The Kuiper belt contains at least three dwarf planets, Pluto, Haumea and Makemake. The largest is Pluto, which was discovered by American astronomer Clyde Tombaugh in 1930 and demoted from planetary status in 2006. Pluto is mostly composed of rock and ice, it is five times less massive than the Moon and has an eccentric orbit, which sometimes takes it closer to the Sun than Neptune. Pluto is orbited by a number of moons including Nix, Hydra and Charon. Charon is the largest with a diameter over half the size of Pluto's.
The Pioneer and Voyager probes are now all in the Kuiper belt but are unlikely to pass close to anything. The first spacecraft sent to explore the Kuiper belt, New Horizons, was launched by NASA in 2006 and will arrive at Pluto in 2015.
Artists' impression of the Kuiper Belt and Oort cloud
The Kuiper belt is an asteroid belt that extends from the orbit of Neptune at about 30 AU to over 50 AU (one AU is the distance between the Earth and the Sun). It was first hypothesised by Dutch-American astronomer Gerard Kuiper in 1951 and the first Kuiper belt object was detected by English astronomer David Jewitt and American astronomer Jane Luu in 1992. Over 70,000 objects over 100 kms in diameter have been found since.
Like the asteroid belt between Mars and Jupiter, the Kuiper belt contains remnants from the Solar System's formation which were not able to form a planet. Unlike the asteroid belt between Mars and Jupiter, which is mostly composed of rock and metal, the Kuiper belt is mostly composed of frozen methane, ammonia and water.
Comparison of 8 Kuiper belt objects and their moons and the Earth, sizes are roughly to scale Image credit: NASA/Lexicon
The Oort Cloud
The Oort cloud is a hypothetical spherical cloud of comets which orbits between about 5,000 AU to the edge of the Solar System at about 100,000 AU. This is over 1.5 light years, two thousand times further than the edge of the Kuiper belt and a third of the distance to the closest extra-solar star, Proxima Centauri.
Oort cloud objects are thought to be composed of frozen water, ammonia, carbon monoxide, hydrogen cyanide, ethane and methane. They can be effected by the gravitational force of nearby stars and this sometimes sends them towards the centre of the Solar System. In 1932 Estonian astronomer Ernst Opik suggested that long-period comets, such as Halley's Comet and Hale-Bopp, may originate from the Oort cloud and this idea was extended by Dutch astronomer Jan Oort in 1950. The Oort cloud is so far away that the Pioneer and Voyager probes will not pass into it for hundreds of years.