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 all passed through the Kuiper belt but did not get close to any large objects. The first spacecraft sent to explore the Kuiper belt, New Horizons, was launched by NASA in 2006, and will arrive at Pluto in 2015.
The Kuiper Belt and Oort Cloud
The Star Garden
The Kuiper belt
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 British astronomer David Jewitt, and American astronomer Jane Luu, in 1992. Over 70,000 objects, over 100 km in diameter, have since been found.
Like the asteroid belt between Mars and Jupiter, the Kuiper belt contains remnants from the Solar System's formation that 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 spherical cloud of comets, which orbits between about 5000 AU and 100,000 AU. This is over 1.5 light years, 2000 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 Ípik 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.
Artists' impression of the Kuiper Belt and Oort cloud