Beyond the gas giant Neptune lies a region of space filled with icy bodies. Known as the Kuiper Belt, this chilly expanse holds trillions of objects — remnants of the early solar system.
In 1943, astronomer Kenneth Edgeworth suggested comets and larger bodies might exist beyond Neptune. And in 1951, astronomer Gerard Kuiper predicted the existence of a belt of icy objects at the far edge of the solar system. Today, the rings predicted by the pair are known as the Kuiper Belt or the Edgeworth-Kuiper Belt.
Despite its massive size, the Kuiper Belt wasn't discovered until 1992 by astronomers Dave Jewitt and Jane Luu. According to NASA, the pair had been "doggedly scanning the heavens in search of dim objects beyond Neptune" since 1987. They dubbed the first object they spotted "Smiley" but it was later cataloged as "1992 QB1."
Since then, astronomers have discovered several intriguing Kuiper Belt objects and potential planets within the region. NASA's New Horizons mission continues to uncover previously hidden planets and objects, helping scientists learn more about this unique solar system relic.
Kuiper Belt formation
When the solar system formed, much of the gas, dust and rocks pulled together to form the sun and planets. The planets then swept most of the remaining debris into the sun or out of the solar system. But objects at the edge of the solar system were far enough away to avoid the gravitational tugs of the much larger planets like Jupiter, and so managed to stay in their place as they slowly orbited the sun. The Kuiper Belt and its compatriot, the more distant and spherical Oort Cloud, contain the leftover remnants from the beginning of the solar system and can provide valuable insights into its birth.
According to the Nice model — one of the proposed models of solar system formation— the Kuiper Belt may have formed closer to the sun, near where Neptune now orbits. In this model, the planets engaged in an elaborate dance, with Neptune and Uranus changing places and moving outward, away from the sun. As the planets moved farther away from the sun, their gravity may have carried many of the Kuiper Belt objects with them, shepherding the tiny objects ahead as the ice giants migrated. As a result, many of the Kuiper Belt objects were moved from the region they were created into the colder part of the solar system.
The most crowded section of the Kuiper Belt is between 42 and 48 times Earth's distance from the sun. The orbit of objects in this region remain stable for the most part, although some objects occasionally have their course changed slightly when they drift too close to Neptune.
Scientists estimate that thousands of bodies more than 100 km (62 miles) in diameter travel around the sun within this belt, along with trillions of smaller objects, many of which are short-period comets. The region also contains several dwarf planets — round worlds too large to be considered asteroids but too small to qualify as a planet.
Kuiper Belt objects
Pluto was the first true Kuiper Belt object (KBO) to be seen, although scientists at the time didn't recognize it as such until other KBOs were discovered. Once Jewitt and Luu discovered the Kuiper Belt, astronomers soon saw that the region beyond Neptune was full of icy rocks and tiny worlds.
Sedna, a KBO that's about three-fourths the size of Pluto, was discovered in 2004. It is so far out from the sun it takes about 10,500 years to make a single orbit. Sedna is about 1,100 miles (1,770 km) wide and circles the sun in an eccentric orbit that ranges between 8 billion miles (12.9 billion km) and 84 billion miles (135 billion km).
"The sun appears so small from that distance that you could completely block it out with the head of a pin," Mike Brown, an astronomer at the California Institute of Technology who discovered this and several other Kuiper Belt objects, said in a statement.
In July 2005, astronomers discovered Eris, a KBO that's slightly smaller than Pluto. Eris orbits the sun approximately once every 580 years, traveling almost 100 times farther from the sun than Earth does. Its discovery revealed to some astronomers the problem of categorizing Pluto as a full-scale planet. According to the International Astronomical Union (IAU)’s 2006 definition, a planet must be large enough to clear its neighborhood of debris. Pluto and Eris, surrounded by the Kuiper Belt, had clearly failed to do so. As a result, in 2006, Pluto, Eris, and the largest asteroid, Ceres, were reclassified by the IAU as dwarf planets. Two more dwarf planets, Haumea and Makemake, were discovered in the Kuiper Belt in 2008.
Astronomers are now reconsidering Haumea's status as a dwarf planet. In 2017, when the object passed between Earth and a bright star, scientists realized it is more elongated than round. Roundness is one of the criteria of a dwarf planet, according to the IAU’s definition. Haumea's elongated shape could be a result of it's rapid spin; a day on the object only lasts about four hours.
"I don't know if this will change the definition [of a dwarf planet]," Santos Sanz, an astronomer at the Instituto de Astrofísica de Andalucía in Granada, Spain, told Space.com. "I think probably yes, but probably it will take time."
Planet Nine is a hypothetical world thought to orbit the sun at a distance that is about 600 times farther from the sun than Earth's orbit, and about 20 times farther out than the orbit of Neptune. (The orbit of Neptune is 2.7 billion miles from the sun at its closest point.)
Scientists have not actually seen Planet Nine. Its existence was inferred by gravitational effects observed on other objects in the Kuiper Belt. Scientists Mike Brown and Konstantin Batygin at the California Institute of Technology in Pasadena described the evidence for Planet Nine in a study published in the Astronomical Journal in 2016.
If there is another world out there, astronomers Scott Sheppard, of the Carnegie Institution for Science in Washington, D.C., and Chadwick Trujillo, of Northern Arizona University, are likely to find it soon. The pair have spent the last six years working on the deepest survey of faint objects at the edge of the solar system, after proposing the existence of Planet X, a small dwarf planet beyond Pluto, in 2014.
So far, Sheppard and Trujillo have found 62 distant objects, which make up about 80 percent of all of those at the edge of the system. Last year, the two discovered dwarf planet 2015 TG387, nicknamed "the Goblin," and the most distant KBO ever reported, 2018 VG18, nicknamed "FarOut." In February 2019, Sheppard unofficially announced the discovery of an even more distant object, informally known as "FarFarOut."
"These distant objects are like breadcrumbs leading us to Planet X," Sheppard said in a statement. "The more of them we can find, the better we can understand the outer solar system and the possible planet that we think is shaping their orbits — a discovery that would redefine our knowledge of the solar system's evolution."
A visit from New Horizons
Because of their small size and distant location, Kuiper Belt objects are a challenge to spot from Earth. Infrared measurements from NASA's space-based Spitzer telescope have helped to nail down sizes for the largest objects.
In order to catch a better glimpse of these remote leftovers from the birth of the solar system, NASA launched the New Horizons mission. The spacecraft reached Pluto in 2015 and continued on with an aim to examine multiple KBOs. On Jan. 1, 2019 New Horizons flew by the Kuiper Belt object called 2014 MU69.
The first images taken of MU69 suggested a snowman-like configuration, with two round balls stuck together. These images seemed to confirm the idea of pebble accretion — a theory of planetary formation that suggests small rocky and icy bodies in the solar system are slowly pulled together by gravity.
However, images released a month after the flyby suggested that the pair were flatter than originally thought, more like two hamburger patties than snowballs. Their formation remains a mystery.
"The new images are creating scientific puzzles about how such an object could even be formed," Alan Stern, the New Horizons principal investigator, said in a statement. "We've never seen something like this orbiting the sun."
MU69 may not be the last object New Horizons visits. The team has already said that the spacecraft has enough fuel to fly by another KBO. NASA would need to approve an extended mission but visiting another object would help scientists gain a broader understanding of the Kuiper Belt.
"We've got fuel, and the spacecraft's in great health," Stern said in a webcast in December 2018. "So, I think there's a bright future ahead."