Are we alone?
The desire to know our place in the universe is a core human yearning, and people have pondered the question for thousands of years. Indeed, hundreds of years before the birth of Christ, Greek philosophers such as Anaximander and Epicurus speculated that the cosmos is teeming with planets, many of which may support life.
We couldn't move beyond mere speculation for a very long time, however — until we invented telescopes and developed a proper understanding of the scientific method. Humanity notched both of those milestones centuries ago, and scientists are now going great guns in the search for alien life.
Well, "going great guns" is a bit of an overstatement. But we've made considerable progress, especially in the last decade or so, and some big discoveries may be coming soon. Let's take a brief look at where the hunt for ET has been and where it's headed.
Related: 6 most likely places for alien life in the solar system
The "cosmic pluralism" theory espoused by Anaximander, Epicurus and their followers didn't take off, unfortunately; it was quashed by the ideas of heavyweights like Plato and Aristotle, who held that Earth is unique.
This latter view aligned nicely with the Christian doctrine that came to dominate Europe throughout the Middle Ages and beyond. So, for a very long time, it was considered strange and was often downright dangerous to hypothesize the existence of life-supporting worlds beyond our own.
The pendulum began swinging back the other way in the 16th century with the dawn of the Copernican Revolution.
"Once it was realized that all the planets go around the sun, it was not hard to imagine that the other planets could be like Earth," then-NASA Chief Historian Steven Dick told writer Michael Schirber back in 2009.
That momentum built as intellectual giants like Johannes Kepler, Galileo Galilei and Isaac Newton continued fleshing out how our solar system works, efforts that reached a near fever pitch during the Enlightenment.
In the 17th century, for example, Czech astronomer Anton Schyrleus considered what creatures on Jupiter might look like. And in the waning years of the 18th century, William Herschel, who discovered both Uranus and the existence of infrared light, postulated that life was widespread throughout the solar system — including on the surface of the sun. (Herschel thought the sun was a giant planet.)
Around this same time, scientists began thinking about how to communicate with our putative cosmic neighbors. One of the pioneers in this nascent field of messaging extraterrestrial intelligence (METI) was the famed German mathematician Carl Friedrich Gauss. In the early 1800s, Gauss proposed carving huge geometric shapes into the Siberian forest to show "lunarians" living on the moon that we're here and know how to do some math.
At the same time, Austrian astronomer Joseph Johann von Littrow suggested digging giant trenches in the Sahara Desert, filling the ditches with water and topping that liquid layer with kerosene. The kerosene would then be lit, producing a fiery signal that would hopefully catch the eye of any aliens keeping tabs on Earth.
These particular ideas were never carried out. But a century later, scientists did start putting some of this alien-hunting talk into action.
Related: 13 ways to search for intelligent aliens
Hello? Is anyone out there?
One of the first actual life-searching projects took place in August 1924, when a team of scientists led by astronomer David Peck Todd used an airship to loft a radio receiver several miles above the ground — a good spot, it was thought, to listen for messages from creatures on Mars, which was making a particularly close approach to Earth at the time.
But the concerted search for extraterrestrial intelligence (SETI) didn't really kick off until 1960. In that year, Cornell University astronomer Frank Drake used a radio telescope in West Virginia to listen for signals coming from the stars Tau Ceti and Epsilon Eridani. Called Project Ozma, this effort incorporated ideas from a seminal 1959 paper by Giuseppe Cocconi and Philip Morrison.
Scientists have been scanning the heavens for "technosignatures" ever since then. Initially, they focused almost exclusively on radio signals, but flashes of light are in play now as well; these are the targets of increasingly common "optical SETI" efforts.
SETI scientists have to keep an open mind; after all, we don't know what sorts of messages an advanced alien civilization might beam out. So, astronomers in the field generally look for signals that appear weird and artificial, something coming from deep space that isn't produced by any known natural astrophysical phenomenon.
It would also be nice if the signal recurs, so it can be studied repeatedly and in detail. One-offs can remain forever and frustratingly mysterious, as 1977's famous "Wow!" signal shows. In that case, a radio dish operated by The Ohio State University picked up something so intriguing that astronomer Jerry Ehman wrote "Wow!" on the data printout. Researchers scoured that same patch of sky again and again, hoping to get another ping, but they never did.
The SETI hunt, it should be noted, has historically been a shoestring operation; finding enough money to keep the telescopes running has been a consistent problem. The U.S. Congress axed a planned NASA SETI project in 1993, and ever since then, ET hunters have mostly had to turn to the private sector for cash.
