How big is Kuiper Belt? | Scale of the Universe

The Kuiper Belt is a doughnut-shaped region of icy bodies in the outer Solar System, beyond the orbit of Neptune. Its main part stretches from about 30 to about 50 astronomical units from the Sun, where an astronomical unit is Earth's average distance from the Sun. That makes the belt's main part about 100 astronomical units, or 15 billion kilometers, across. Beyond the main part, the belt has no single outer edge, and a thinner scattering of objects reaches much farther out.

Size

One astronomical unit (au) is exactly 149,597,870,700 meters, about 150 million kilometers. According to NASA, the belt's inner edge begins at the orbit of Neptune, about 30 au from the Sun, and its main region ends around 50 au. Objects on calm, nearly circular orbits become abruptly scarcer beyond about 47 au, at an edge called the Kuiper Cliff. Measured across the outer edge of the main belt, the belt is about 100 au, or 15 billion kilometers, wide.

The belt is not flat like a ring, but thick, like a puffed-up disk. Many of its objects, called classical Kuiper Belt objects, orbit at an average distance of 40–50 au. Overlapping the outer edge is a second region, the scattered disk, whose objects follow stretched, tilted orbits out to nearly 1,000 au. Some astronomers treat the two as separate regions, but their boundaries overlap, so any single figure for the belt's size is approximate.

Size comparisons

Across its outer edge, the belt is more than three times the distance from Neptune to the Sun.
Light takes about 14 hours to cross it, a little more than half a light-day.
The orbit of the dwarf planet Pluto, which is 2,377 kilometers across, takes it from about 30 to 49 au from the Sun, across nearly the whole main belt.
If the Sun were shrunk to the size of a basketball, Neptune would orbit about 780 meters (half a mile) away. The belt's outer edge would be about 1.3 kilometers (0.8 miles) away, so the belt would be about 2.6 kilometers (1.6 miles) across.

Discovery and name

The belt is named for the astronomer Gerard Kuiper, who published a paper in 1951 that speculated about objects beyond Pluto. Kenneth Edgeworth had also written about such objects in the 1940s, so the region is sometimes called the Edgeworth-Kuiper Belt. Pluto, found in 1930, was the first Kuiper Belt object discovered. Astronomers began finding many more in the early 1990s.

What is in the belt

Astronomers think the belt holds millions of small, icy objects, including hundreds of thousands wider than 100 kilometers (60 miles). Besides rock and water ice, they contain other frozen substances such as ammonia and methane. The dwarf planets Pluto, Eris, Haumea and Makemake are all Kuiper Belt objects with moons of their own. Even so, all the material in the belt together is estimated to have no more than about a tenth of Earth's mass.

According to NASA, as of 2025 only one spacecraft has visited the Kuiper Belt: New Horizons. It flew past Pluto in July 2015, and on January 1, 2019, it passed a small object called Arrokoth. Its pictures showed two lobes joined together, like a partly flattened red snowman.

How the belt formed

The belt's icy objects are thought to be leftovers from the formation of the Solar System. Neptune's gravity stirred up the region so much that they could not come together into a planet. According to one well-supported theory, the orbits of the giant planets shifted early on, and Neptune moved outward through a dense disk of icy bodies. Most of the original material, likely 7 to 10 times Earth's mass, may have been lost.

The belt is still slowly wearing away. Its objects sometimes collide and break into smaller pieces, and some of those become comets.