The Star Garden
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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. NASA launched its first mission to Pluto in 2006 and it is due to arrive in 2015.
The Oort Cloud
The Oort cloud is a hypothetical spherical cloud of comets which orbits between five thousand AU to the edge of the Solar System at one hundred thousand AU. This is over one and a half 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.
References
See NASA's profiles of Pluto and The Kuiper belt and Oort Cloud.
The Kuiper Belt and Oort Cloud
The Kuiper belt
The Kuiper belt is an asteroid belt that extends from the orbit of Neptune at about thirty AU to over fifty 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 seventy thousand objects, over one hundred kilometres in diameter, have been found since then.
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, which is mostly composed of rock and metal, the Kuiper belt is mostly composed of frozen methane, ammonia and water.