought Sputnik Planitia was of impact origin, but nobody had been able to explain its distinctive pear shape," Harry Ballantyne, an astronomer at the University of Bern in Switzerland, told Space.com.

Sputnik Planitia's shape and colossal size — roughly 2,000 kilometers long (1,243 miles long) and 1,600 kilometers wide (994 miles wide) — are not the only reasons planetary scientists have looked at it with curiosity. Whatever created this formation managed to carve out a dent as deep as s 4 kilometers (2.5 miles); and at the bottom of the chasm appears to be a frozen expanse of nitrogen ice. Gravity ought to have slowly rotated Pluto such that the dent and its missing mass ended up at one of Pluto's poles — but, oddly enough, Sputnik Planitia remains around the equator.

One popular theory suggested that Sputnik Planitia was actually a hint of a global ocean buried under Pluto's surface. After a massive impact, liquid water from the ocean might have risen to fill the gap, which would have then been iced over with a nitrogen coating — explaining why Sputnik Planitia stayed on the equator. Still, some scientists remained unconvinced.

"I had never bought into the idea that Sputnik ending up on the equator requires a global ocean," Erik Asphaug, an astronomer at the University of Arizona, told Space.com. "In my view it's much easier to explain it if you start and end with a solid body that can sustain this kind of splat."

So, Asphaug, Ballantyne and their colleagues set about conducting three-dimensional simulations to learn what circumstances might have given rise to Sputnik Planitia. The feature's strange shape was a hint that the dent's creator struck Pluto obliquely, rather than head-on. That led them to simulate a 700-kilometer-wide (435-mile-wide) object — a ball of ice and rock — striking a Pluto-like world. This object was made to exhibit a rocky core within a water-ice shell too, and the impact was imparted at a shallow 30-degree angle. Indeed, not only did this scenario lead to a pear-shaped crater, but the team's simulations indicated that the core of the impactor remained buried under Sputnik Planitia, giving it the extra mass it needed to remain on the equator.

The authors say that future research is still needed to expand on these concepts, both to understand how Sputnik Planitia evolved over billions of years and how collisions work in this distant corner of the solar system. 

"There's still a lot to learn about planetary collisions," Adeene Denton, a planetary scientist at the University of Arizona and another of the paper’s authors, told Space.com. "And for the Kuiper Belt in particular, there are many uncertainties for what happens when icy and rocky bodies collide."

Pluto's interior may help us understand those dynamics at last, but for more definite answers, we may have to wait for another mission. Although New Horizons' visit to Pluto was a remarkable feat, the probe did not do any more than simply fly by, meaning that the mission, very literally, only scratched the former planet's surface. A future mission may be able to orbit Pluto and probe its interior by, for instance, measuring the world's gravitational field.

"We need the kind of geophysical data that's been collected around the Moon and Mars that have allowed us to determine the internal structure of those bodies in such detail," Denton told Space.com. "There's a lot of information to be gained about Pluto's interior, and orbiting Pluto is the best way to do it."

The researchers published their work in the journal Nature Astronomy on April 15.

Pluto may have lost its status as a full-fledged planet in 2006, but that doesn't mean it's a joke of a world this April Fools' Day and the folks behind Pluto TV want to make that clear. 

On April 1, the free Pluto TV streaming service will host a "sit-in" to revisit the Pluto planethood debate while also celebrating the 10th anniversary of the live TV streaming service. The event, dubbed "Pluto TV's Rally for Pluto! Make Pluto A Planet!," runs from 11:30 a.m. to 1 p.m. Pacific Time at the University of California Los Angeles' Bruin Plaza. 

You can follow along live online via Pluto TV's Instagram, X (formerly Twitter), Facebook, Threads and TikTok channels. And if you can't make it in person, you can sign a Change.org petition for Pluto's planethood to make your voice heard.

Related: Why is Pluto not a planet?

"Join the citizens of Pluto for a rally to spark the conversation around Pluto's planetary status," Pluto TV said in a statement. "It's been 18 years since Pluto was demoted and we think it's time for Pluto to have a vote in its own destiny. "

Pluto TV has enlisted Pluto pioneer Alan Stern, principal investigator for NASA's New Horizons mission (which made the first Pluto flyby in 2015) to speak on Pluto's behalf. Space.com has confirmed with Stern that's it's no April Fools prank. He actually will speak at the event. 

A video message from astrophysicist Neil DeGrasse Tyson, director of the Hayden Planetarium at the American Museum of Natural History in New York City, who supported Pluto's demotion, is also expected to be played at the rally during the debate. Tyson recently launched his StarTalk TV streaming channel on Pluto TV.

Related: What is a planet?

Pluto is currently classified as a dwarf planet after losing its planethood status in 2006 during a vote by astronomers with the International Astronomical Union. The vote came amid a larger debate over what makes a planet, and if being round with moons (like Pluto is - it has five moons) is enough to qualify. Ultimately, scientists opted to demote Pluto to a dwarf planet rather than raise other dwarf planets in our solar system (like Ceres, the largest asteroid in the asteroid belt) up to planetary status. 

Since then, Pluto supporters and naysayers have been vocal about the decision, but that hasn't stopped true Pluto planet lovers from continuing their fight for its status. 

Pluto TV's Pluto rally aims to both celebrate the streaming service's 10th birthday while supporting what it calls a planetary underdog.

"The sit-in will feature high-profile guest speakers, fun chants, snacks and more all in the name of scientific fun!" Pluto TV event organizers wrote. "If you’re a believer in Pluto’s right to planet-hood, sign the petition online to show your support."

NASA's New Horizons mission, which encountered Pluto in 2015 is now riding through the deepest depths of the Kuiper Belt, is encountering a cosmic dust storm that hints there may be more going on in the outermost reaches of the solar system than we imagine.

Space is filled with dust formed of tiny particles just microns — millionths of a meter — in size. Much of the dust in our solar system is leftover residue from the formation of the planets, which was a violent affair that saw a multitude of objects smash into one another. Today, this ancient dust is also joined by fresh dust sputtered off the surfaces of asteroids and comets by micrometeorite impacts. This dust content, both fresh and ancient, gives rise to the enigmatic "Zodiacal light." The dust extends into the farthest reaches of the solar system. Astronomers still are not entirely sure of the make-up of this final frontier. 

Related: NASA extends New Horizons mission through late 2020s

The Kuiper Belt (or the Kuiper–Edgeworth Belt, named after astronomers Gerard Kuiper and Kenneth Edgeworth, who independently proposed its existence) is so far away, and its icy inhabitants are so small and faint, that it wasn't until 1992 that the first Kuiper Belt Object (KBO) beyond Pluto was discovered. That discovery was made by University of Hawaii astronomers Dave Jewitt and Jane Luu. But since then, thousands of KBOs have been spotted, and astronomers have tentatively been able to begin mapping the outer solar system.

Beyond the Kuiper Belt is the Scattered Disk, populated by KBOs that have been scattered from the Kuiper Belt by gravitational tides coming from the solar system's outermost planet, Neptune. Objects in the Scattered Disk tend to have highly elliptical orbits that are steeply inclined to the plane of the solar system and can go out to hundreds of AU's from the sun. One AU, or astronomical unit, is equal to the distance between Earth and the sun.

Far beyond the Kuiper Belt and the Scattered Disk is the Oort Cloud, a vast spherical region of frozen objects extending over a light-year from the sun. While its distance means that the Oort Cloud has never been directly observed, scientists know it exists because the orbits of long-period comets can be traced back to there.

Now, however, new findings from New Horizons threaten to upend much of what we thought we knew about the outer solar system to begin with.

"New Horizons is making the first direct measurements of interplanetary dust far beyond Neptune and Pluto, so every observation could lead to a discovery," astronomer Alex Doner of the University of Colorado, Boulder said in a statement.

The distance between the outer edge of the Kuiper Belt and the sun was thought to be about 50 astronomical units ( one AU is also equal to 149.5 million kilometers, or 93 million miles.) On Jan. 1, 2019, New Horizons encountered the KBO named Arrokoth, which sits at a distance of 44.5 AU from the sun; today, New Horizons is at a distance of 58.25 AU from the Sun, having passed the 50 AU mark in April of 2021. Over the past five years, New Horizons should have sailed over the edge of the Kuiper Belt. With KBOs separated by millions of miles, however, New Horizons wouldn't visually notice that it had left them behind. Instead the sign would be a drop-off in levels of interplanetary dust.

Yet, the spacecraft's Venetia Burney Student Dust Counter (SDC), named after the little girl who named Pluto in 1930, has not observed this drop-off. In fact, there's as much dust as ever out there, bewildering astronomers.

The SDC is mounted on the leading face of the New Horizons spacecraft. It consists of 14 plastic film detectors, each 14.2 by 6.5 centimeters (5.6 by 2.6 inches) in size and just 28 microns thick. A dozen of the detectors are exposed to space, while the other two are shielded so that they can act as reference detectors, recording any events not related to dust impacts to help rule out false positives. Whenever a dust particle strikes one of the detectors, the impact leaves a little pit in the plastic film that subtly alters the way their surfaces conduct electricity.

One possibility is that the excess dust was actually produced closer to the sun and was blown out of the Kuiper Belt thanks to the pressure of sunlight acting on the particles. However, Doner's team has deemed this theory unlikely. Instead, a more enticing possibility is favored, they say. 

There may simply be more to the Kuiper Belt than astronomers had realized.

The continued presence of dust implies New Horizons is still within the Kuiper Belt, and that the Kuiper Belt is far more extensive than anyone knew, stretching across billions of miles farther from the sun than our maps presently estimate. 

It's not just the dust counts telling us this. Astronomers have been using machine learning algorithms to search observations made by the 8.2-meter Subaru Telescope on Mauna Kea in Hawaii, and the Victor M. Blanco Four-Meter Telescope at the Cerro Tololo Inter-American Observatory in Chile to hunt for more icy objects out there that New Horizons could investigate. They've so far found 154 objects in the direction that New Horizons is headed, including about 20 that the spacecraft will come within a few million miles of, close enough for some rudimentary observations. Yet some of those 154 objects appear to be located beyond the Kuiper Belt, and not on the kind of eccentric orbit suggestive of the Scattered Disk, but in the ecliptic plane that's also shared by the Kuiper Belt. 

Are they members of a wider Kuiper Belt, or perhaps even a second belt? 

"The idea that we might have detected an extended Kuiper Belt — with a whole new population of objects colliding and producing more dust — offers another clue in solving the mysteries of the solar system's most distant regions," said Doner.

New Horizons is sailing through uncharted waters. Only four spacecraft have passed this way before — Pioneer 10 and 11, and Voyager 1 and Voyager 2; none were equipped with a dust counter like New Horizons. 

While the Pioneers are long since inactive and with the Voyager 1 spacecraft beginning to falter, New Horizons has enough fuel and power to survive well into the 2040s, when it could be well beyond 100 AU from the Sun. By the time its power dwindles, it will likely have completely redrawn the map of the outer solar system.

The new results from the SDC were published on Feb. 1 in the Astrophysical Journal Letters. 

Close to 18 years ago, astronomers spotted a miniature, icy world named Eris billions of miles beyond Neptune. But unlike its dwarf planet cousin Pluto — which New Horizons promoted to a rich, dynamic world after its visit in 2015 — Eris has not had any robotic visitors. It is so far away from Earth, in fact, that it shows up in observations just as a single pixel of light. 

All in all, scientists know very little about what happens on Eris.

Though what we do know is Eris is known to have an atmosphere that freezes and snows onto the surface below, thanks to its place near the edge of the solar system. It's about 68 times farther from the sun than Earth is. And now, new models based on data from an array of radio telescopes in Chile have revealed more about Eris. Heat leftover from the dwarf planet's birth seems to be oozing out and slowly flexing its icy surface.

Related: Dwarf Planet Eris is 'Almost Perfect' Pluto Twin

The process is causing Eris to behave less like a solid, rocky planet and "more like a soft cheese or something like that," study co-author Francis Nimmo of the University of California Santa Cruz said in a statement. "It has a tendency to flow a bit."