Without steady funding, progress was slow for several years. But private money has flowed more freely into the SETI field recently. Most of it comes from one man: tech billionaire Yuri Milner. Passionate about the search for alien life, Milner established an ambitious program in 2015 called Breakthrough Initiatives to seek out extraterrestrials.
Among the projects under the Breakthrough umbrella are the $100 million Breakthrough Listen SETI campaign and the $100 million Breakthrough Starshot, which aims to develop the technology required to send tiny robotic probes to nearby exoplanet systems at about 20% the speed of light.
There's also Breakthrough Message, which aims to both help humanity craft the best possible message to send out into the cosmos and encourage debate and conversation about SETI in general.
And there is considerable debate within the scientific community about SETI.
Some people, including the late physicist Stephen Hawking, have argued that it's unwise to advertise our presence to aliens, whose nature and intent are complete mysteries to us; these creatures may pillage our planet after picking up our ping, after all. But other researchers think any creatures advanced enough to travel to Earth to enslave or eat us would already know we're here anyway.
Breakthrough Message pledges not to actually broadcast any SETI signals until this debate has played itself out. But humanity has already beamed out messages on multiple occasions, most famously in 1974 with the Arecibo message. And those are just the intentional, directed missives; we're leaking radio signals in all directions at all times, providing cosmic bread crumbs for anyone close enough to find them.
Related: Would aliens really kill us, 'Independence Day'-style?
Looking for life on Mars
Around the same time that SETI was getting off the ground, planetary scientists began getting their first good looks at alien worlds.
In 1964, Mariner 4 flew by Mars, returning the first up-close images of the Red Planet. Those photos revealed a dry, heavily cratered and seemingly desolate world, forcing many scientists to recalibrate previously optimistic notions of Mars' habitability. (Hopes of a life-supporting Mars had been famously stoked around the turn of the 19th century by astronomer Percival Lowell, who claimed that channels on the planet were actually canals built by intelligent creatures.)
But the optimists got some good news in 1969, after Mariner 9 arrived in orbit around Mars, becoming the first spacecraft to circle another planet in the process. This probe spotted river channels and other evidence of past liquid-water activity on the Martian surface. These discoveries helped spur NASA to develop two ambitious life-hunting Mars missions, Viking 1 and 2, which launched a few weeks apart in 1975.
The identical Viking landers each carried four biology experiments, which hunted for signs of microbial life in the red dirt. One of those experiments, called Labeled Release (LR), returned data consistent with evidence of microbial life. Indeed, LR principal investigator Gil Levin argued (and continues to argue today) that the Vikings found evidence of Mars life. However, most scientists who studied the data disagreed with Levin, determining that the data could be explained by abiotic (non-life-based) chemical reactions.
The Viking results taught NASA and astrobiologists some valuable lessons — chiefly, that they didn't know enough about Mars to mount a proper life hunt there. So, the space agency eventually embarked on a long-term "follow the water" exploration strategy, seeking to learn more about ancient environmental conditions on the Red Planet and how they changed over time.
Related: Water on Mars: Exploration and evidence
This strategy gave us many prominent Mars missions over the past few decades, including the orbiters Mars Odyssey, Mars Reconnaissance Orbiter, and Mars Atmosphere and Volatile Evolution (MAVEN); the rovers Spirit, Opportunity and Curiosity; and the Phoenix lander.
These robotic explorers did their jobs well, finding lots of evidence that ancient Mars was quite wet and helping scientists better understand why, how and when the Red Planet transitioned to the frigid desert world it is today. Curiosity has taken this work the furthest, finding that its landing site, the 96-mile-wide (154 kilometers) Gale Crater, hosted a long-lived lake-and-stream system billions of years ago that could have supported Earth-like life.
Related: Long window for life on Mars: Hundreds of millions of years?
Meanwhile, some scientists continued the hunt for Mars life, focusing on aliens that may have fallen fortuitously to Earth. Over the eons, billions of Red Planet rocks have made their way here, after being blasted into space by powerful asteroid or comet impacts. A lot of Earth material has ended up on Mars as well, but the ledger is decidedly unbalanced; the sun's powerful gravity pulls more stuff inward, toward Earth. (This extensive rock-swapping, by the way, has led some scientists to postulate that life actually arose first on Mars, then made its way to Earth later.)
In 1996, researchers announced that they'd found potential signs of life in one such Mars meteorite, known as Allan Hills 84001 (ALH84001). It was a very big deal; the result was published in the prestigious journal Science, and President Bill Clinton held a press conference about the news on the White House lawn.
