Pluto was a planet for 76 years until it was reclassified as a dwarf planet in 2006.
Pluto is now thought to be an object in the Kuiper Belt, a disc-shaped area of icy, dark objects beyond Neptune and has one large moon, Charon, and two much smaller moons, Nix and Hydra. In June 2011, Hubble spotted a fourth moon around Pluto that will be called S/2011 (134340) 1 for the time being.
NASA's New Horizons mission is scheduled to reach Pluto in 2015.
Photo: Pluto and its moons (NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI), and the HST Pluto Companion Search Team)
Once a planet, Pluto is now a Kuiper Belt dwarf planet.
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Now demoted to dwarf planet status, Pluto is still an important world.
Astronomer Clyde Tombaugh discovered Pluto in 1930 by meticulously comparing images of the night sky. Pluto held its planet status until 2006 when it was reclassified as a dwarf planet, one of many bodies orbiting in an area known as the Kuiper Belt. It is now known that Pluto does not mark the edge of the Solar System.
Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond Neptune. It was the first Kuiper belt object to be discovered. It is the largest and second-most-massive known dwarf planet in the Solar System and the ninth-largest and tenth-most-massive known object directly orbiting the Sun. It is the largest known trans-Neptunian object by volume but is less massive than Eris, a dwarf planet in the scattered disc. Like other Kuiper belt objects, Pluto is primarily made of ice and rock and is relatively small—about one-sixth the mass of the Moon and one-third its volume. It has a moderately eccentric and inclined orbit during which it ranges from 30 to 49 astronomical units (4.4–7.3 billion km) from the Sun. This means that Pluto periodically comes closer to the Sun than Neptune, but a stable orbital resonance with Neptune prevents them from colliding. In 2014, Pluto was 32.6 AU from the Sun. Light from the Sun takes about 5.5 hours to reach Pluto at its average distance (39.4 AU).
Pluto was discovered in 1930 by Clyde Tombaugh, and was originally considered the ninth planet from the Sun. After 1992, its status as a planet fell into question following the discovery of several objects of similar size in the Kuiper belt. In 2005, Eris, which is 27% more massive than Pluto, was discovered, which led the International Astronomical Union (IAU) to define the term "planet" formally for the first time the following year. This definition excluded Pluto and reclassified it as a member of the new "dwarf planet" category (and specifically as a plutoid). Some astronomers think that Pluto, as well as the other dwarf planets, should be considered planets.
Pluto has five known moons: Charon (the largest, with a diameter just over half that of Pluto), Styx, Nix, Kerberos, and Hydra. Pluto and Charon are sometimes considered a binary system because the barycenter of their orbits does not lie within either body. The IAU has not formalized a definition for binary dwarf planets, and Charon is officially classified as a moon of Pluto.
On 14 July 2015, the New Horizons spacecraft became the first spacecraft to fly by Pluto. During its brief flyby, New Horizons made detailed measurements and observations of Pluto and its moons.