While a lot still remains unknown about Eris, it is considered an "almost perfect" twin of Pluto — both dwarf planets are nearly exactly the same size. Actually, when it was first spotted in 2005, it appeared to be slightly bigger than Pluto, triggering a debate among scientists. This had led the International Astronomical Union (IAU) to clarify its definition of a planet and demote Pluto to a dwarf planet. It was thanks to this contention in the scientific community that the IAU in 2006 named the dwarf planet Eris, after the Greek goddess of discord.

For the latest study, Nimmo and his colleague Mike Brown, a Caltech astronomer who had led the discovery of Eris in 2005 and is best known as the man who killed Pluto, estimated the mass for Eris' very small moon Dysnomia. Eris and its satellite are mutually tidally locked, which means both face the same way toward each other. Scientists think this is because the tiny moon "raises" tides on Eris, causing the dwarf planet to spin down over 4.5 billion years.

The new findings show Eris likely has a rocky core enveloped by a convecting icy shell. 

"The rock contains radioactive elements, and those produce heat. And then that heat has to get out somehow," said Nimmo. "So as the heat escapes, it drives this slow churning in the ice."

He and Brown suspect the surface of Eris should be "pretty smooth" as any surface features will likely be erased by flowing ice. 

"So it would be nice to get some measurements of what shape Eris is because if it's very irregular, that would not agree with our model."

This research is described in a paper published Nov. 15 in the journal Science Advances. 

When Alan Stern launches to suborbital space this week on Virgin Galactic's fifth commercial flight, he'll accomplish the goal of a lifetime. But it won't be all play — Stern will be working during almost the entire mission.

That won't dilute the joy of fulfilling his childhood dream, however. Growing up in the Apollo era, Stern watched flights to the moon launch with some regularity. Like many of his generation, he expected crewed spaceflight to become routine, expecting many more trips to the moon and, in relatively short order, voyages to Mars as well. Instead, he watched as only a handful of people climbed out of Earth's atmosphere.

Stern, a planetary scientist at the Southwest Research Institute (SwRI) in Boulder, Colorado and principal investigator of NASA's New Horizons mission to Pluto and the outer solar system, will soon join their ranks. On Thursday (Nov. 2), he will take a brief trip to suborbital space on Virgin Galactic's Galactic 05 mission.

"I've done a lot of things over the last 30 years that have led me to be prepared," Stern told Space.com. To be so close to heading into space is "really gratifying [and] a little surreal," he said.

Related: Destination Pluto: NASA's New Horizons mission in pictures

Preparation

Preparing for spaceflight isn't like packing for a family vacation. As part of his training for Galactic 05, Stern has completed three centrifuge sessions and three high-performance jet flights. 

The jet flights weren't like his previous research trips, during which he performed airborne astronomy on board F-18 Hornets and other combat aircraft. Instead, the flights reached high levels of acceleration, or Gs, to help prepare the body for the rapid speeds achieved at launch.

"I can tell you, at the end of those flights, it's just like that scene in the original 'Top Gun' movie in the locker room," Stern said. "You're raining sweat."

But he doesn't expect his actual flight to be quite so physically demanding. "Just like athletes, you train harder than the event," Stern said.

A lifetime accomplishment

Stern's fast-moving training isn't the only way he has prepared for spaceflight. He has been working toward that goal throughout his career.

"As a young engineer, and then as a young scientist, I did everything I could think of to make myself a top candidate to be a NASA engineer," he wrote on Oct. 23, in a piece for The Space Review.

That included scuba diving, traveling to the South Pole, becoming a commercial pilot and even working as a state-certified Emergency Medical Technician. He applied six times to become a NASA space shuttle mission specialist and was at one point approved to fly as a shuttle payload specialist, though he was later replaced by another researcher.

Over the last decade, Stern has been encouraging scientists to travel into space via commercial space flight. Since 2010, Stern has helped to organize the Next-Generation Suborbital Researchers Conference, which brings together hundreds of suborbital researchers, educators, flight providers, spaceport operators and others.

Stern has also campaigned tirelessly for the research and education marketplace in suborbital flight.

"When Virgin and Blue Origin started their business, they had only one interest, and it was flying tourists," Stern said. "I started talking to them about flying researchers and educators. They both do that now."

"All the suborbital flight providers in this commercial era are doing research in addition to tourist flights," he said.

For Stern's short flight, he has eight objectives laid out by SwRI. Most of those will help set the stage for his next suborbital flight, which will be funded by NASA. That upcoming flight will be purely research, Stern said.

While in the air, he will be working to familiarize himself with how the actual experience differs from training simulations, a process that will make future research flights flow more smoothly. He'll be wearing a bioharness modeled after a design flown on more than two dozen space shuttle missions, according to his flight team member Dan Durda, also of SwRI. The harness will measure his blood pressure and heart rate and compare those figures to data from his centrifuge runs and zero-g flights, as well as to the Virgin-Galactic-provided biomedical band.

As preparation for his upcoming NASA flight, Stern will also carry a functional mockup of the camera that he will carry on his second trip into space.

But all of the objectives aren't strictly business. One thing Stern will be doing is to "mitigate the overview effect distractions" — a fancy way of saying, getting a look out the window now so it won't distract him on the next flight.

"When we're there to get a very specific job done…we pretty much can't 'afford' the 'distractions' of the rest of this amazing experience," Durda told Space.com by email. "So, getting that all 'out of the way,' so to speak, is an important risk-reduction aspect of a first flight like this."

Related: Virgin Galactic launches 3 of its original space tourist customers to the final frontier (video)

'A damn good time'

Does Stern have any concerns about his upcoming trip?

"You mean, other than I could be killed?" he said.

He acknowledged that the risk is higher than it would be on a commercial airline flight. At the same time, with more than 24 zero-g flights under his belt, Stern has experience with potentially dangerous situations.

His bigger fear is smaller than that. In the week before his flight, Stern flew to Boston for a meeting with more than 100 people to discuss the New Horizons mission. "I don't want to get something from somebody and be sick next week," he said.

That fear is well founded. During Apollo 13, NASA astronaut Thomas Kenneth Mattingly was kept from launching into space due to exposure to German measles, which he never contracted. (Mattingly later flew on Apollo 16.) 

While exposure wouldn't be enough to keep Stern from going to space, being sick likely would affect his flight capability. "I'm masking a fair amount," Stern said.

After several decades of trying to get into space, what is Stern most excited about?

"Accomplishing the objectives and having a damn good time," he said.

Although it has been close to a decade since NASA's New Horizons spacecraft visited Pluto, the dwarf planet continues to reveal itself as a surprisingly complex world.

Scientists studying spacecraft data of an unusual crater near a bright, heart-shaped region on Pluto called Sputnik Planitia say they may have found a supervolcano that likely erupted just a few million years ago. That might sound like an incredibly long time ago, but cosmically speaking, it's pretty recent. For context, the solar system is more than 4.5 billion years old. 

Instead of molten rock that blasts out of Earth's volcanoes, however, the 44-km-wide Kiladze crater appears to have spewed ice lava onto Pluto's surface in a process known as cryovolcanism. The process, which also unfolds on the moons of gas giants in our solar system and likely created other mystifying terrains on Pluto, is thought to have thrusted water from the world's hidden subsurface ocean onto its surface, reshaping it across millions of years.

Volcanoes require some sort of heat source to blow up, so the recent activity on Pluto suggests there is more heat remaining in the dwarf planet's interior than previously thought.

Related: Cracks on Pluto's moon Charon may be evidence of a frozen subsurface ocean

Researchers analyzed images New Horizons had clicked of the Kiladze crater, which resides northeast of Sputnik Planitia. While at first the crater looked similar to those left behind by meteorite impacts, it appeared to be missing a central peak otherwise expected for these geological phenomena. It also seemed slightly elongated, consistent with movements caused by tectonic forces from within Pluto, according to the new study, which is yet to be peer-reviewed.

Most of Pluto's surface is blanketed with methane and nitrogen ice, so the "tip-off that Kiladze is different" from rest of Pluto's surface is the strong presence of water ice around the crater, study lead author Dale Cruikshank, a professor at University of Central Florida, told Space.com. "The water ice stands out clearly from the methane ice that covers much of the planet's surface."

Pluto's axis is tilted at a sharp 120 degrees, which means it spins almost on its side, leading to dramatic changes in climate as it circles the sun. As a result, methane ice sublimates into a haze of hydrocarbons in the dwarf planet's atmosphere, some of which rain down as snow and coat its surface. 

Over Pluto's 4.6 billion-year lifetime, scientists estimate this methane ice blanket must have gotten  at least 14 meters (46 feet) thick. "Even a centimeter or two of this organic smog would mask the water ice spectral signature we observe," Cruikshank said. 

Such a layer would have formed in just three million years. This led Cruikshank and his team to conclude Kiladze was "alive" only a few million years ago. 

Scientists don't fully understand how cryovolcanic activity works on Pluto. The world is so small that it would have lost all heat from its formation long ago. One possibility is the dwarf planet contained radioactive elements in its core which released heat while decaying, although previous research suggested there just may not be enough of these elements to power-up Pluto. 

Whatever the heat source might be, however, something appears to be keeping Pluto's subsurface ocean from freezing.

"As the planet cooled, it is plausible that pockets of liquid water were left behind, and perhaps our eruptions of water onto the surface tap into those pockets," Cruikshank said. 

But, the researcher added: "We don't know."

NASA's New Horizons spacecraft will be able to keep exploring its exotic environs for at least another five years.

The agency announced on Friday (Sept. 29) that it will keep New Horizons' lights on while it's still zooming through the Kuiper Belt, the expansive ring of icy bodies beyond Neptune's orbit.

"The New Horizons mission has a unique position in our solar system to answer important questions about our heliosphere and provide extraordinary opportunities for multidisciplinary science for NASA and the scientific community," Nicola Fox, associate administrator for NASA's Science Mission Directorate in Washington, said in a statement on Friday. (The heliosphere is the big bubble of magnetic fields and charged particles that the sun blows around itself. Beyond it lies interstellar space.)

"The agency decided that it was best to extend operations for New Horizons until the spacecraft exits the Kuiper Belt, which is expected in 2028 through 2029," Fox added. 

Related: NASA may shift New Horizons Pluto probe to sun-studying mission

New Horizons launched in January 2006, on a mission to provide the first-ever up-close looks at Pluto, the Kuiper Belt's most famous resident. The spacecraft delivered, zooming past the dwarf planet in July 2015 and revealing a world of stunning diversity and beauty.

And the probe kept cruising along, on an extended mission that centered on a flyby of another Kuiper Belt object (KBO) — the 21-mile-wide (34 kilometers) Arrokoth. That encounter went swimmingly as well, returning great imagery and intriguing science. For example, New Horizons' observations suggest that Arrokoth formed via a very gentle merger of two smaller objects, mission team members said.

But the mission's future became murky this year. NASA extended New Horizons' mission through 2024, but proposed a big shift after that — a move to the Heliophysics Division, from Planetary Science. New Horizons' principal investigator, Alan Stern, objected to this idea, arguing that the best scientific return would come from staying the course, allowing the probe to continue studying the little-explored Kuiper Belt. 

The newly announced mission extension appears to be a compromise of sorts. It will be funded primarily by Planetary Science but jointly managed by that division and Heliophysics.

Beginning in 2025, New Horizons will focus on gathering heliophysics data — but team members can still hold out hope for another KBO encounter.

"While the science community is not currently aware of any reachable Kuiper Belt object, this new path allows for the possibility of using the spacecraft for a future close flyby of such an object, should one be identified," NASA officials said in the same statement. "It also will enable the spacecraft to preserve fuel and reduce operational complexity while a search is conducted for a compelling flyby candidate."

Stern seems happy to have secured at least five more years of New Horizons operations. "I want to thank everyone who supported us in getting #NASA to continue the exploration of the Kuiper Belt by its @NASANewHorizons spacecraft — You did it!" he wrote via X (formerly known as Twitter) on Sunday (Oct. 1).

