What is the Kuiper Belt?
Quick answer
The Kuiper Belt is a huge, doughnut-shaped region of icy objects beyond the orbit of Neptune, about 30 to 50 times Earth’s distance from the Sun. It contains Pluto and other dwarf planets, along with countless smaller bodies left over from the formation of the solar system. Many short-period comets come from here.
Like the asteroid belt, but bigger
| Asteroid belt | Kuiper Belt | |
|---|---|---|
| Location | Between Mars and Jupiter | Beyond Neptune |
| Distance from Sun | About 2–3.5 AU | About 30–50 AU |
| Made of | Mostly rock and metal | Mostly ices (water, methane, ammonia) and rock |
| Size | Smaller | About 20 times wider, and perhaps up to 200 times more massive |
An AU (astronomical unit) is the average Earth–Sun distance, about 150 million km.
Famous Kuiper Belt objects
| Object | Notes |
|---|---|
| Pluto | Reclassified as a dwarf planet in 2006 |
| Haumea | Spins so fast it’s stretched into an egg shape; has rings |
| Makemake | One of the brightest after Pluto |
| Arrokoth | A “snowman” of two joined lobes; the most distant object ever visited |
Eris, a dwarf planet about as large as Pluto, belongs to the related scattered disc farther out.
Why Pluto was reclassified
After 1992, astronomers began finding many Pluto-like objects. In 2005, the discovery of Eris forced a decision. In 2006, the International Astronomical Union said a planet must clear its orbit of other objects, which Pluto hasn’t done, so Pluto became a dwarf planet.
Visiting the Kuiper Belt
NASA’s New Horizons flew past Pluto in July 2015, revealing a heart-shaped plain of nitrogen ice and mountains of water ice. On January 1, 2019, it flew past Arrokoth, about 6.6 billion km from Earth.
Who was Kuiper?
The belt is named after Dutch-American astronomer Gerard Kuiper, who discussed such objects in 1951, though he thought they might no longer exist. Irish astronomer Kenneth Edgeworth made similar suggestions earlier, so it’s sometimes called the Edgeworth–Kuiper Belt.
Even farther out
Beyond the Kuiper Belt lies the hypothetical Oort Cloud, a vast sphere of icy bodies thought to be the source of long-period comets. Neptune’s moon Triton may be a captured Kuiper Belt object; see the planet with the Great Dark Spot.
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