The Kuiper Belt

The Kuiper Belt is a huge ring of icy worlds out beyond the planet Neptune. It contains over a hundred thousand objects bigger than a kilometre across, plus probably trillions of smaller ones. The most famous Kuiper Belt object is Pluto, which used to be called the ninth planet but is now classed as a "dwarf planet".

• Where is it?Past Neptune's orbitAbout 30 to 50 times further from the Sun than Earth
• How wide?About 3 billion kmAround 20 times wider than the Asteroid Belt
• Most famous residentPlutoReclassified as a dwarf planet in 2006
• Other dwarf planetsEris, Haumea, MakemakeAll icy worlds in the Kuiper Belt
• Discovered1992First Kuiper Belt object (other than Pluto) found in 1992
• Visited byNew HorizonsFlew past Pluto in 2015 and Arrokoth in 2019

The biggest Kuiper Belt objects

Diameter in km. Pluto is the biggest, but only just.

Diameter (km)

Sednaapprox. 995 km

Haumeaapprox. 1,632 km

Makemakeapprox. 1,430 km

Erisapprox. 2,326 km

Plutoapprox. 2,377 km

For decades Pluto was thought to be much bigger. The New Horizons flyby in 2015 measured it more accurately at 2,377 km across. Eris, discovered in 2005, is almost the same size and slightly heavier.

What is the Kuiper Belt?

The Kuiper Belt is a flat doughnut-shaped region of frozen worlds beyond the orbit of Neptune. It sits roughly 30 to 50 times further from the Sun than the Earth does. (Compare this to the Asteroid Belt, which is just 2 to 3 times further out than Earth.) The Kuiper Belt is much, much bigger than the Asteroid Belt and contains many more objects.

The objects in the belt, called Kuiper Belt objects or KBOs, are mostly made of frozen water, frozen methane, frozen ammonia and rock. The whole region is freezing cold, with temperatures down to around -240 degrees Celsius.

How was it discovered?

The Kuiper Belt was predicted long before it was found. In the 1940s, Dutch-American astronomer Gerard Kuiper suggested that there might be a swarm of small icy bodies just past Neptune, left over from the building of the Solar System. The first Kuiper Belt object (other than Pluto, found in 1930) was discovered in 1992. Since then, telescopes have found over 3,000 of them, and there are probably hundreds of thousands more waiting to be spotted.

Fact When the Hubble Space Telescope and ground-based surveys started finding Kuiper Belt objects nearly as big as Pluto, astronomers had to decide whether Pluto was a planet or not. In 2006 the International Astronomical Union decided it was a "dwarf planet" instead. Pluto fans have been arguing about it ever since.

Where do short-period comets come from?

Comets that come back to the Sun every few decades or centuries (like Halley's Comet) are called short-period comets. They mostly come from the Kuiper Belt. Every now and then, the gravity of Neptune or one of the other outer planets nudges a Kuiper Belt object onto a new orbit that sends it falling towards the Sun. As it gets close to the Sun, its ice starts to melt, it grows a tail and becomes a comet.

Visiting the Kuiper Belt

The Kuiper Belt is so far away that only one spacecraft has ever been there: NASA's New Horizons. Launched in 2006, it flew past Pluto in 2015 and returned the first detailed photos of its surface, revealing a young icy world with mountains made of frozen water and a heart-shaped plain of frozen nitrogen. New Horizons then visited a much smaller, more typical KBO called Arrokoth in 2019, the most distant object ever visited by spacecraft.

Did you know? Pluto has five moons, including one called Charon that is more than half its size. Pluto and Charon actually orbit each other around a point between them, like two ice dancers swinging round.

Deeper dive: the scattered disc, the cold and hot classical belt, and the Nice model

The "Kuiper Belt" is actually several different populations of objects. The classical Kuiper Belt sits between roughly 42 and 48 AU from the Sun and contains objects in nearly circular, stable orbits. It splits into two: the cold classical belt (objects with low inclinations, probably formed in place) and the hot classical belt (more tilted orbits, probably pushed outward from closer to Neptune). The resonant Kuiper Belt contains objects locked in orbital resonance with Neptune; the largest group, including Pluto, has a 3:2 resonance and is called the "Plutinos".

Beyond the Kuiper Belt proper lies the scattered disc, an even larger and more sparsely populated region containing objects on highly eccentric, tilted orbits. Eris and Sedna both live here. Their orbits suggest gravitational scattering by Neptune in the past, and Sedna's extremely distant orbit hints at influence from an unknown body even further out, the so-called Planet Nine that astronomers are still searching for.

The structure of the Kuiper Belt provides one of the strongest pieces of evidence for the Nice model of Solar System evolution. Under this model, the giant planets all formed closer to the Sun than their current orbits, then migrated outward over hundreds of millions of years through gravitational interactions with a vast disc of icy planetesimals. Neptune's outward migration scattered most of the original Kuiper Belt material into the inner Solar System (causing the Late Heavy Bombardment) or out into the Oort Cloud, leaving only a small remnant in the cleared-out structure we see today.

For the much larger comet reservoir even further out, see the Oort Cloud. For more on icy visitors from these regions, see what is a comet.