The ALH84001 story ended up going down a Viking path. Other scientists picked at the claim, and a consensus emerged that the meteorite evidence was ambiguous at best. But, like Levin, the ALH84001 team held firm in its findings, and continues to do so today.
The ocean moons
NASA and the broader exploration community weren't focused solely on Mars for all these years, of course. The Cassini-Huygens mission, which ended in September 2017, transformed scientists' understanding of the Saturn system and our solar system's potential to host alien life. That mission found that Titan, Saturn's largest moon, has a hydrocarbon-based weather system and that the frigid moon's surface harbors lakes and seas of liquid ethane and methane. Life could swim around in these seas, though it would have to be very different than the life we know here on Earth.
And the Cassini orbiter spotted geysers blasting from the south pole of another Saturn moon, the ice-covered Enceladus. This discovery, and other Cassini observations, revealed that Enceladus harbors a big ocean of salty liquid water beneath its shell.
The geysers produce a huge plume of water ice and other material, a cloud so substantial that it creates Saturn's E ring. Cassini flew through this plume on multiple occasions, gathering samples that scientists analyzed for clues about the moon's subsurface environment. The researchers found carbon-containing organic compounds and free hydrogen, the latter of which suggests the existence of a hydrothermal system in Enceladus' buried ocean. Undersea hydrothermal vents are one popularly invoked environment for the origin of life on Earth. (Cassini didn't look for signs of life in this plume material; the spacecraft wasn't equipped to do so, because nobody knew about the plume before that mission launched.)
Buried oceans are relatively common in the outer solar system, scientists have come to realize. Multiple ice-covered Jupiter moons seem to have these oceans — Ganymede, Callisto and, most intriguingly, Europa. Europa's huge subsurface sea seems to be in contact with the moon's rocky core, like the ocean of Enceladus is, making possible a range of complex chemical reactions that could theoretically have led to life. (Scientists think the oceans of Ganymede and Callisto are more boring, sandwiched between layers of ice.)
Titan seems to have a buried ocean of salty water as well, meaning the moon likely has two very different potentially habitable environments. Observations by NASA's New Horizons spacecraft indicate that liquid water may slosh beneath Pluto's surface, too.
And the list goes on. Indeed, the abundance of water worlds in the outer solar system suggests that looking for "Earth 2.0" may not be the best life-hunting strategy; most of the habitable real estate in the cosmos may be buried under ice.
Related: 7 theories on the origin of life
Life on exoplanets?
These revelations about our celestial backyard have come in parallel with big news about the cosmos at large. Over the past decade or so, we have learned that our Milky Way galaxy is teeming with potentially life-supporting worlds, as Anaximander and Epicurus surmised so many centuries ago.
Much of this knowledge comes courtesy of NASA's pioneering Kepler space telescope, which operated from 2009 through November 2018. Kepler is responsible for nearly two-thirds of the 4,400 confirmed exoplanet discoveries to date, and mission data reveal that planets outnumber stars in our galaxy.
Many of those planets might bear more than a passing resemblance to Earth. Kepler found that at least 20% of Milky Way stars probably host rocky planets in their habitable zones, the just-right range of orbital distances where liquid water could persist on a world's surface.
Some of these potentially habitable worlds are just a stone's throw away in the cosmic scheme of things. For example, the nearest star to the sun — Proxima Centauri, which is about 4.2 light-years away from us — hosts a roughly Earth-size planet in the habitable zone. (This world, called Proxima b, is a prime Breakthrough Starshot target.) And the TRAPPIST-1 system, which lies 39 light-years from us, boasts seven rocky worlds, three of which may be capable of supporting life as we know it.
However, both Proxima Centauri and TRAPPIST-1 are red dwarfs, like 70% of the Milky Way's stellar population. Red dwarfs are small but very active stars, and their intense flaring may severely dampen their planets' habitability.
Kepler's legacy is being carried on by other exoplanet missions, such as NASA's Transiting Exoplanet Survey Satellite (TESS), which is expected to find thousands more alien worlds circling nearby stars, and ESA's CHEOPS probe, which aims to characterize some of these neighboring worlds.
Related: NASA's TESS exoplanet-hunting mission in pictures
Mars rover Perseverance, James Webb and more
The avalanche of exoplanet discoveries, as well as finds much closer to home, have brought astrobiology from the scientific fringe firmly into the mainstream. NASA is openly prioritizing the search for alien life these days, as some current and coming missions show.
