The Kuiper belt and beyond

The Kuiper belt is a ring of icy bodies beyond the orbit of Neptune, a region of leftovers from the solar system's early history much like the asteroid belt, but colder, more distant and far more massive in number.^[1] It is home to Pluto and most of the known dwarf planets, and to some comets.^[2] Anything that orbits the Sun out there is a trans-Neptunian object.

Where it is

The belt's inner edge begins at the orbit of Neptune, about 30 astronomical units from the Sun, and its main region ends around 50 astronomical units.^[1] Overlapping that outer edge is a second region, the scattered disk, which continues outward to nearly 1,000 astronomical units, with some bodies on orbits that go even farther.^[1] The region is named for Gerard Kuiper, who published a paper in 1951 speculating about objects beyond Pluto; Kenneth Edgeworth had mentioned such objects in the 1940s, so it is sometimes called the Edgeworth-Kuiper belt.^[1] Pluto, found in 1930, was the first Kuiper belt object discovered, at a time when nobody expected a large population of icy worlds beyond Neptune.^[1]

What is out there

Astronomers think the belt holds millions of small icy objects, including hundreds of thousands larger than 100 km across, yet the total mass of everything in it is no more than about 10 per cent of the mass of Earth.^[1] Some objects, Pluto among them, are over 1,000 km wide.^[1] Besides rock and water ice they contain other frozen compounds such as ammonia and methane.^[1]

Catalogues lag the estimates by a wide margin. NASA's own page counts more than 2,000 trans-Neptunian objects catalogued by observers, a tiny fraction of what is thought to be there.^[1] JPL's Small-Body Database, which holds an orbit for every object the Minor Planet Center has designated, listed 8,337 trans-Neptunian objects on 5 November 2026, plus 1,087 Centaurs on orbits between Jupiter and Neptune.^[3][4]

Families of orbits

Once astronomers started discovering Kuiper belt objects in the early 1990s, one early surprise was that they could be grouped by the shapes and sizes of their orbits.^[1]

• The classical belt holds a large fraction of them, all with a similar average distance from the Sun of between about 40 and 50 astronomical units.^[1] They divide into two groups, called cold and hot, terms that refer to the tilt and shape of their orbits rather than to temperature.^[1]
• Resonant objects are on orbits tightly controlled by Neptune, in several groupings: 1:1, 4:3, 3:2 and 2:1.^[1] Pluto is in a 3:2 resonance, circling the Sun twice for every three times Neptune goes around, and there are enough objects on such orbits that they have their own name, the plutinos.^[1]
• The scattered disk stretches far beyond the main belt and holds objects that Neptune has scattered into orbits that are highly elliptical and highly inclined to the plane of the planets.^[1] Eris is the largest known member.^[1]
• Detached objects never come closer to the Sun than about 40 astronomical units.^[1] Sedna is the example: the closest it comes to the Sun is 76 astronomical units, while at its farthest it travels out to about 1,200.^[1]

Centaurs

Centaurs travel through the space between the orbits of Jupiter and Neptune, and the most likely explanation is that they are relatively recent escapees from the Kuiper belt.^[1] Their orbits are not stable: most are fated to be ejected from the solar system or pushed inward, where they become comets, a process that takes tens of millions of years for a typical Centaur.^[1]

The Oort cloud

The Kuiper belt should not be confused with the Oort cloud, a much more distant region of icy, comet-like bodies that surrounds the whole solar system, the Kuiper belt included.^[1] Scientists think it is a giant spherical shell around the Sun, the planets and the Kuiper belt objects, lying far beyond Pluto and the most distant edges of the belt.^[5] Both regions are thought to be sources of comets: most long-period comets on highly tilted orbits come from the Oort cloud.^[1]

How they are found

Most of what is known about the Kuiper belt comes from ground-based telescopes and the Hubble Space Telescope.^[2] Discovery is a matter of photographing the same sky repeatedly and finding the faint points that move. The Rubin Observatory's ten-year survey, which began in June 2026, is the largest such effort yet: in April, before it formally started, its team had submitted observations of over 11,000 new asteroids to the Minor Planet Center, including roughly 380 new trans-Neptunian objects.^[6] Its first dwarf-planet candidate, 2026 ED138, was found on 13 October 2026 by a search code that compared billions of detections and linked 30 observations of one object over 20 nights.^[6] Archival images from another telescope extended its track back to 2016, and the discovery paper reports an orbit followed for more than ten years.^[6][7] It had been missed because it lies far south and near the crowded regions of the Galactic plane, and the same paper estimates that about three more objects as bright remain undiscovered away from the plane, with more likely within it.^[7]

Visited once

Only one spacecraft has visited the Kuiper belt.^[2] NASA's New Horizons flew past Pluto on 14 July 2015, sending back the first clear close-up images of it, and on 1 January 2019 flew past a small Kuiper belt object later named Arrokoth, the most distant and most primitive object ever explored by a spacecraft.^[8][2][1] Arrokoth is a contact binary, two lobes joined together.^[1]

Questions people ask

Is Pluto in the Kuiper belt?

Yes. Pluto was the first Kuiper belt object to be discovered, in 1930, and it is the largest object in the region.^[1] Dwarf planets Pluto, Eris, Haumea and Makemake are all Kuiper belt objects.^[1]

Will more dwarf planets be found there?

Very likely. The IAU says there may be more than a hundred dwarf planets waiting to be discovered.^[9] The Rubin Observatory will image the whole southern sky repeatedly for ten years, and its first candidate turned up in data from its first months.^[6]

Could a Kuiper belt object reach Earth?

Not as it is. Objects that leave the belt first become Centaurs between Jupiter and Neptune, and some of those are eventually pushed into the inner solar system, where they become comets.^[1] Comets, and how they are tracked, are covered in Comets.