The Kuiper Belt: Home of Pluto and Short-Period Comets

Where the Kuiper Belt is

NASA places the inner edge of the Kuiper Belt at about 30 AU, which is the orbit of Neptune. The main part of the belt ends around 50 AU. In kilometers, that is a band from about 4.5 billion km (2.8 billion miles) to about 7.5 billion km (4.7 billion miles) from the Sun.

The scattered disk is a related population. Neptune's gravity flung its members into highly elliptical, highly tilted orbits. NASA lists Eris, a dwarf planet slightly more massive than Pluto, as the largest known member.

The Oort Cloud is a different place. It is a far more distant, roughly spherical shell, and the Oort Cloud article covers it. The Kuiper Belt is much closer and shaped more like a flattened disk.

What lives there

Pluto is the largest object in the region by size, at over 1,000 km (600 miles) wide. NASA lists Pluto, Eris, Haumea and Makemake as Kuiper Belt objects that have moons. Many other bodies are smaller. NASA says astronomers estimate millions of small icy objects, including hundreds of thousands wider than 100 km (60 miles).

The count of cataloged bodies is far smaller. NASA's Kuiper Belt facts page says more than 2,000 trans-Neptunian objects have been cataloged, though it gives no date and the number grows as surveys continue. A trans-Neptunian object is any body that orbits beyond Neptune.

The belt may sound crowded, but NASA estimates its total mass at no more than about 10 percent of Earth's. The early belt was heavier. NASA says it likely held 7 to 10 times Earth's mass until shifts in the giant planets' orbits removed most of it. Neptune's gravity also kept the leftover bodies from ever gathering into a planet.

Some objects are locked into patterns with Neptune. Pluto is in a 3:2 resonance, a locked orbital pattern: for every three orbits Neptune completes, Pluto completes two. That rhythm keeps Pluto from ever getting close enough to Neptune to be strongly disturbed. The story of how Pluto's status changed is told in why Pluto is a dwarf planet.

How the belt supplies short-period comets

Gerard Kuiper published a paper in 1951 that speculated about objects beyond Pluto, and NASA notes that Kenneth Edgeworth wrote about such objects in the 1940s, so the region is sometimes called the Edgeworth-Kuiper Belt.

The link to comets works like a relay. NASA says Neptune's gravity pushes fragments from the belt toward the Sun. Jupiter then takes over and holds them in loops of 20 years or less. These are the short-period Jupiter-family comets. More broadly, NASA defines short-period comets as those that circle the Sun in less than 200 years. ESA describes Comet 67P, the target of the Rosetta mission, as a Jupiter-family comet, and the Rosetta mission story shows what one of them looks like up close.

Long-period comets are different. They have orbits that last far longer and arrive from the Oort Cloud. The anatomy of a comet explains what happens when either kind warms up near the Sun.

The New Horizons visits

Most of what astronomers know about the belt comes from ground telescopes and the Hubble Space Telescope. NASA says New Horizons is the only spacecraft to have visited it so far. The key dates are short.

• 1951: Gerard Kuiper publishes a paper about objects beyond Pluto.
• 2014 (June 26): Marc Buie finds Arrokoth with the Hubble Space Telescope.
• 2015 (July): New Horizons flies past Pluto and returns the first clear, close-up images.
• 2019 (January 1): New Horizons passes Arrokoth.

NASA's facts page gives Arrokoth's length as about 35 km (22 miles), with a flattened larger lobe of 22 by 20 by 7 km and a rounder smaller lobe of 14 by 14 by 10 km. It is very red, redder than Pluto. It follows a nearly circular orbit and takes about 293 Earth years to go around the Sun. The name means "sky" in the Powhatan and Algonquian language, and it was chosen in 2019 with the consent of Powhatan tribal elders. Before that, the object carried the nickname Ultima Thule.

As of May 2026, NASA lists New Horizons as an active mission. NASA says its extended mission has focused on gathering data about the Sun's environment, and the page lists no new Kuiper Belt flyby.

Common mistakes and myths

"The Kuiper Belt and the Oort Cloud are the same thing." They are not. The belt lies at roughly 30 to 50 AU. The Oort Cloud is far more distant and is not shaped like a disk.

"Pluto is the Kuiper Belt's only large body." Eris is slightly more massive than Pluto, and Haumea and Makemake also live in the region.

"New Horizons has mapped the belt." It has visited two bodies, Pluto and Arrokoth. The rest of the belt is known from telescopes.

"Comets all come from the Kuiper Belt." Only the short-period group is tied to it. The long-period group comes from farther out.

"Arrokoth was named from the start." It was nicknamed Ultima Thule during the mission. The official name came later, in 2019.

Frequently asked questions

How far away is the Kuiper Belt?

NASA places it at about 30 to 50 AU from the Sun, or roughly 4.5 to 7.5 billion km (2.8 to 4.7 billion miles). The scattered disk extends to nearly 1,000 AU.

Why is it called the Kuiper Belt?

It is named for Gerard Kuiper, who wrote about such a region in 1951. Because Kenneth Edgeworth had written about similar objects earlier, some people say Edgeworth-Kuiper Belt.

Is Pluto in the Kuiper Belt?

Yes. NASA calls Pluto the largest object in the belt by size. It sits in a 3:2 resonance with Neptune.

How many Kuiper Belt objects are there?

NASA estimates millions of small icy objects. More than 2,000 have been cataloged so far, according to NASA's Kuiper Belt facts page, and the cataloged number keeps growing.