NASA spent $9.5 million on New Horizons in fiscal year 2022, according to Space News. The agency is still working on how to pay for the mission's next five or so years.

"As a starting point, funding within the New Frontiers program (including science research and data analysis) will be rebalanced to accommodate extended New Horizons operations, and future projects may be impacted," NASA officials wrote in the same statement. (New Frontiers is NASA's program of medium-cost missions; New Horizons was the first New Frontiers project to get off the ground.)

Additional extensions are possible, if the resources and will are there; New Horizons has enough nuclear fuel remaining to continue flying through at least 2040, mission team members have said.

For 17 years, the New Horizons spacecraft has hurtled at unprecedented speed through the solar system. Launched in 2006, it flew past Pluto in July 2015, returning the first close-up images of the planet and its moon. Then, in 2019, the probe reconnoitered the Kuiper Belt Object (KBO) Arrokoth. Both encounters returned astonishing images and a treasure trove of transformative data.

With its budget being trimmed for 2024, NASA is making some weighty decisions... and one includes drastically trimming New Horizons funds by replacing the current science staff with a new team in an effort to save about $3 million—a rounding error in terms of the planetary science budget. 

The principal investigator of the mission from its earliest days, Dr. Alan Stern, is not happy about the situation. "New Horizons is the only spacecraft in the Kuiper Belt, and the only one currently planned to go there. We have valuable new Kuiper Belt observations, and a search for a new flyby target, still to complete every year until we leave the Belt. Quitting this exploration prematurely, after spending nearly $1 billion to get New Horizons to the Kuiper Belt, seems to many of us to be tragically mistaken, a poor use of taxpayer money, and a lost scientific opportunity that can never be recovered from."

Related: NASA may shift New Horizons Pluto probe to sun-studying mission

As it stands, New Horizons will exit the Kuiper Belt around 2028 and should continue operating until 2050. "The spacecraft continues flying in excellent health today, returning data from the outer reaches of the solar system and preparing for a possible encounter with another KBO. Recent events, however, threaten the continued progress of this Kuiper Belt mission," Stern added. 

While we can expect cuts across a variety of NASA programs and missions, the current New Horizons budget is about on par with other long-serving veterans of deep space explorations, Voyagers 1 and 2. We're talking well under $10 million per year, which in government terms is a pittance ... especially when you consider the expense of getting the spacecraft where they are today. That is a sunk cost, and the potential, congoing science returns from the mission are the dividends. 

When New Horizons flew past Pluto, it returned an unprecedented look at the previously mysterious world and its large moon, Charon. Previous to this, the best images of Pluto came from the Hubble Space Telescope and were mere smudges just a few pixels across. The imagery and data returned from New Horizons upended many assumptions about not just Pluto and Charon, but also about the outer solar system.

"New Horizons literally wrote the book on Pluto and its system of moons. Before New Horizons, almost nothing was known. Today the New Horizons spacecraft and team have made Pluto one of the best known worlds in the solar system," Stern said.

New Horizons' next adventure was in the Kuiper Belt, the region beyond Neptune's orbit which is filled with rock and ice left over from the earliest era of the solar system. Stern's team targeted the KBO 486958 Arrokoth, which turned out to be another golden opportunity.

"Arrokoth may have been more important than our discoveries at Pluto," Stern commented. "For decades, there have been two warring computer models about how planets got their start and planetesimal formation. One was through high-speed collisions in their orbit around the sun, and the other was through local collapse clouds with very gentle collisions." After a thorough examination of Arrokoth, the latter model was validated. "It turns out these objects collided at very low speeds, more like how you might walk into a wall as opposed to supersonic impacts. When we saw Arrokoth up close, we realized from its shape and general surface geology that it had been built through gentle accretion," he added. Only though direct observation could this argument have been settled. 

If NASA ends up cutting funding to this modestly budgeted mission, a new team would be installed—presumably one with no direct experience in outer solar system operations—with the focus shifted to low-level data gathering about heliophysics, the distant plasma environment of the sun. This reduced mission would utilize just a small fraction of the spacecraft's ongoing science capability.

"We've been in flight for going on 18 years, and we have every prospect of flying as long as the Voyagers," now going on 46 years, Stern said. "At that point we would be well beyond Kuiper Belt exploration and into interstellar space." Notably, while the Voyagers are currently exploring that same interstellar region, they are doing so with ancient 1970s technology and instruments that are being shut down as they age out, with mission operations scheduled to be slimmed down until they cease within a decade or so. "New Horizons should be able to continue until at least the middle of the 21st century," Stern added, "with nearly state-of-the-art instrumentation that will be capable of shifting our view of interstellar space just as it has changed our understanding of Pluto and KBOs. But none of that justifies curtailing the current exploration of the Kuiper Belt."

With a nearly $1 billion spacecraft and modern suite of instruments at stake, it seems incredibly wasteful to ignore the vast trove of valuable, one-of-a-kind data that would be lost with a mission shift—data that includes Kuiper Belt clues about the very formation of our solar system.

Groups like the National Space Society and others have placed a petition, to be forwarded to NASA and Congress, on Change.org. Prominent voices supporting the continuation of the mission as currently implemented include famed rock musician and astrophysicist Sir Brian May, former NASA Deputy Administrator Lori Garver, and science communicator Bill Nye. 

Only through public action will our voices be heard and the mission continued at its full capability. It may be many decades before another robotic craft is sent this way; perhaps longer. Your voice counts. Go to the Save New Horizons petition page to let NASA know that science matters.

NASA's New Horizons probe flew by Pluto nearly eight years ago, but the epic encounter is still bearing scientific fruit.

New Horizons gave humanity its first up-close looks at Pluto on July 14, 2015, when the probe zoomed just 7,800 miles (12,500 kilometers) above the dwarf planet's frigid surface. The mission team is still analyzing the stockpile of data New Horizons gathered during the flyby — and still making intriguing discoveries, as new results show.

New Horizons researchers shared their latest findings on Tuesday (March 14) at the Lunar and Planetary Science Conference (LPSC) being held in Texas and virtually. Among the discoveries presented, one tied Pluto's puzzling flip to its ice-filled basin, another found interesting but puzzling landscapes on the dwarf planet's surface, and a third unveiled the building blocks that formed the snowman-like object Arrokoth, which New Horizons flew by on Jan. 1, 2019.

Related: NASA's New Horizons mission in pictures

Pluto's flip tied to Sputnik Planitia

While scientists know that Pluto, like Earth, flipped on its side sometime in its past, Pluto's orientation before the flip and the degree to which it reoriented itself has not been well understood. Scientists who use New Horizons data to study Pluto's geologic past hope to find clues that explain this event. 

Now, a group of researchers has attributed Pluto's flip to the formation of Sputnik Planitia, a 620-mile-wide (1,000 km) basin that makes up half of the iconic heart-shaped region on Pluto. Researchers previously knew that Sputnik, which is filled with nitrogen ice, played a profound role in realigning the dwarf planet's surface.

Using images that New Horizons sent home from the 2015 flyby, they are now trying to trace the path of Pluto's flip. In doing so, they found parallel mountain ranges and deep valleys that form what they think is a global tectonic system. These features are more than 186 miles (300 km) wide and span a similar distance from Pluto's north pole. 

However, the fact that Pluto changed its orientation in the past reveals that none of the terrains scientists now see is in its original location. 

"We can't really explain that in Pluto's current configuration," Oliver White, a New Horizons co-investigator at the SETI (Search for Extraterrestrial Intelligence) Institute in California, said during a presentation Tuesday at LPSC. Instead, these features likely existed along Pluto's equator early on and migrated to their current locations nearer the poles because of the flip, he said.

White's team also found that Pluto's subsurface ocean likely provided some push to Sputnik and helped shift a bulk of the dwarf planet's mass toward its equator.

Related: Why is Pluto not a planet?

Massive knife-like methane ice deposits extend to Pluto's far side

In addition to helping scientists study ancient landscapes on Pluto, New Horizons data is providing clues about its more recent features.

The spacecraft had previously spotted massive methane deposits near Pluto's equator, many as tall as Earth's skyscrapers. Scientists announced Tuesday that they had a new line of evidence suggesting that these knife-like landforms also extend to the far side of Pluto — beyond what New Horizons was able to see during its 2015 flyby.

"The discovery of these features just adds to our understanding of the processes that shape Pluto and other icy planets in our solar system and highlights the complexity, dynamic nature and diversity of planetary surfaces like Pluto's," Ishan Mishra, a postdoctoral researcher at NASA's Jet Propulsion Laboratory in California, said in a statement. 

On Earth, such pillars are called penitentes; they are made of water ice and stretch for just a few meters. On Pluto, however, these features primarily exist on the highest points on its surface and soar for hundreds of meters. At such heights, methane freezes out of Pluto's wispy atmosphere in cold weather and evaporates back into its gaseous state during warmer spells. 

The team behind the latest study used images snapped by the Long Range Reconnaissance Imager (LORRI) onboard New Horizons, and studied how the light reflected from surfaces changes with different viewing angles. 

Doing so, they found similar methane absorption features on the far side of Pluto, thanks to the surfaces being "rougher than average roughness of Pluto," Mishra said during his presentation. Such "bladed" terrains are likely one of the most common landforms on Pluto, he added.

Related: Far beyond Pluto: What's next for NASA's New Horizons probe?

Piecing together Arrokoth's long-ago merger

On Jan. 1, 2019, New Horizons passed a small object in the Kuiper Belt called Arrokoth, which looks like a partially flattened snowman. Located 4 billion miles (6.6 billion km) from Earth, it became the farthest object ever explored by a spacecraft. It is also the most primitive, thanks to its distance from the sun that keeps in a "deep freeze."

Arrokoth is a gentle fusion of two objects that once orbited each other. The larger of the two lobes, called Wenu, is itself a pile of 12 rocks huddled around a bigger slab, scientists announced on Tuesday. The latest findings show that Wenu formed not as a whole, but by pieces of rock that already existed in the outer reaches of the solar system.

"This is surprising, and a new piece in the puzzle of how planetesimals — building blocks of the planets — like Arrokoth and other Kuiper Belt objects come together," New Horizons principal investigator Alan Stern, from the Southwest Research Institute in Colorado, said in a statement.

Early in the history of the solar system, millions of kilometer-sized icy objects made up a vast, donut-shaped region at its edge called the Kuiper Belt. A few of them amalgamated to form Wenu, Stern said, but these tiny objects did not merge at high speeds, which explains why Wenu is elongated the way it is. (When objects merge at high speeds, their spin throws away material, forming circular bodies.) Since the rocks have retained their shape even after merging, Stern's team estimates that they would have been traveling less than 1 meter per second when they coalesced.

Previous research showed that Wenu tidally interacted with the smaller of the two objects; both lost some angular momentum by ejecting material and eventually merged to form today's Arrokoth. 

The individual rocks look like "Lego pieces" and have similar sizes, compositions and colors, all of which tell us "something very important about the formation of Arrokoth," Stern said during his presentation at the conference.

Stern's team found that each of Arrokoth's 12 rocks are more than 3 miles (5 km) wide. Given that Wenu is only 6 miles (10 km) thick, scientists think the 12 rocks clustered around Wenu's equator make up a bulk of its body and also extend to its far side, which wasn't seen by New Horizons.

Related: The icy 'space snowman' Arrokoth just got names for its best features

Looking inward: A unique view of Uranus and Neptune

Scientists also announced on Tuesday that future New Horizons observations will include clicking color images of both Uranus and Neptune. From its distinct vantage point in the Kuiper Belt, the spacecraft will be well placed for observations that "can only be done by a spacecraft far beyond Uranus and Neptune," said Stern.

Spacecraft within the solar system can only see light reflected from the ice giants inward, or from their sides facing Earth. New Horizons, though, will be able to collect data about the light scattered from the far sides of the planets. 