In July 2020, for example, the agency launched the Perseverance rover, which landed in February 2021 to hunt for signs of ancient Mars life and collect samples for eventual return to Earth. Finding evidence of long-dead microbes is expected to be a very tricky task, one ideally carried out by teams of scientists in well-equipped labs studying pristine pieces of Mars specifically selected for their life-preserving potential. (The European Space Agency had planned to launch its own life-hunting Mars rover, called Rosalind Franklin, in July 2020 as well, but technical issues pushed the launch back to the next window, the fall of 2022.)
In 2024, NASA's Europa Clipper mission is scheduled to launch toward the Jupiter system. Clipper will orbit the gas giant but make dozens of flybys of Europa, characterizing the moon's subsurface ocean and scouting out good touchdown sites for a future life-hunting lander, among other tasks.
And in 2027, NASA plans to launch Dragonfly, a probe that will fly through Titan's thick, smoggy skies. Dragonfly's main goals involve investigating the complex chemistry that could set the stage for life's emergence and assessing Titan's habitability, but the rotorcraft will also search for biosignatures.
The agency will also soon start hunting for aliens much farther afield. NASA's $9.7 billion James Webb Space Telescope, the oft-delayed successor to the iconic Hubble Space Telescope, is scheduled to launch in late 2021.
One of the many things the powerful new telescope will do once aloft is probe the atmospheres of nearby exoplanets for potential biosignatures — gases such as oxygen and methane, whose simultaneous presence in a world's air would provide a strong case for life.
Three highly anticipated megascopes will begin doing similar work from the ground in the mid- to late 2020s, if all goes according to plan. The Giant Magellan Telescope and the Extremely Large Telescope will do their observing from the mountains of Chile, whereas the Thirty Meter Telescope will sit atop Hawaii's Maunakea volcano, if the telescope team and the local community can come to an agreement.
Related: Controversy over giant telescope roils astronomy conference in Hawaii
SETI activities may ramp up considerably soon, too, and not just because of Breakthrough Listen. The biggest radio telescope ever built, China's Five-hundred-meter Aperture Spherical radio Telescope (FAST), came fully online in early 2020, and searching for technosignatures is one of its many charges.
This is just a partial list of the coming life-hunting activities, of course. And the full list may eventually become gloriously ungainly, thanks to the continuing drop in the cost of building and launching spacecraft. This trend could eventually make astrobiology missions feasible for a variety of interested parties, from university groups to private citizens. Indeed, Milner has already mused about launching a life-hunting mission to Enceladus or Europa.
Some of this alien searching will continue to occur in Earth-based studies, and it won't just involve inspection of Mars meteorites. There's an ongoing search for a "shadow biosphere" on our planet — an entire tree of life separate from the one that includes bacteria, bats, birds and everything else we currently recognize as alive.
This pursuit isn't so crazy if you think about it. After all, life appeared on Earth about 4 billion years ago — very quickly, considering that our planet formed just 4.5 billion years ago and remained hot and inhospitable for a long time thereafter. So, life's emergence doesn't seem miraculous, which, in turn, implies that it could have happened here more than once.
Photos: Europa, mysterious icy moon of Jupiter
The Fermi Paradox
Given the incredible abundance of potentially habitable real estate — and that's just for Earth-like life, to say nothing of the environments that could support "strange life" of various types — why haven't we found ET yet?
Nobel Prize-winning physicist Enrico Fermi famously posed this question in 1950, specifically referring to intelligent aliens. Seven decades later, the answer to the Fermi paradox remains elusive.
"Answers" is probably a better formulation, however, because multiple factors are probably working together to keep us from finding intelligent aliens. Among the foremost is the vastness of space, which makes it difficult for two civilizations to touch base. Consider: Proxima b is just 4.2 light-years away, in a galaxy 100,000 light-years wide. But 4.2 light-years is about 26 trillion miles (42 trillion km), an expanse that would take humanity's current spacecraft tens of thousands of years to cross.
Contact with intelligent aliens would require temporal and temperamental alignments as well; their civilization would have to rise in sync with ours, no mean feat in a universe that's 13.82 billion years old. And ET would have to want to reach out. That's no given, either; there are many reasons some aliens may want to keep quiet, as the METI pessimists have pointed out.
Or maybe intelligence is rare throughout the cosmos, even if life isn't. Earth has been inhabited for about 4 billion years, after all, but we've been sending out radio waves for just a century or so and launching spacecraft only since 1957. And it's tough to find faraway microbes, which presumably have not yet invented the radio.
Our technological youth may be the biggest factor of all: We've just begun the search for our cosmic neighbors, after all. And that search has mostly been halting and haphazard, conducted by small teams of dedicated researchers who've had to scrounge money to keep the lights on.
But that's changing, as the exciting new missions and instruments currently in development show. So we may start getting some answers very soon.