The Discovery and Characteristics of the Icy, Remote Kuiper Belt

The "Third Zone" of the solar system houses a treasure trove of its ancient past

Pluto is the most famous member of the Kuiper Belt, a region beyond the orbit of Neptune. courtesy NASA/SWRI/APL. NASA/New Horizons/JHU-APL

There's a vast, unexplored region of the solar system out there that lies so far from the Sun that it took a spacecraft about nine years to get there. It's called the Kuiper Belt and it covers the space that stretches out beyond the orbit of Neptune to a distance of 50 astronomical units from the Sun. (An astronomical unit is the distance between Earth and the Sun, or 150 million kilometers). 

Some planetary scientists refer to this populated region as the "third zone" of the solar system. The more they learn about the Kuiper Belt, the more it appears to be its own distinct region with specific characteristics that scientists are still investigating.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). 

How the Kuiper Belt Was Formed

The early solar system
An artist's concept of the birth of star similar to our own. After the birth of the Sun, the icy materials that make up the Kuiper Belt migrated to the distant reaches of the Kuiper Belt region, or were slingshotted there after interactions with the planets as they formed and migrated to their current positions. NASA/JPL-Caltech/R. Hurt

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. Those objects 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.

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 altered.

Discovery of the Kuiper Belt

Kuiper
Gerard Kuiper was one of several scientists who theorized the existence of the Kuiper Belt. It is named in his honor and is often also called the Kuiper-Edgeworth belt, honoring astronomer Ken Edgeworth. NASA

The Kuiper Belt is named after planetary scientist Gerard Kuiper, who did not actually discover or predict it. Instead, he strongly suggested that comets and small planets could have formed out in the chilly region known to exist beyond Neptune. The belt is also often called the Edgeworth-Kuiper Belt, after planetary scientist Kenneth Edgeworth. He also theorized that there could be objects beyond the orbit of Neptune that never coalesced into planets. These include small worlds as well as comets. As better telescopes were built, planetary scientists have been able to discover more dwarf planets and other objects out in the Kuiper Belt, so its discovery and exploration are an ongoing project.

Studying the Kuiper Belt from Earth

KUIPER BELT OBJECT 2000 FV53
Kuiper Belt object 2000 FV53 is very small and distant. However, Hubble Space Telescope was able to spot it from Earth orbit and use it as a guide object while searching for other KBOs. NASA and STScI

 Objects that make up the Kuiper Belt are so distant that they cannot be seen with the naked eye. The brighter, larger ones, such as Pluto and its moon Charon can be detected using both ground-based and space-based telescopes. However, even their views are not very detailed. Detailed study requires a spacecraft to go out there to take close-up images and record data. 

The New Horizons Spacecraft

new_horizons.jpg
An artist's idea of what New Horizons looked like as it passed by Pluto in 2015. NASA

The New Horizons spacecraft, which swept past Pluto in 2015, is the first spacecraft to actively study the Kuiper Belt. Its targets also include Ultima Thule, which lies much farther out from Pluto. This mission has given 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.

The Realm of Dwarf Planets

Makemake and its moon as seen by HST
Makemake and its moon (upper right) as seen by Hubble Space Telescope. This artist's concept shows what the surface might be like. NASA, ESA, A. Parker and M. Buie (Southwest Research Institute), W. Grundy (Lowell Observatory), and K. Noll (NASA GSFC)

 In addition to Pluto and Eris, two other dwarf planets orbit the Sun from the distant reaches of the Kuiper Belt: Quaoar, Makemake (which has its own moon), and Haumea.

Quaoar was discovered in 2002 by astronomers using the Palomar Observatory in California. This distant world is about half the size of Pluto and lies about 43 astronomical units away from the Sun. (An AU is the distance between Earth and the Sun. Quaoar has been observed with Hubble Space Telescope. It appears to have a moon, which is named Weywot. Both take 284.5 years to make one trip around the Sun.

KBOs and TNOs

kuiper belt
This schematic of the Kuiper Belt shows the relative locations of four of the region's dwarf planets. The line from the inner solar system is the trajectory taken by the New Horizons mission. NASA/APL/SWRI

Objects in the disk-shaped Kuiper Belt are known as “Kuiper Belt Objects” or KBOs. Some 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”. The Kuiper Belt is thought to contain hundreds of thousands of icy objects that are larger than a hundred kilometers across.

Comets and the Kuiper Belt

This region is also the origin point of many comets that periodically leave the Kuiper Belt on orbits around the Sun. There may be nearly a trillion of these cometary bodies. The ones that leave on orbit are called short-period comets, which means they 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.