The Kuiper belt

The Kuiper belt is a vast region of the solar system beyond Neptune's orbit. It is best described as shaped like a flat doughnut around the Sun that extends from 30 AU to 50 AU. It is similar to the asteroid belt, although it is much larger and more massive. Like the asteroid belt, it contains mainly small bodies. Whereas the asteroids are composed mainly of rock and metal, the objects in the Kuiper belt are made mostly of rock and ices such as methane, ammonia and water. The temperature of objects within the belt is around -230°C, so they are very cold.

The belt is named after Dutch-born American astronomer Gerard Kuiper, who first predicted its existence in 1951. The objects in this region are called Kuiper belt objects or KBOs. The orbits of many of these objects are highly elliptical and are destabilised by Neptune's gravity.

The most famous KBO is the plutoid Pluto. Apart from Pluto, discovered in 1930, the first KBO was discovered by David Jewitt and Jane Luu in Hawaii in August 1992. This object is called 1992 QB1. It was found 42 AU from the Sun. Six months later, these two astronomers discovered a second object, 1993 FW, in the same region.

It is suspected that there may be as many as 35 000 objects in the Kuiper belt with diameters of 100 km or greater, as well as many more smaller objects. Most objects in this belt probably formed at the same distance from the Sun as we find them today.

The Kuiper belt is also thought to be the home of short-period comets (those with periods less than 200 years). Comets are small bodies of ice and rock in orbit around the Sun. Many comets have highly elliptical orbits that occasionally bring them close to the Sun. When this happens the Sun's radiation vaporises some of comet's icy material, and a long tail is seen extending from the comet's head and pointing away from the Sun.

The plutoid Pluto and its companion Charon are two of the larger KBOs. Several other large KBOs have been discovered, including Quaoar (2002 LM60), Makemake (2005 FY9) and Orcus (2004 DW).

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