The Kuiper Belt: Our Solar System's Deep Freeze

a Kuiper Belt object
An artist's concept of a Kuiper Belt object to be visited by New Horizons in 2018. The Kuiper Belt is populated with many small worlds that can tell a story about the earliest history of the solar system. ASA, ESA, and G. Bacon (STScI)

There's a huge unexplored region of the solar system out there that lies so distant it took a spacecraft about 9 years to get there. It's called the Kuiper Belt. It covers the space that stretches from the orbit of Neptune and tapers off at a distance of 50 astronomical units from the Sun. (The astronomical unit is the distance between Earth and the Sun, 150 million kilometers (93 million miles)).


The Kuiper Belt is named after planetary scientist Gerard Kuiper, who did not actually discover or predict it, but strongly suggested that comets and small planets could have formed out in this region. He is often referred to as the father of modern planetary science. The belt is also often called the Edgeworth-Kuiper Belt, after planetary scientist Kenneth Edgeworth. He theorized that there could be objects beyond the orbit of Neptune that never coalesced into planets. These include small worlds as well as comets. Now that it is being explored, some planetary scientists refer to it as the "third zone" of the solar system. The other two zones are the realm of the rocky planets (Mercury, Venus, Earth, and Mars) and the outer, icy gas giants (Jupiter, Saturn, Uranus, and Neptune). 

What’s the Kuiper Belt Like?

The Kuiper Belt is disk-shaped and contains such worlds as Pluto and Eris, as well as other dwarf planets.

It’s also the origin point of many comets that have left the Kuiper Belt on orbits around the Sun. These short-period comets have orbits that last less than 200 years; comets with periods longer than that seem to emanate from the Oort Cloud, which is a spherical collection of objects that extends out about a quarter of the way to the nearest star.

The Kuiper Belt is thought to contain hundreds of thousands of icy objects that are larger than 100 kilometers across, plus many smaller ones. There may also be up to a trillion cometary bodies in the Belt, although no one knows the exact number.

Objects in the Kuiper Belt are called “Kuiper Belt Objects” or KBOs. Some objects are also referred to as “trans-Neptunian Objects” or TNOs. The planet Pluto is the first “true” KBO, and is sometimes referred to as the “King of the Kuiper Belt”.

How Did the Kuiper Belt Form?

In the standard explanation of solar system formation, the Kuiper Belt is explained as a “treasure chest” of icy materials left over after the Sun and planets formed. This is true but is not the whole story. As the planets formed, their orbits changed over time. The large gas- and ice-giant worlds of Jupiter, Saturn, Uranus, and Neptune, formed much closer to the Sun and then migrated out to their present places. As they did, their gravitational effects “kicked” smaller objects out to the outer solar system. That populated the Kuiper Belt and Oort Cloud, placing a great deal of primordial solar system material out in a place where it could be preserved by the cold temperatures.

Kuiper Belt: Frozen Remnants of the Early Solar System

When planetary scientists say that comets (for example) are treasure chests of the past, they are absolutely correct. Each cometary nucleus, and perhaps many of the Kuiper Belt objects such as Pluto and Eris, contains material that is literally as old as the solar system and has never been changed.

New Horizons in the Kuiper Belt

The New Horizons spacecraft, which swept past Pluto in 2015, is now destined to travel through the Kuiper Belt, perhaps studying at least one other object in the next few years. The spacecraft is following a path that will take it past a world called 2015 MU69. That will give planetary scientists a second look at some of the rarest real estate in the solar system. After that, the spacecraft will continue on a trajectory that will take it out of the solar system later in the century.