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The Kuiper Belt
A doughnut of icy worlds beyond Neptune, home to Pluto, Haumea and Makemake
Key facts
- Where it is
- Beyond Neptune, from about 30 AU out to roughly 50 to 55 AU from the Sun
- Shape
- A thick doughnut (a torus) rather than a thin flat ring
- Biggest members
- Pluto (about 1,480 miles / 2,377 km), Makemake (about 1,430 km) and Haumea (about 1,600 km long)
- How many
- Probably hundreds of thousands of bodies wider than 62.1 miles / 100 km, and a trillion or more comets
- Named after
- Gerard Kuiper, who wrote about a belt like this in 1951
- First small member found
- 1992 QB1, now called Albion, by David Jewitt and Jane Luu in 1992
- Visited by
- New Horizons, which flew past Pluto on 14 July 2015 and Arrokoth on 1 January 2019
- Temperature
- About minus 220 degrees Celsius, cold enough for ices of nitrogen and methane
- What it gives us
- Most short-period comets, which return to the inner solar system every few years or decades
About The Kuiper Belt
Far past Neptune, where it is very cold and dark, there is a huge ring of icy worlds called the Kuiper belt. Pluto lives there, and so do Haumea and Makemake. Most of the worlds are small and frozen. Some comets come from here and swing in close to the Sun.
The Kuiper belt is a doughnut-shaped ring of icy bodies beyond the orbit of Neptune, from about 30 to 55 times as far from the Sun as Earth. Pluto, Haumea and Makemake are its biggest known members. It is named after the astronomer Gerard Kuiper, but the first small Kuiper belt object was not found until 1992. Many short-period comets, the ones that return every few years or decades, start their journeys here. In 2019 NASA's New Horizons flew past a small Kuiper belt object called Arrokoth, the most distant world ever visited.
The Kuiper belt is a flattened disc of icy planetesimals extending from Neptune's orbit at about 30 AU out to roughly 50 to 55 AU, left over from the formation of the solar system. Gerard Kuiper discussed such a belt in 1951; David Jewitt and Jane Luu found the first object beyond Pluto, 1992 QB1, in 1992. Neptune's gravity shapes the belt, and Pluto sits in a 3:2 resonance with it. The belt is believed to contain hundreds of thousands of bodies wider than 100 km and perhaps a trillion comets, and it supplies most Jupiter-family comets.
Eris, farther out on a tilted orbit, belongs to the scattered disc rather than the belt itself. New Horizons passed Pluto in July 2015 and Arrokoth on 1 January 2019, and Neptune's moon Triton is probably a captured Kuiper belt object.
Did you know?
- Arrokoth, the most distant world ever visited, looks like a flattened snowman only about 36 km long.
- Neptune's big moon Triton probably started life as a Kuiper belt object before Neptune captured it.
- The Kuiper belt is about 20 times wider than the asteroid belt and could hold hundreds of times more material.
Questions children ask
Is Pluto in the Kuiper belt?
Yes. Pluto was the first Kuiper belt object ever found, in 1930, though nobody knew about the belt then. Haumea and Makemake are in it too.
Is Eris in the Kuiper belt?
Not quite. Eris is on a tilted orbit that swings out to nearly 100 AU, so astronomers put it in the scattered disc, a region just beyond the belt.
What is the Kuiper belt made of?
Frozen things: water ice, and ices of methane, ammonia and nitrogen, mixed with rock and dark organic material.
Has anything visited it?
Only New Horizons so far. It flew past Pluto in 2015 and the small object Arrokoth in 2019, and is still travelling outward through the belt.
Quick quiz
Three questions for space cadets. Nothing is recorded.
- Where is the Kuiper belt?
- Which dwarf planet lives in the Kuiper belt?
- Which spacecraft has visited the Kuiper belt?
Pluto
Neptune
Eris