Unlike the test images it clicked in 2019, future observations will be taken as the planets rotate, scientists announced on Tuesday. They say the new images, despite being low resolution that show the two ice giants no clearer than pale blue dots, will help researchers understand more about how cloud structures evolve on the two ice giants.

On June 1, 2022, scientists put New Horizons "to sleep" to save fuel, and the spacecraft woke up from its 10-month hibernation on March 1. From the third week of April, scientists expect the spacecraft to begin studying distant Kuiper Belt objects as well as the two outer gas giants.

The observations about Uranus and Neptune are "going to be very exciting when they do come along," said Will Grundy, a New Horizons co-investigator from Lowell Observatory in Arizona. New Horizons researchers will collaborate with those working with the Hubble Space Telescope, he added.

"The science return is better than either spacecraft can provide on its own," Grundy said in a statement. "It also sets the stage for observations of similar ice giant planets around other stars."

Follow Sharmila Kuthunur on Twitter @Sharmilakg. Follow us on Twitter @Spacedotcom or on Facebook.  

The icy volcanism of Pluto's large moon Charon and a belt of fractures across its surface may have been caused by a subsurface frozen ocean bursting through a thin ice shell.

New models suggest that when Charon's internal ocean froze it could have formed deep, elongated depressions along its midsection, but this might imply the outer shell was thinner than currently predicted at some point in the moon's history. The models also suggest that cryovolcanoes erupting with ice, water, and other materials are less likely in Charon's northern hemisphere.

The icy geological features of Charon came as a shock to scientists when NASA's New Horizons spacecraft visited the Pluto-Charon system in 2015, previously believing Charon to be an inert ball of ice. Since then a science team led by Southwest Research Institute (SwRI) researchers has been investigating the New Horizons data, attempting to discover the cause of these frigid geological features. 

Related: Charon: Pluto's Largest Moon

Team member and SwRI researcher Alyssa Rhoden is an expert in the geophysics of icy satellites, especially ones that host their own oceans.

"A combination of geological interpretations and thermal-orbital evolution models implies that Charon had a subsurface liquid ocean that eventually froze," she said in a statement. "When an internal ocean freezes, it expands, creating large stresses in its icy shell and pressurizing the water below. We suspected this was the source of Charon's large canyons and cryovolcanic flows."

Rhoden modeled how fractures formed in the ice shell of Charon as the ocean beneath it froze to better understand the evolution of this moon's surface and interior. The oceans factored in the models were composed of water, ammonia, and a combination of the two. Even though ammonia can act as antifreeze and high concentrations could help preserve the duration of liquid oceans, Rhoden found the different compositions of the ocean didn't have substantial effects on their results. 

When the frozen ocean exerts pressure on the outer shell of Charon it caused fractures to penetrate the entire shell. As the volume of the ocean increases it placed pressure on overlying liquid causing it to erupt through the fractures to Charon's surface. 

The team searched for conditions that would allow fractures to fully penetrate the icy shell of Charon to link surface and subsurface water to allow for ocean-sourced cryovolcanism. This revealed that current theories surrounding the evolution of Pluto's moon could be incorrect. This is because these theories suggest that Charon ice shells were far too thick to be fully cracked by the stresses associated with ocean freezing.

"Either Charon's ice shell was less than 6 miles (10 km) thick when the flows occurred, as opposed to the more than 60 miles or 100 km indicated, or the surface was not in direct communication with the ocean as part of the eruptive process," Rhoden said. "If Charon's ice shell had been thin enough to be fully cracked, it would imply substantially more ocean freezing than is indicated by the canyons identified on Charon's encounter hemisphere."

These canyons run along the global tectonic belt of ridges across the face of Charon, separating the northern and southern geological regions of the moon. The team's model suggests the canyons may have started at fractures in the moon's ice shell that don't reach all the way to its ocean, meaning they formed after the cryovolcanism-causing fractures and when the shell of Charon had thickened.

The idea that Charon's cryovolcanism originates from a frozen ocean could be confirmed if a future mission spots additional larger extended features across the moon's hemisphere. These features, not spotted by New Horizons, would support the idea that the ocean of Charon was thicker than expected and its shell thinner. 

"Ocean freezing also predicts a sequence of geologic activity, in which ocean-sourced cryovolcanism ceases before strain-created tectonism," Rhoden said. "A more detailed analysis of Charon's geologic record could help determine whether such a scenario is viable."

The team's research is published in the journal Icarus. 

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Textbooks had to be rewritten. Members of the public were outraged. Our understanding of the solar system itself was forever changed on Aug. 24, 2006, when researchers at the International Astronomical Union (IAU) voted to reclassify Pluto, changing its status from a planet to a dwarf planet — a relegation that was largely seen as a demotion and which continues to have reverberations to this day. 

Today, the debate about Pluto exposes difficulties in the definition of "planet." The IAU defines a planet as a celestial body orbiting the sun, with a nearly spherical appearance, and that has (for the most part) cleared debris from its orbital neighborhood. But even this set of metrics is not universally agreed upon. 

Earth, and even Jupiter, have not cleared many asteroids from their orbital regions despite their large size. Moreover, there are small worlds that are circular and that orbit the sun and yet are not considered planets, such as Ceres.

Related: 15 stunning places on Earth that look like they're from another planet

Pluto's so-called demotion from planetary status raises larger issues about how to define any object in the solar system, or even in space more generally. It shows that science cannot, sometimes, slot objects into easy categories. Because if the definition of a planet once again widens, it is unclear how to assess the numerous non-circular objects that circle our sun. This may even put the asteroid belt into question, referring to the huge band of small objects between Mars and Jupiter. Or what happens if a planet is somehow broken up into pieces?

All the same, as the Pluto debate took place almost 20 years ago, many still don't quite understand all the fuss, nor why Pluto was knocked from its planetary position. But the solar system's transformation from nine planets to eight (at least by the standard IAU definition) was a long time in the making and helps encapsulate one of the greatest strengths of science — the ability to alter seemingly steadfast definitions in light of new evidence.

What is a planet, anyway?

The word planet (in English) stretches back to antiquity, deriving from the Greek word "planetes," which means "wandering star." The five classical planets — Mercury, Venus, Mars, Jupiter and Saturn — are visible to the naked eye and can be seen shifting in strange pathways across the sky compared with the more distant background stars. 

After the advent of telescopes, astronomers discovered two new planets, Uranus and Neptune, which are too faint to spot with the naked eye (Note that this definition of "planet" is following the Greco-Roman tradition on which the International Astronomical Union or IAU's community definitions are based. The names of planets vary by culture and the naked-eye planets were observed around the world during antiquity.)

When astronomers discovered Ceres (today considered a dwarf planet), they initially categorized it as a "planet" among scientific communities of the day. But that began to change as further measurements showed it was smaller than other planets ever seen at the time. Eventually, Ceres was lumped into a group of rocky bodies, called "asteroids", of which we now know of hundreds of thousands of these in the asteroid belt alone.

Pluto was found and classified as a planet in 1930 (note the IAU was formed in 1919) when astronomer Clyde Tombaugh of the Lowell Observatory in Arizona compared photographic plates of the sky on separate nights and noticed a tiny dot that drifted back and forth against the backdrop of stars. Right away, the solar system's newest candidate was considered an oddball, however. Its orbit is so eccentric, or far from circular, that it actually gets closer to the sun than Neptune for 20 of its 248-years-long trip. It also is tilted to the ecliptic, which is the plane upon which the other solar system planets orbit.

In 1992, scientists discovered the first Kuiper Belt object, 1992 QB1, a tiny body orbiting out in Pluto's vicinity and beyond the orbit of Neptune. Many more such objects were soon uncovered, revealing a belt of small, frozen worlds similar to the asteroid belt between Mars and Jupiter. Pluto remained the king of this region, but in July 2005, astronomers found the distant body Eris, which at first was thought to be even larger than Pluto.

A planetary conundrum

Researchers had to ask themselves these questions: If Pluto was a planet, then did that mean Eris was one as well? What about all those other icy objects out in the Kuiper belt, or the littler objects in the asteroid belt? Where exactly was the cut-off line for classifying a body as a planet? A word that had seemed straightforward and simple was suddenly shown to be oddly slippery.

An intense debate followed, with many new proposals for the definition of a planet being offered. 

"Every time we think some of us are reaching a consensus, then somebody says something to show very clearly that we're not," Brian Marsden, a member of the IAU Executive Committee in charge of coming up with a new meaning for the word planet, told Space.com in 2005. 

A year later, astronomers were no closer to a resolution, and the dilemma hung like a dark cloud over the IAU General Assembly meeting in Prague in 2006. At the conference, researchers endured eight days of contentious arguments, with four different proposals being offered. One controversial suggestion would have brought the total number of planets in the solar system to 12, including Ceres, the largest asteroid, and Pluto's moon Charon. 

The suggestion was "a complete mess," astronomer Mike Brown of Caltech, Eris' discoverer, told Space.com. 

Near the end of the Prague conference, the 424 astronomers who remained voted to create three new categories for objects in the solar system. From then on, only the worlds Mercury through Neptune would be considered planets. Pluto and its kin — round objects that shared the neighborhood of their orbit with other entities — were henceforth called dwarf planets. All other objects orbiting the sun would be known as small solar system bodies.  

The New Horizons mission and the planet debate

A contingent of professionals did not take the decision lightly. "I'm embarrassed for astronomy," Alan Stern, a leader of NASA's New Horizons mission, which flew past Pluto in 2015, told Space.com, adding that less than 5 percent of the world's 10,000 astronomers participated in the vote.

New Horizons was a significant turning point in the planet debate, as its swift flyby by Pluto showed a world that is far more dynamic than anyone imagined. Large mountains, battered craters and signs of liquid flowing upon its surface all point to a world that underwent massive geological change since its formation. On this basis alone, people like Stern have said, Pluto should be considered a planet since it is a dynamic place, a place that is not so static that only micrometeorites disturb its surface.

Views of Charon, Pluto's moon, also show a very dynamic place, including a red cap on its pole that appears to change appearance with the slow seasonal change that far out in the solar system. Notably, Pluto possesses several moons while two established planets, Mercury and Venus, do not. (Numerous asteroids and dwarf planets also have moons, which make the definition of a planet even more complicated.)

Such views are shared by many in the public. In 2014, shortly before the flyby, experts at the Harvard & Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, debated different definitions of a planet. Science historian Owen Gingerich, who chaired the IAU's planet-definition committee, asserted that "planet is a culturally defined word that changes over time." But the audience watching the CfA debate overwhelmingly chose a different participant's definition — one that would have brought Pluto back into the planetary fold. 

Alternative classification schemes continue to pop up. A 2017 proposal defined a planet as "a round object in space that's smaller than a star." This would make Pluto a planet again, but it would do the same to the Earth's moon as well as many other moons in the solar system, and bring the total number of officially recognized planets up to 110. A year later, Stern, along with planetary scientist David Grinspoon, wrote an opinion article in The Washington Post arguing that the IAU's definition was "hastily drawn" and "flawed" and that astronomers should reconsider their ideas. 

Will Pluto ever become a planet again?

Such pleas have fallen on deaf ears so far, and it seems unlikely the IAU will revisit the controversy any time soon. Astrophysicist Ethan Siegel responded to Stern and Grinspoon in Forbes by writing: "The simple fact is that Pluto was misclassified when it was first discovered; it was never on the same footing as the other eight worlds."

Mike Brown also chimed in. "So, hey, Pluto is still not a planet. Actually, never was. We just misunderstood it for 50 years. Now, we know better. Nostalgia for Pluto is really not a very good planet argument, but that's basically all there is. Now, let's get on with reality," Brown wrote on Twitter, where he has embraced his role in the redefinition with the handle @plutokiller. 

Why does it matter?

With kids these days (who weren't even born when Pluto was a planet) asking why the definition even matters, astronomers have said that it is no easy answer and we may have to look beyond our solar system in considering what makes a planet and what does not.

