Pluto Is Just One of Millions of Objects in the Kuiper Belt

By: Valerie Stimac  | 
Kuiper Belt, space probes
The Kuiper belt region is a ring-shaped collection of icy bodies beyond the outer edge of Neptune's orbit. This illustration shows some of the space probes NASA has launched over the years. NASA

When we're young children in school, many of us learn about the planets, the sun and maybe the asteroid belt. There's much more to our solar system than that, though, including a region of the outer solar system called the Kuiper belt.

Sitting beyond the orbit of Neptune, the Kuiper (pronounced Ky-per) belt is home to millions (or billions, it's unclear) of icy objects that orbit our sun just like the planets and asteroids we know about. Here's how NASA puts it: "Similar to the asteroid belt, the Kuiper belt is a region of leftovers from the solar system's early history. Like the asteroid belt, it has also been shaped by a giant planet, although it's more of a thick disk (like a doughnut) than a thin belt."


Neptune is one of the giant planets in our solar system. The others are Jupiter, Saturn and Uranus. Giant planets are so-called because of their size in relation to Earth. They are also known as the outer planets because they are farthest away from the sun.

Models and diagrams of the solar system often seem to end at the orbit of Neptune — or perhaps Pluto if you're looking at an older version. Putting aside the controversy about Pluto's status as a dwarf planet, there are actually millions of objects beyond the orbit of Neptune — and some of them are sizable. These are called Kuiper belt objects or KBOs. So far, around 2,000 Kuiper belt objects have been categorized.

In addition to Pluto, there are several dwarf planets that are part of the Kuiper belt, including Eris, Makemake, and Haumea. These large solar system objects orbit the sun like the planets but aren't quite large enough to be categorized as such. There are millions of other, smaller objects, like comets and moons, many of which are generally clustered in a band of space between 30-55 Astronomical Units (AU) from the sun (1 AU equals the distance from Earth to the sun on average, which is 90 million miles or 150 million kilometers).

Beyond the Kuiper belt, there's also the Oort cloud, another area of space containing even more objects. "The Oort Cloud is believed to be a giant spherical shell surrounding the rest of the solar system. It is like a big, thick-walled bubble made of icy pieces of space debris the sizes of mountains and sometimes larger," according to NASA. All this to say, there's a lot more to our solar system than the eight planets most of us learn about.


Who Is the Kuiper Belt Named For?

the Kuiper Belt and Solar System
The dwarf planet Pluto is part of the Kuiper belt. Scientists are still discovering Kuiper belt objects. bogadeva1983/Shutterstock

The Kuiper belt was named, not after the person who discovered the first objects in this part of the solar system, but for astronomer Gerard Kuiper, who published a scientific paper in 1951 that speculated about objects beyond Pluto. These objects weren't even proven, and his paper didn't say much about how many objects there were or where they might be.

Later research showed that the Kuiper belt is defined by the orbit of Neptune; the inner boundary of the Kuiper belt is Neptune's orbit, and it stretches out from there. The Kuiper belt is slowly eroding away, and this erosion produces comets via colliding KBOs. Neptune’s gravity usually pushes these comets toward the sun. Most of these become dormant or die pretty quickly due to the frequent trips to the inner solar system.


NASA estimates that there are 1 trillion comets in the Kuiper belt, as well as hundreds of thousands of objects with diameters of 62 miles (100 kilometers) or wider. For comparison, the asteroid that killed the dinosaurs was only 6.2 miles (10 kilometers) in diameter!

Why Is the Kuiper Belt Important?

This composite image of the primordial Kuiper belt object 2014 MU69 (now called Arrokoth) was compiled from data obtained by NASA's New Horizons spacecraft as it flew by the object Jan. 1, 2019. NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Roman Tkachenko

Since the Kuiper belt is thought to be composed of leftovers from the formation of the outer planets, studying it can help scientists better understand how our solar system was formed. In 2018, New Horizons spacecraft sent back the first images from the Kuiper belt and continues to travel farther from the sun, documenting this part of the solar system. New Horizons also transmitted images of an ancient Kuiper belt object called Arrokoth (or 2014 MU69), the most distant and primitive object seen by a space probe so far.

Before making its way to the Kuiper belt, the New Horizons spacecraft flew past Pluto in 2015 and delivered some incredible close-up photos of everyone's favorite dwarf planet.


Pluto actually crosses through the Kuiper belt as part of its 248-year orbit but was within the orbit of Neptune (and thus closer to the sun than the Kuiper belt) when New Horizons did its fly-by. This means that New Horizons' next destination is the outer solar system.

Originally Published: Dec 26, 2022