Pluto is an outer solar system body that was formerly known as a planet, but since a 2006 resolution by the International Astronomers Union is now known as a dwarf planet, along with Ceres and Eris. It is the 2nd largest member of the Kuiper belt, an icy asteroid belt outside the orbit of Neptune. Pluto's orbit varies between 29 and 49 AUs (Earth-Sun distances) from the Sun. It has a diameter one-fifth of Earth, smaller than the Moon at one-fourth of Earth. Pluto has a mean surface temperature of 44 K (-380.47 C).
Pluto was discovered in 1930 by Clyde Tombaugh, by comparing variations in two photographic plates. In the early 20th century, scientists began looking for a Planet X, which was said to have caused perturbations in Neptune's orbit, much the way that the existence of Neptune was predicted by perturbations in Uranus' orbit. Although Pluto was found near the location where Planet X was predicted to be, it could not have been the predicted planet, because it was far too small to influence the orbit of Neptune. It turned out later that the perturbations in Neptune's orbit could be explained with Einstein's new theory of general relativity.
Pluto has a brownish color with a large black splotch in its center. It was not until the launch of the Hubble space telescope that astronomers saw Pluto as much more than a faint point. Pluto has three moons; Charon, Hydra, and Nix. Charon is substantially larger than the others. When Pluto was discovered, many people submitted recommendations for its name. The winner was an eleven-year old schoolgirl from Oxford, Venetia Burney, who submitted Pluto.
In 2015, the New Horizons spacecraft will reach Pluto, observing it and its moons up close. This will be the most distant object ever to be approached by an artificial satellite. After New Horizons passes Jupiter, it will become the 5th probe to exit the solar system.