More than 5,000 exoplanets, or planets outside our solar system, have been discovered to date and they are showing a huge set of worlds. From "super-Earths" between the size of Earth and Uranus, to "hot Jupiters" orbiting close to suns, to a range of other planetary sizes, the types of planetary environments to consider are changing rapidly.

What this proliferation of planet-forming options shows us is that each solar system may be its own unique environment. While we can say more generally that stars can form planets from collapsing gas and dust in their environment, the individual dynamics that control planet formation are far more complicated. For example, are there multiple stars involved? How much dust is available? Is there a black hole or a supernova blowing the valuable dust and gas away that planets need to grow?

Even if planets are lucky enough to grow up, how they are connected with other planets early in their formation is poorly understood. As worlds interact with each other, their mutual gravities appear to shift planets closer and further from their parent star or in some cases, to fling worlds out of the solar system together.

What this all shows is that our definition of a planet may have to become more situational to account for the number of scenarios in which a world may form. Perhaps planets may be tied to particular formation circumstances or particular zones. All that seems to be known for sure is that as we collect data, planethood and the debate that Pluto has engendered will continue to be debated for quite some time yet.

Additional resources

Learn more about the Kuiper Belt from NASA's Solar System Exploration website. See images of Pluto from NASA's New Horizons spacecraft flyby. 

References

International Astronomical Union. "Pluto and the Developing Landscape of Our Solar System." https://www.iau.org/public/themes/pluto/

Johns Hopkins Applied Research Laboratory. (2022.) "New Horizons." http://pluto.jhuapl.edu/ 

NASA. (2019, Dec. 19.) "What is a Planet?" https://solarsystem.nasa.gov/planets/in-depth/ 

NASA Science Solar System Exploration. "Kuiper Belt." https://solarsystem.nasa.gov/solar-system/kuiper-belt/overview/

Siegel, Ethan. (2018, May 8). "You Won't Like The Consequences Of Making Pluto A Planet Again." Forbes. https://www.forbes.com/sites/startswithabang/2018/05/08/you-wont-like-the-consequences-of-making-pluto-a-planet-again/

You would think plunging into deep space, to the outer banks of our solar system, would be a person's career ascension.

But for Alan Stern, principal investigator of the New Horizons mission that explored Pluto and more recently the Kuiper Belt, he also recently descended in a deep sea submersible to view the sunken remains of the RMS Titanic.

A luxury steamship, the Titanic sank on April 15, 1912 off the coast of Newfoundland in the North Atlantic. Its sinking came after the ship ran into an iceberg during its maiden cruise. The tragedy involved 2,240 passengers and crew. More than 1,500 lost their lives in the catastrophe.

For planetary scientist Stern, who has purged deep space of its secrets in the 21st century, witnessing the 20th century deep sea sarcophagus that is the Titanic serves up analogies to spaceflight. But there are lots of differences too.

Related: James Cameron's 'Titanic' Correction May Impress Astronomers

Ocean space, with a difference

"I have been doing a lot of thinking about this. A 100 years from now, somebody like me could be piloting down into Europa's ocean or Enceladus's or Pluto's deep ocean in submersibles," Stern told Space.com. "I thought a lot about this expedition to Titanic. There are so many layers to the whole thing that made it interesting: science, history, tragedy, archeology, exploration and adventure."

Stern said that these deep ocean submersibles here on Earth can go down deep and take 6,000 pounds per square inch (PSI) of pressure. "These are vehicles that can take the pressure that's four times that of the bottom of the atmosphere of Venus. They are high-tech but they are much lower tech than what it takes to do spaceflight and they are much less expensive."

The underwater sojourn to the Titanic came via OceanGate, a private company based in Everett, Washington that owns and operates three 5‑person submersibles for site survey, scientific research, film production, and exploration travel to depths as great as 4000 meters (2.5 miles).

The Titanic voyage from St. John's, Newfoundland and returning back to port was July 17-26, inclusive.

"The people at OceanGate are very talented and it's a very multifaceted operation. There's the mothership, the submersible itself, three other boats and scuba divers. Launch and recovery is a complex operation," Stern said. When undersea you've got to communicate acoustically once you are more than a meter below the surface, he said.

"So everything is traveling at the speed of sound — which means it's like a 30-second two-way time delay when you want to say something to the surface. Even if they answer almost immediately it's a minute later when they respond," Stern said. The sound waves have to travel up to the ship, 2.5 miles (4 kilometers) above, and then back at the speed of sound in the salt water, he added.

Crewmates

"It was a very interesting experience with three dives during the week. But two days we couldn't dive due to high seas; the remainder of the 8-day voyage was steaming down there and back from port at St. John's, Newfoundland, another 40 hours each way," said Stern.

The surface vessel — the Horizons Arctic carrier ship — looks like a maritime pick-up truck. Its long and enormous back deck is like a cab with cabins and bridge in front and a big bed in the back. "The ship had like 53 people made up of the ship's crew, the submersible crew, and those of us that were diving, along with a British Broadcasting Corporation film crew," Stern said.

The submersible itself is rated to be occupied by six people. "We went with five and it was definitely cramped, Two of us were fairly small men, like me and the others were average or a little bigger. If you put five line backers in there, it would be very, very tight," Stern said.

On his recent dive to the Titanic, onboard were two Blue Origin suborbital space crewmates: Dylan Taylor and Evan Dick; Dick has flown twice on that firm's rocket, Taylor once. Joining that threesome was Denver businessman Randy Brunschwig and the pilot of the submersible, Stockton Rush, the chief executive officer and founder of OceanGate Inc.

Space odyssey

The Titanic-diving craft is equipped with a single window, about 16 inches across. Those onboard took turns viewing out the port. "But you can see, no matter wherever you are in the vehicle because there's a 4K video resolution camera that's bore sighted with the window. It is displaying its view on a big light-emitting diode (LED) display that's roughly 35 or 40 inches (89 to 101 centimeters) across. You are seeing the same thing that the camera sees. If people are crowded around the window — you can look at the display. And there's a whole bunch of other computer displays with information about the submersible's systems, navigation, communications, and more," Stern said.

The actual cockpit of the submersible looks a little bit like the EVA Pod on the 1968 epic movie, "2001: A Space Odyssey." "It's like that," Stern added, "but the window is at the back end and it's opposite where the pilot station is and is basically a rear view mirror, along with this 4K camera."

Titanic dive day

Stern and his colleagues were below water for nine hours and 20-plus minutes. Sitting inside the craft, they were outfitted in a jump suit called a flight suit. Half of that dive, on the order of 4.5 hours, was down on the abyssal plain, 2.5 miles (4,000 meters) down where the Titanic ruins rest. 

Stern was chosen to be the voice com, communicating to mission control on the surface. All the better for the pilot that could pay steel-eyed attention to the driving and checking out sub systems. Stern was also the designated mission scientist on the dive, making sure sampling and photo documentation duties were performed. 

"We did science all the way down and back collecting water samples, photographing the sea life, and the Titanic itself. The biologists gave us a checklist of things to do. We brought back mud and water samples from the bottom, including the metal content of what's coming off the Titanic using mass spectrometers and other devices to study. We took lots of photography of the many kinds of sea life down there at the bottom," Stern explained. "It's kind of a desert down there. It is not like dense fields of coral reef, it's very here and there, spotty vegetation. The fish, crabs and the shrimp are pretty sparse. They are down at 6,000 PSI so it's amazing there's life down there; it's so dark, so cold, so high pressure."

Perfectly perched and looking brand new

From his undersea vantage point, Stern was struck by this thought: "1,500 people lost their lives. Some of them, maybe in some cases their clothes are still in the Titanic. The bodies dissolved in the salt water over time. But lots of things don't," he said.

The submersible was equipped with floodlights, but normally the Titanic is engulfed in darkness. "It is like being in a very dark cave down there. There's no light whatsoever; except when a vehicle comes down," Stern said. The experience of seeing the Titanic is very select; about 600 people have been to space, and only less than 250 have been down to the Titanic, he said.

One thing that surprised Stern was spotting a porcelain coffee cup, sitting on the sandy bottom, perfectly perched and looking brand new. "I guess the biology can't glom onto it. It's perfectly white like you bought it yesterday. It has a little bit of sand in it, I would say the bottom 10 percent of the cup. It made me wonder whose coffee cup was that? Did they live? Did they die?" 

Scattered about as well were some porcelain plates, even a beer bottle, Stern said.

Disappearing act

The metal of the Titanic is rusted. All the wood is rotted after a century in salt water. The metal is barnacle encrusted, Stern said, things dubbed "rusticles" — barnacles that are made of rust. "It looks like an ancient ruin," he said.

The Titanic debris field is massive, stretching out more than a kilometer. The ship broke in half, and when it 'porpoised' up, because the back end was so heavy, it broke the keel and the two halves went down separately.

Another item that Stern emphasized is that the Titanic is collapsing and decaying, Year to year the people who go back and make the dive detect the difference.

"The collapse and decay of the ship is accelerating. All the bolts and rivets are being attacked by the salt water. And they are all being attacked equally and they are all built to the same tolerance in the same salt water at same temperature and pressure together. To first order — they all start to break and let go within a short period of time, a few years of each other. These bolts and rivets that are decaying in the salt water mean that whole decks will crash down onto one another as those fail. And we saw some of that," Stern said. "That's one reason I wanted to make the time to go. It was a special opportunity but it was also a perishable opportunity."

Somber feelings

"I had a lot running through my mind. First of all I had responsibilities to work," Stern said, and then to see the Titanic as an historical artifact. "It's quite emotional to be down there. The little children of the lower classes never made it. Most of the people that died were just abandoned when it sank. They were the peasants; the lower classes as they had back then that didn't have access to the life boats. The aristocrats got to the boats. It is emotional. I wasn't reduced to tears but I was certainly somber about that. We all were," he said.

The biology at the Titanic site is fascinating, Stern said. "These 'beings' that live in total darkness, some of them light their own way. If you turn off the ship lights on the sub, you see some of them glowing. The temperature and pressures are like the conditions in Europa's ocean or Pluto's ocean, or the ocean of Enceladus," said Stern. "It really makes you think that perhaps life could evolve in those situations — if there is a source of nutrients and energy — like the thermal vents."

Personal exploration

In his above underwater judgment, what does his Pluto experience of seeing a new world contrasted to seeing the Titanic for the first time compare?

"In this case I was going to a place I hadn't been, but others have been," Stern said. Maybe it's kind of like being the 200th person to step on the moon, very select but not at all a first. But it's still very select among us humans," Stern said. "Of course, other people have been to the sea floor and other locales. Still, it's extremely rare that people get to do this. Whereas the exploration of Pluto by New Horizons was revealing a whole new type of Pluto for the first time, it was its own thing. I would call that a whole order of magnitude or two more raw exploration," he said.

For Stern the Titanic dive was personal versus the first time human or robotic exploration sensed an unexplored destination. "Yes we were doing science and archeology and photographing the Titanic wreck for certain purposes, certain angles, looking at the biology. But this was very much more personal exploration, like climbing Everest or trekking across Antarctica and being among the select few to do it and to see it." 

Follow us on Twitter @Spacedotcom or on Facebook.   

Pluto is the largest known dwarf planet in the solar system and used to be considered the ninth and most distant planet from the sun. 

The strange world is located in the Kuiper Belt, a zone beyond the orbit of Neptune brimming with hundreds of thousands of rocky, icy bodies each larger than 62 miles (100 kilometers) across as well as 1 trillion or more comets. 

Pluto stopped being a planet in 2006 when it was reclassified as a dwarf planet, a demotion that attracted controversy and stirred debate in the scientific community and among the general public. 

Related: Why isn't Pluto a planet anymore?

American astronomer Percival Lowell first suggested that Pluto existed in 1905 when he observed strange deviations in the orbits of Neptune and Uranus. Lowell thought there must be another whose gravity is tugging on these ice giants, causing discrepancies in their orbits. Lowell proceeded to predict the mystery planet's location in 1915 but died 15 years before its discovery. Pluto was eventually discovered in 1930 by Clyde Tombaugh at the Lowell Observatory, based on predictions by Lowell and other astronomers.

