Pluto - dwarf planet, trans-Neptunian Object and member of the Kuiper Belt
Until recently, Pluto was considered the last of the planets. However, Pluto fails one crucial criterion in the definition of a planet - it is locked in an orbital "relation" with Neptune and is therefore not the gravitationally dominant object in its neighbourhood. Many other objects have similar orbits to Pluto in that they orbit twice round the Sun for every three orbits of Neptune. These objects are called Kuiper Belt Objects (KBOs) and Pluto is rightly the first of these objects to be discovered.
The discovery of Pluto in 1930 by Clyde Tombaugh at Lowell Observatory was the result of a long detailed search of photographic plates. The planet is too faint to be readily found by visual observation. Pluto is a relatively small object; indeed it is smaller than some of the moons of the other planets. Situated about forty times the Earth's distance from the Sun, Pluto is a cold, dark place, with a surface temperature close to absolute zero.
Pluto is almost certainly a very light body, made up of "rocky methane ice", with a density similar to that of water. It also has a very thin atmosphere of methane gas - this was discovered indirectly in 1988. With a diameter of only 2,302 km, Pluto is much smaller than our Moon (3,476 km). Compare these with the size of Australia, about 4,200 km across.
Pluto has a moon, Charon, which is large in relation to Pluto: 1151 km in diameter, and about one seventh of Pluto's volume. Discovered in 1978, it orbits very close to its parent body: only 8 Pluto diameters away as opposed to the 30 Earth diameters that separate the Earth from the Moon. Another two small moons of Pluto, Nix and Hydra, were discovered in 2005.