Pluto got its name from 11-year-old Venetia Burney of Oxford, England, who suggested to her grandfather that the new world get its name from the Roman god of the underworld. Her grandfather then passed the name on to Lowell Observatory. The name also honors Percival Lowell, whose initials are the first two letters of Pluto. 

Pluto FAQs answered by an expert

We asked Emily Safron, an astronomy instructor at Case Western Reserve University, who received her Ph.D. in physics from Louisiana State University under Dr. Tabetha Boyajian, a few frequently asked questions about Pluto. 

What does Pluto look like?

Since Pluto is so far from Earth, little was known about the dwarf planet's size or surface conditions until 2015, when NASA's New Horizons space probe made a close flyby of Pluto. New Horizons showed that Pluto has a diameter of 1,473 miles (2,370 km), less than one-fifth the diameter of Earth, and only about two-thirds as wide as Earth's moon.

Observations of Pluto's surface by the New Horizons spacecraft revealed a variety of surface features, including mountains that reach as high as 11,000 feet (3,500 meters), comparable to the Rocky Mountains on Earth. While methane and nitrogen ice cover much of the surface of Pluto, these materials are not strong enough to support such enormous peaks, so scientists suspect that the mountains are formed on a bedrock of water ice. 

Pluto's surface is also covered in an abundance of methane ice, but New Horizons scientists have observed significant differences in the way the ice reflects light across the dwarf planet's surface. The dwarf planet also possesses ice ridge terrain that appears to look like a snakeskin; astronomers spotted similar features to Earth's penitentes, or erosion-formed features on mountainous terrain. The Pluto features are much larger; they are estimated at 1,650 feet (500 m) tall, while the Earth features are only a few meters in size.  

Another distinct feature on Pluto's surface is a large heart-shaped region known unofficially as Tombaugh Regio (after Clyde Tombaugh; regio is Latin for region). The left side of the region (an area that takes on the shape of an ice cream cone) is covered in carbon monoxide ice. Other variations in the composition of surface materials have been identified within the "heart" of Pluto.

In the center-left of Tombaugh Regio is a very smooth region unofficially known by the New Horizons team as "Sputnik Planum," after Earth's first artificial satellite, Sputnik. This region of Pluto's surface lacks craters caused by meteorite impacts, suggesting that the area is, on a geologic timescale, very young — no more than 100 million years old. It's possible that this region is still being shaped and changed by geologic processes.

These icy plains also display dark streaks that are a few miles long and aligned in the same direction. It's possible the lines are created by harsh winds blowing across the dwarf planet's surface.

NASA's Hubble Space Telescope has also revealed evidence that Pluto's crust could contain complex organic molecules.

Related: Photos of Pluto and its moons

Pluto's surface is one of the coldest places in the solar system. Temperatures here can drop to around minus 375 to minus 400 degrees Fahrenheit (minus 226 to minus 240 degrees Celsius). When compared with past images, pictures of Pluto taken by the Hubble Space Telescope revealed that the dwarf planet had apparently grown redder over time, apparently due to seasonal changes.

Pluto may have (or may have had) a subsurface ocean, although the evidence is still out on that finding. If the subsurface ocean existed, it could have greatly affected Pluto's history. For example, scientists found that the zone of Sputnik Planitia redirected Pluto's orientation due to the amount of ice in the area, which was so heavy it affected Pluto overall; New Horizons estimated the ice is roughly 6 miles (10 km thick). A subsurface ocean is the best explanation for the evidence, the researchers added, although looking at less likely scenarios, a thicker ice layer or movements in the rock may be responsible for the movement. If Pluto did have a liquid ocean, and enough energy, some scientists think Pluto could harbor life.

What is Pluto made of?

Some of Pluto's parameters, according to NASA:

Atmospheric composition: Methane, nitrogen. Observations by New Horizons show that Pluto's atmosphere extends as far as 1,000 miles (1,600 km) above the surface of the dwarf planet.

Magnetic field: It remains unknown whether Pluto has a magnetic field, but the dwarf planet's small size and slow rotation suggest it has little to no such field.

Chemical composition: Pluto probably consists of a mixture of 70 percent rock and 30 percent water ice.

Internal structure: The dwarf planet probably has a rocky core surrounded by a mantle of water ice, with more exotic ices such as methane, carbon monoxide and nitrogen ice coating the surface.

Pluto's orbital characteristics

Pluto's highly elliptical orbit can take it more than 49 times as far out from the sun as Earth. Since the dwarf planet's orbit is so eccentric, or far from circular, Pluto's distance from the sun can vary considerably. The dwarf planet actually gets closer to the sun than Neptune is for 20 years out of Pluto's 248-Earth-years-long orbit, providing astronomers a rare chance to study this small, cold, distant world.

As a result of that orbit, after 20 years as the eighth planet (in order going out from the sun), in 1999, Pluto crossed Neptune's orbit to become the farthest planet from the sun (until it was demoted to the status of dwarf planet).

When Pluto is closer to the sun, its surface ices thaw and temporarily form a thin atmosphere, consisting mostly of nitrogen, with some methane. Pluto's low gravity, which is a little more than one-twentieth that of Earth's, causes this atmosphere to extend much higher in altitude than Earth's. When traveling farther away from the sun, most of Pluto's atmosphere is thought to freeze and all but disappear. Still, in the time that it does have an atmosphere, Pluto can apparently experience strong winds. The atmosphere also has brightness variations that could be explained by gravity waves, or air flowing over mountains. 

While Pluto's atmosphere is too thin to allow liquids to flow today, they may have streamed along the surface in the ancient past. New Horizons imaged a frozen lake in Tombaugh Regio that appeared to have ancient channels nearby. At some point in the ancient past, the planet could have had an atmosphere roughly 40 times thicker than on Mars. 

In 2016, scientists announced that they might have spotted clouds in Pluto's atmosphere using New Horizons data. Investigators saw seven bright features that are near the terminator (the boundary between daylight and darkness), which is commonly where clouds form. The features are all low in altitude and roughly about the same size, indicating that these are separate features. The composition of these clouds, if they are indeed clouds, would likely be acetylene, ethane and hydrogen cyanide. 

Does Pluto have any moons?

Pluto has five moons: Charon, Styx, Nix, Kerberos and Hydra, with Charon being the closest to Pluto and Hydra the most distant.

In 1978, astronomers discovered that Pluto had a very large moon nearly half the dwarf planet's own size. This moon was dubbed Charon, after the mythological demon who ferried souls to the underworld in Greek mythology.

Because Charon and Pluto are so similar in size, their orbit is unlike that of most planets and their moons. Both Pluto and Charon orbit a point in space that lies between them, similar to the orbits of binary star systems, For this reason, scientists refer to Pluto and Charon as a double dwarf planet, double planet or binary system.

Pluto and Charon are just 12,200 miles (19,640 km) apart, less than the distance by flight between London and Sydney. Charon's orbit around Pluto takes 6.4 Earth-days, and one Pluto rotation — a Pluto-day — also takes 6.4 Earth-days. This is because Charon hovers over the same spot on Pluto's surface, and the same side of Charon always faces Pluto, a phenomenon known as tidal locking.

While Pluto has a reddish tint, Charon appears more grayish. In its early days, the moon may have contained a subsurface ocean, though the satellite probably can't support one today. Compared with most of the solar system's planets and moons, the Pluto-Charon system is tipped on its side in relation to the sun.

Observations of Charon by New Horizons have revealed the presence of canyons on the moon's surface. The deepest of those canyons plunges downward for 6 miles (9.7 km). A long swatch of cliffs and troughs stretches for 600 miles (970 km) across the middle of the satellite. A section of the moon's surface near one pole is covered in a much darker material than the rest of the planet. Similar to regions of Pluto, much of Charon's surface is free of craters — suggesting the surface is quite young and geologically active. Scientists saw evidence of landslides on its surface, the first time such features have been spotted in the Kuiper Belt. The moon may have also possessed its own version of plate tectonics, which cause geologic change on Earth. 

In 2005, scientists photographed Pluto with the Hubble Space Telescope in preparation for the New Horizons mission and discovered two other tiny moons of Pluto, now dubbed Nix and Hydra. These satellites are two and three times farther away from Pluto than Charon. Based on measurements by New Horizons, Nix is estimated to be 26 miles (42 km) long and 20 miles (32 km) wide, while Hydra is estimated at 34 miles (55 km) long and 70 miles (113 km) wide. It is likely that Hydra's surface is coated primarily in water ice.

Scientists using Hubble discovered a fourth moon, Kerberos, in 2011. This moon has a doubled-lobed shape and the larger lobe is around 5 miles (8 km) across and the smaller lobe is around 3 miles (5 km) across. On July 11, 2012, a fifth moon, Styx, was discovered (with an estimated width of 6 miles or 10 km), further fueling the debate about Pluto's status as a planet.

The leading hypothesis for the formation of Pluto and Charon is that a nascent Pluto was struck with a glancing blow by another Pluto-size object. Most of the combined matter became Pluto, while the rest spun off to become Charon, this idea suggests. The other four moons may have formed from the same collision that created Charon. 

Missions to Pluto

NASA's New Horizons mission is the first probe to study Pluto, its moons and other worlds within the Kuiper Belt up close. It was launched on January 2006, and successfully made its closest approach to Pluto on July 14, 2015. 

The New Horizons probe carries some of the ashes of Pluto's discoverer, Clyde Tombaugh.

The limited knowledge of the Pluto system created unprecedented dangers for the New Horizons probe. Prior to the mission's launch, scientists knew of the existence of only three moons around Pluto. The discovery of Kerberos and Styx during the spacecraft's journey fueled the idea that more satellites could orbit the dwarf planet, unseen from Earth. Collisions with unseen moons, or even small bits of debris, could have seriously damaged the spacecraft. But the New Horizons design team equipped the space probe with tools to protect it during its journey.

In October 2021, New Horizons made history when it returned the first close-up images of Pluto and it's moons.  

Additional resources

Explore Pluto in more detail with NASA's 10 cool things we learned about Pluto article. Read more about Pluto and the developing landscape of our Solar System with the International Astronomical Union. Discover the six things dwarf planets have taught us about the solar system with the American Geophysical Union's science news magazine Eos. 

Bibliography

International Astronomical Union. IAU. Retrieved July 28, 2022, from https://www.iau.org/public/themes/pluto/

NASA. (2021, August 5). Pluto. NASA. Retrieved July 28, 2022, from https://solarsystem.nasa.gov/planets/dwarf-planets/pluto/overview/

Talbert, T. (Ed.). (2020, July 14). 10 cool things we learned about pluto. NASA. Retrieved July 28, 2022, from https://www.nasa.gov/feature/five-years-after-new-horizons-historic-flyby-here-are-10-cool-things-we-learned-about-plut-0

Wendel, J. A. (2016, August 17). Six things dwarf planets have taught us about the solar system. Eos. Retrieved July 28, 2022, from https://eos.org/articles/six-things-dwarf-planets-have-taught-us-about-the-solar-system 

Get your observing equipment ready for some world-hunting.

Dwarf planet Pluto will reach its opposition, or brightest appearance in the sky, late on Wednesday (July 20). Stay up until dawn, and you can watch the third-quarter moon approach Mars in the brightening sky Thursday (July 21).

Be forewarned that Pluto is an observing challenge, as it is only visible in the largest of backyard telescopes. Its maximum magnitude will be 14.3, compared with naked-eye stars that shine at -6. 

Related: See the rare alignment of 5 planets and the moon in this stunning night sky photo

Still, we can at least point you to the area of sky Pluto currently occupies. According to astronomer Chris Vaughan of Astrogeo.ca, who prepares Space.com's monthly Night Sky calendar in cooperation with Simulation Curriculum, look for the medium-bright star Omega Sagittarii, which shines well to the left (east) of Sagittarius' teapot-shaped asterism. 

Pluto is roughly three finger-widths (3.5 degrees) north of the star. Or you can seek out the bright globular star cluster Messier 75; Pluto is two degrees to the lower right. 

Happily, the moon and Mars are much easier to spot and will be visible to the naked eye.

The moon will rise early in the morning on Thursday as a waning crescent, starting at 12:32 a.m. local time in New York City. (Consult Timeanddate.com to find the rise time for your locality.) Mars should rise a few minutes later.

Wait until the moon and Mars are well above the horizon, around 2 a.m. local time. Find the moon, then look several finger-widths (4 degrees) to the upper right to spot Mars. The two worlds will share a single binocular field.

You'll be able to follow them up at the sky through dawn, around 5 a.m. If you're clouded out, you'll be able to spot the two worlds again on Friday morning (July 22), although the moon will now be at the lower left of Mars.

In between, observers in eastern China, Japan and northeastern Russia can watch the moon go in front of (occult) Mars around 1500 GMT on Thursday, according to Vaughan.

Looking for a telescope or binoculars to observe Pluto, Mars or the moon? Our guides for the best binoculars deals and the best telescope deals now can help. Our best cameras for astrophotography and best lenses for astrophotography can also help you prepare to capture the next skywatching sight on your own. 

Follow Elizabeth Howell on Twitter @howellspace. Follow us on Twitter @Spacedotcom and on Facebook. 

Scientists have figured out why Pluto's moon Charon wears a red cap.

New laboratory work shows that the color results when ultraviolet light and the solar wind interact with hydrocarbons and turn them into a soup of organic compounds dubbed tholins.

Until NASA's New Horizons spacecraft flew past Pluto in 2015, little was known about Charon, which is Pluto's largest moon, with a diameter of about 750 miles (1,200 kilometers). But New Horizons' close-up images revealed a frozen world with a ruddy-colored polar cap in the moon's northern hemisphere.

Related: Pluto flyby photos: New Horizons mission leader reveals 10 of his favorite epic views

At the time, scientists thought the red cap was produced when ultraviolet light from the faraway sun broke down molecules of methane that had escaped Pluto and been captured by Charon. That reaction resulted in "tholins," a term Carl Sagan coined to refer to a soup of organic compounds. However, new research described in two papers by scientists at the Southwest Research Institute (SwRI) in Texas shows that the process that forms the tholins is a little more convoluted.

"Our findings indicate that drastic seasonal surges in Charon's thin atmosphere, as well as light breaking down the condensing methane frost, are key to understanding the origins of Charon's red polar zone," SwRI's Ujjwal Raut, a planetary scientist and lead author of one of the new papers describing the new findings, said in a statement. 

In experiments at SwRI's Center for Laboratory Astrophysics and Space Science Experiments, scientists recreated the surface conditions on Charon, such as its ability to trap methane on its surface, and methane’s sublimation rate when warmed, and then plugged their results into a dynamic model describing Charon's thin methane atmosphere, or exosphere. 

They found that Charon's atmosphere is inactive for centuries as Pluto and its five moons laboriously continue on their long orbit around the sun. During this time, Charon's surface temperatures linger as low as minus 433 degrees Fahrenheit (minus 258 degrees Celsius). Those conditions change around the time of Pluto's equinox, which most recently occurred in 1988 to 1989.

During the equinox period, Charon experiences a surge of activity that lasts just four Earth years, with the moon's surface warming to minus 406 F (minus 243 C). Methane frozen at the pole in the hemisphere experiencing spring rapidly sublimates, creating a short-lived exosphere. The methane gas in the exosphere migrates to the hemisphere experiencing autumn, where it refreezes. (That's the theory, anyway, since scientists have only gotten a good look at the north pole.)

When ultraviolet light from the distant sun interacts with the frozen methane at the poles before it sublimates, it turns some of the methane into ethane. Because of its higher sublimation point, ethane remains frozen while the methane turns into a gas. However, ethane is colorless, so it cannot be the direct cause of the red color. Once the methane has sublimated, though, the solar wind can interact directly with the ethane left behind and catalyze chemical reactions that turn the ethane into a cornucopia of organic compounds — the tholins.

"Exposure to the solar wind may convert ethane into persistent reddish surface deposits, contributing to Charon's red cap," Raut said. "This is one of the most illustrative and stark examples of surface-atmospheric interactions so far observed at a planetary body."

Understanding the chemistry of objects in the distant outer solar system has relevance to life here on Earth. Life's building blocks include many of the organic compounds formed through the interaction of hydrocarbons with energy from the sun. Indeed, many of those organic compounds may have been brought to Earth by the impacts of comets hailing from the region near Pluto. By learning how Charon produces those compounds, we may catch a glimpse into the origin of the molecules that helped make life on Earth.

The research was published in two papers: one on April 15 in the journal Geophysical Research Letters, and the other on June 17 in the journal Science Advances.

Follow Keith Cooper on Twitter @21stCenturySETI. Follow us on Twitter @Spacedotcom and on Facebook.

This article was originally published at The Conversation. The publication contributed the article to Space.com's Expert Voices: Op-Ed & Insights.

Samantha Lawler, Assistant professor, Astronomy, University of Regina

"Why does it matter if Pluto is a planet or a dwarf planet? Because for me it just makes it more confusing in our solar system. I know that some things in space are planets and some are stars and some are other names like moons or comets. Dwarf planet is a more different name and I think it just makes it more confusing." — Timmy, 11, Kitchener, Ont.

"Comet," "star" and "planet" are category names that immediately tell you something important about what they describe.

Our solar system consists of the sun, planets (which orbit around the sun) and small bodies (which either orbit around the sun or planets). The "small bodies" category is divided up into even smaller categories, mostly depending on the shape and size of orbits.

In 1801, astronomers discovered Ceres, which was initially categorized as a "planet." Astronomers measured that it was much smaller than the other known planets. Soon, a lot of smaller objects were discovered on orbits very close to Ceres. These small bodies were categorized as "asteroids" and we have since discovered hundreds of thousands of these in the asteroid belt.

Pluto flyby anniversary: The most amazing photos from NASA's New Horizons

New discoveries

A similar process of discovery and re-categorization happened for small bodies further out in the solar system.

Pluto was discovered in 1930 and was called the ninth planet in our solar system for many decades. But astronomers soon learned that Pluto was pretty different from the other eight planets: it's on a tilted orbit and it’s way, way smaller than the other planets.

Over the years, astronomers discovered more and more small, planet-like objects that crossed Pluto's orbit. These are now categorized as "Kuiper Belt objects." It was looking more and more like Pluto might fit in better with the category of Kuiper Belt objects than with planets.

In 2005, a new object was discovered in the outer solar system, Eris, that is even heavier than Pluto. This led astronomers to consider if both Eris and Pluto are planets or not. Astronomers thought this was an important enough decision that the International Astronomical Union voted on it in 2006. Astronomers decided that rather than demoting Pluto to a plain old Kuiper Belt object, they would make a new category of small body called a "dwarf planet." Pluto and Eris would both be part of this new category.

How planets form

Solar systems like ours form from big clouds of dust and gas that collapse into disks around young stars, but astronomers are still learning exactly how that process works. We use telescopes to look carefully at forming solar systems far away, but they are so far that it’s really hard to see the planets forming directly.

A planetesimal — a baby planet — first forms from clumps of dust in a disk orbiting a young star. Planetesimals then grab nearby pebbles, dust and sometimes even smaller planetesimals with their gravity, which gets stronger as they get bigger. When they get to be a few hundred kilometers across, they have enough gravity to pull themselves into a round shape, which is the definition of a dwarf planet.

Measuring small bodies in our solar system, including dwarf planets, and comparing them with computer simulations is another way to see how our solar system formed. Our current theory is that there must have been a lot of dwarf planets that formed in our solar system.

Ceres, in the asteroid belt, and Pluto, Eris and about a dozen other Kuiper Belt objects are big enough to be in the dwarf planet category. This means that while they are planetesimals that grew big enough to be round, they did not develop a gravity strong enough to grab all the other planetesimals near their orbit.

Other solar systems

Astronomers have now measured more than 5,000 exoplanets, planets in other solar systems. We won't be able to measure dwarf planets there for a very long time, but the ones we've found in our own solar system can teach us about how planets form everywhere.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Follow all of the Expert Voices issues and debates — and become part of the discussion — on Facebook and Twitter. The views expressed are those of the author and do not necessarily reflect the views of the publisher.

This article was originally published at The Conversation. The publication contributed the article to Space.com's Expert Voices: Op-Ed & Insights.

Andrew Gunn, Lecturer, Monash University

What is it like to be on the surface of Mars or Venus? Or even further afield, such as on Pluto, or Saturn's moon Titan?

This curiosity has driven advances in space exploration since Sputnik 1 was launched 65 years ago. But we're only beginning to scratch the surface of what is knowable about other planetary bodies in the solar system.

Our new study, published May 19 in Nature Astronomy, shows how some unlikely candidates — namely sand dunes — can provide insight into what weather and conditions you might experience if you were standing on a far-off planetary body.

Related: Weird 'blue' dunes speckle the surface of Mars in NASA photo

What’s in a grain of sand?

English poet William Blake famously wondered what it means "to see a world in a grain of sand."

In our research, we took this quite literally. The idea was to use the mere presence of sand dunes to understand what conditions exist on a world's surface.

For dunes to even exist, there are a pair of "Goldilocks" criteria that must be satisfied. First is a supply of erodible but durable grains. There must also be winds fast enough to make those grains hop across the ground — but not fast enough to carry them high into the atmosphere.

So far, the direct measurement of winds and sediment has only been possible on Earth and Mars. However, we have observed wind-blown sediment features on multiple other bodies (and even comets) by satellite. The very presence of such dunes on these bodies implies the Goldilocks conditions are met.

Our work focused on Venus, Earth, Mars, Titan, Triton (Neptune’s largest moon) and Pluto. Unresolved debates about these bodies have gone on for decades.

How do we square the apparent wind-blown features on Triton's and Pluto's surfaces with their thin, tenuous atmospheres? Why do we see such prolific sand and dust activity on Mars, despite measuring winds that seem too weak to sustain it?

And does Venus's thick and stiflingly hot atmosphere move sand in a similar way to how air or water move on Earth?

Furthering the debate

Our study offers predictions for the winds required to move sediment on these bodies, and how easily that sediment would break apart in those winds.

We constructed these predictions by piecing together results from a host of other research papers, and testing them against all the experimental data we could get our hands on.

We then applied the theories to each of the six bodies, drawing on telescope and satellite measurements of variables including gravity, atmospheric composition, surface temperature, and the strength of sediments.

Studies before ours have looked at either the wind speed threshold required to move sand, or the strength of various sediment particles. Our work combined these together — looking at how easily particles could break apart in sand-transporting weather on these bodies.

For example, we know Titan's equator has sand dunes — but we aren't sure of what sediment encircles the equator. Is it pure organic haze raining down from the atmosphere, or is it mixed with denser ice?

As it turns out, we discovered loose aggregates of organic haze would disintegrate upon collision if they were blown by the winds at Titan's equator.

This implies Titan's dunes probably aren’t made of purely organic haze. To build a dune, sediment must be blown around in the wind for a long time (some of Earth's dune sands are a million years old).

We also found wind speeds would have to be excessively fast on Pluto to transport either methane or nitrogen ice (which is what Pluto's dune sediments were hypothesized to be). This calls into question whether "dunes" on Pluto's plain, Sputnik Planitia, are dunes at all.

They may instead be sublimation waves. These are dune-like landforms made from the sublimation of material, instead of sediment erosion (such as those seen on Mars's north polar cap).

Our results for Mars suggest more dust is generated from wind-blown sand transport on Mars than on Earth. This suggests our models of the Martian atmosphere may not be effectively capturing Mars's strong "katabatic" winds, which are cold gusts that blow downhill at night.

Potential for space exploration

This study comes at an interesting stage of space exploration.

For Mars, we have a relative abundance of observations; five space agencies are conducting active missions in orbit, or in situ. Studies such as ours help inform the objectives of these missions, and the paths taken by rovers such as Perseverance and Zhurong.

In the outer reaches of the solar system, Triton has not been observed in detail since the NASA Voyager 2 flyby in 1989. There is currently a mission proposal which, if selected, would have a probe launched in 2031 to study Triton, before annihilating itself by flying into Neptune’s atmosphere.

Missions planned to Venus and Titan in the coming decade will revolutionize our understanding of these two. NASA's Dragonfly mission, slated to leave Earth in 2027 and arrive on Titan in 2034, will land an uncrewed helicopter on the moon's dunes.

Pluto was observed during a 2015 flyby by NASA’s ongoing New Horizons mission, but there are no plans to return.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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Pluto is lucky to exist in the orbit that it does, according to new computer simulations that show how the dwarf planet skirts near a chaotic orbit that could potentially destroy the beloved world.

The former ninth planet, now considered a dwarf planet, has an unusual orbit that is highly elongated and tilted with respect to the orbits of the planets. For 20 years of its 248-year journey around the sun, Pluto actually moves inside Neptune's orbit. That they never collide is a consequence of two properties of Pluto's orbit, known as azimuthal libration and latitude libration.

Azimuthal libration describes how, whenever Pluto crosses Neptune's orbit, it is always at least 90 degrees away from Neptune. Meanwhile, latitude libration ensures that when Pluto reaches its closest point to Neptune or the other giant planets, it is always high above them and the plane of the solar system. Combined, these two factors keep Pluto out of harm's way.

Related: The most amazing photos from NASA's New Horizons

Now planetary scientists Renu Malhotra of the Lunar and Planetary Laboratory at the University of Arizona and Takashi Ito of the National Astronomical Observatory of Japan have developed a greater understanding of why Pluto survives in this particular orbit.

In computer simulations they found that Neptune, unsurprisingly, has the greatest influence on Pluto's azimuthal libration, a consequence of the two planets' 3:2 orbital resonance, which dictates that for every three times Neptune orbits the sun, Pluto orbits exactly twice. However, the simulations suggest that Neptune does not greatly influence Pluto's latitudinal libration. 

Throw the gravity of Uranus into the mix, and things take a downturn — Uranus acts to destabilize both the azimuthal and latitudinal constraints. If Pluto's orbit were a consequence of the effects of just these two planets, it would have become unstable after just tens or hundreds of millions of years, resulting in Pluto either colliding with Neptune, — or more likely — being ejected out of the solar system entirely.

It's Jupiter, and to a lesser degree Saturn, that come to Pluto's rescue. Despite being farther from Pluto than Neptune and Uranus, their gravity is so great that they can still dominate. Jupiter alone provides sufficient gravitational influence to keep Pluto's orbit stable for at least 5 billion years — the length of time that the simulations ran for.

What's striking about this new work is how narrow this zone of stability is for Pluto, and how the dwarf planet has only been able to achieve it thanks to the chance arrangement of planets in the solar system. 

The findings also have consequences for objects in similar orbits to Pluto's that also have a 3:2 resonance with Neptune. Since the orbits of various populations of small objects in the Kuiper Belt beyond Neptune retain the imprint of how they were pushed around by the migrating gas giants in the early days of the solar system, they provide a unique window into the history of the solar system. Their orbits could potentially hide evidence for the existence of lost planets that were ejected from the Solar System billions of years ago, say Malhotra and Ito.

The results were published on March 31 in Proceedings of the National Academy of Sciences.

Follow Keith Cooper on Twitter @21stCenturySETI. Follow us on Twitter @Spacedotcom and on Facebook.  

This article was originally published at The Conversation. The publication contributed the article to Space.com's Expert Voices: Op-Ed & Insights.

Vahe Peroomian, Professor of Physics and Astronomy, USC Dornsife College of Letters, Arts and Sciences

"What is a dwarf planet?" — Myranda, age 8, Knoxville, Tennessee

The word "planet" came from the ancient Greek words that mean "wandering star." That makes sense, because for thousands of years, people have watched planets change position in the night sky — unlike stars, which appear fixed and unmoving to the naked eye.

That's how the ancients discovered five of the planets: Mercury, Venus, Mars, Jupiter and Saturn. Astronomers using telescopes found Uranus in 1781, Neptune in 1846, and Pluto in 1930.

Solar system leftovers

I'm a space scientist with a passion for astronomy and the exploration of the solar system. I received my Ph.D. in physics in 1994, about the time astronomers began to find more and more objects beyond Neptune, in the Kuiper belt. That's a place in space that holds the "leftovers" of the solar system — particularly small icy bodies.

Three of those icy bodies — Eris, Haumea and Makemake — were discovered in the early to mid 2000s. They seemed large enough to be planets; all of them are roughly the same size as Pluto.

Astronomers then surmised that there were likely many more of these icy bodies in the Kuiper belt. They began to wonder: How many planets might we end up identifying in our solar system? Twenty? Thirty? A hundred? More?

Dwarf planet defined

In 2006, and after much debate, the International Astronomical Union came up with a new definition for a planet. And for the first time, the term "dwarf planet" was used.

Here's what the IAU said: A planet has to orbit the sun directly. It also must be large enough to have a round, or spherical, physical shape.

And the planet must "clear its neighborhood." That means, aside from any moons it might have, the planet can't share its orbit with other objects of comparable size.

An object that satisfies only the first two criteria — but not the last — is now called a dwarf planet.

Pluto is demoted

That's why Pluto lost its status as a planet and is now classified as a dwarf planet. It failed the final item on the checklist — other icy Kuiper belt bodies are within its orbital path. The decision, a controversial one to be sure, is debated by scientists to this very day.

At the same time Pluto got demoted, another solar system object was promoted. Ceres, once considered an asteroid, is now classified as a dwarf planet. It's nowhere near the Kuiper belt; instead, Ceres is in the main asteroid belt, orbiting between Mars and Jupiter.

Add them up — Pluto, Ceres, Eris, Haumea and Makemake — and that brings the number of dwarf planets in our solar system to five. But that list is sure to grow. Already, hundreds of candidates, nearly all in the Kuiper belt, potentially satisfy the criteria to be a dwarf planet.

About the dwarf planets

Dwarf planets are nothing like Earth.

As their name implies, they are much smaller. Pluto and Eris, the largest of the dwarfs, have less than one-fifth the diameter of the Earth.

They have less mass, too. For example, Earth has about 6,400 times more mass than Ceres. That’s like comparing two killer whales to a guinea pig.

And dwarf planets are cold. Pluto's average temperature is around minus 400 degrees Fahrenheit (minus 240 Celsius).

Could life exist on a dwarf planet?

Three things are needed for life: liquid water, an energy source and organic molecules — that is, molecules containing carbon.

More than 100 miles (161 kilometers) below Pluto's surface, an enormous ocean of liquid water may exist; this might also be true for other Kuiper belt worlds. Ceres also has subsurface water, remnants of what might have been an ancient global ocean.

Organic molecules, in abundance everywhere in our solar system, have been found on Ceres and Pluto.

But the one missing ingredient for all the dwarf planets is a source of energy.

Sunlight won't work, particularly for the Kuiper belt dwarfs; they are simply too far away from the sun. To reach the belt, the light must travel more than 2.7 billion miles (4.4 billion km). By the time the sunshine arrives at these distant worlds, it's too weak to significantly heat any of them.

And all the dwarf planets are too small to hold the inner heat that remains from the solar system's formation.

Yet scientists have discovered life on Earth in the most hostile places imaginable – near the bottom of the ocean, miles deep in the soil and even inside an active volcano. When it comes to life in our solar system, never say never.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Follow all of the Expert Voices issues and debates — and become part of the discussion — on Facebook and Twitter. The views expressed are those of the author and do not necessarily reflect the views of the publisher.

An area of Pluto that researchers think was formed from the eruption of ice volcanoes is unique on the dwarf planet and in the solar system, a new study suggests.

NASA's New Horizons mission, which launched in 2006, took detailed photos of the surface of Pluto, a dwarf planet and the largest object in the Kuiper Belt. Now, a new analysis examines images of an area containing two main mounds that scientists have proposed are ice volcanoes. In the study, the researchers conclude that the surface around these mounds was likely formed by fairly recent activity of the ice volcanoes, or cryovolcanoes.

The finding raises the possibility that these volcanoes may still be active and that liquid water, or something like it, flows or recently flowed under the surface of Pluto. Recent activity also means that there is likely more heat in Pluto's interior than scientists previously thought. Given other recent research, the scientists say their work could even raise the possibility of life existing under Pluto's surface.

Related: The most amazing photos from NASA's New Horizons 

The researchers analyzed photographs of a region dominated by two large mounds, called Wright Mons and Piccard Mons, which scientists think are cryovolcanoes. Wright Mons is a mount 2.5 to 3 miles (4 to 5 kilometers) high and about 90 miles (150 km) wide, while Piccard Mons is about 4 miles (7 km) high and 150 miles (250 km) wide. 

The suspected ice volcanoes also have extremely deep depressions at their peaks — the one on Wright Mons is about as deep as the mount is high. Many parts of the area also have an unusual, lumpy or "hummocky" appearance, made up of undulating, rounded mounds. The researchers think smaller mounds, formed from ice volcanoes, could have accumulated over time to form these two main mounds.

"There was no other areas on Pluto that look like this region," Kelsi Singer, a planetary scientist at the Southwest Research Institute in Boulder, Colorado and the lead author of the study, told Space.com. "And it's totally unique in the solar system."

Unlike other areas of Pluto, this area has few to no impact craters, indicating that the surface was formed relatively recently in geological time. Based on the lack of craters, the area is likely no older than one or two billion years old, with some areas likely being less than 200 million years old, Singer said.

In some ways, cryovolcanoes are analogous to volcanoes on Earth, since much of Pluto's surface is made of ice, and temperatures on Pluto are far below water's freezing point. That means that liquid water, or something like it which is at least partially fluid or mobile, would be like magma on Earth, coming up to the surface after an eruption and freezing, or hardening, into a solid.

"It's probably not coming up completely liquid — it's probably more like a slushy thing where you have some liquid and some ice, or it could even be more like a flowing solid," Singer said, which could be "more like ketchup or silly putty." It could even be more solid ice that can still flow.

"We all know that ice can flow because we have glaciers that flow on Earth," she said.

Though scientists don't totally understand how cryovolcanic activity on Pluto might work, it is likely powered by radiogenic heat created by the decay of radioactive elements in the dwarf planet's interior. A similar phenomenon is also one of the sources of heat in the Earth's interior, although Pluto does not have plate tectonics, the complex system of shifting continental crust that underlies geologic activity on Earth. Scientists call geologic activity like that on Pluto "general tectonics," which can still create features like faults in rock but does not have tectonic plates.

Pluto's cryovolcanoes display some similarities to shield volcanoes on Earth, which are low-profile volcanoes which form from the steady accumulation of lava flows into rounded structures. (Think of the Hawaiian island volcanoes, rather than an eruption like Mount St. Helens or Vesuvius.) But shield volcanoes usually form from very liquid lava, unlike what scientists think happened on Pluto. 

Some volcanoes on Earth and other planets also have a depression in their middle called a caldera, formed when a newly erupted volcano collapses into the void left by all the material it spewed out. But the depression on Wright Mons is so deep that the volcano would have had to lose about half of its volume to be similar in shape to Mauna Loa, a shield volcano in Hawaii that is one of the largest volcanoes on Earth and has a comparatively small caldera, though the two structures are similar in volume, Singer said.

There's still a lot researchers don't know about these features, how they were formed, and how cryovolcanism works on Pluto. The idea that liquid water could exist beneath the surface of Pluto raises the chances of life existing on Pluto from practically non-existent to slightly more plausible, given other research suggesting that Pluto was hot when it first formed and could still have a liquid ocean under its icy surface.

"I think that it is a little more promising, and that there might be some heat and liquid, potentially liquid water closer to the surface," Singer said. "But there's still some big challenges for poor microbes that want to live on Pluto."

The research is described in a paper published Tuesday (March 29) in the journal Nature Communications.

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