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Pluto, in astronomy, the 9th and most distant known planet from the sun, with an elliptical orbit usually lying beyond that of Neptune. Pluto's mean distance from the sun is 3.67 billion mi (5.91 billion km), and its period of revolution is about 248 years. Since Pluto has the most elliptical and tilted orbit of any planet (eccentricity .250, inclination 17°), at its closest point to the sun it passes inside the orbit of Neptune; between 1979 and 1999 it was closer to the sun than Neptune was. It will remain farther from the sun for 220 years, when it will again pass inside Neptune's orbit. Its surface consists largely of frozen nitrogen. It is thought to have a rocky, silicate core; its thin atmosphere probably contains nitrogen, carbon monoxide, and methane. Its surface temperature is estimated to be about -360°F (-218°C), a temperature at which most gases exist in the frozen state.
The existence of an unknown planet was first proposed by Percival Lowell on the basis of observed perturbations of the orbits of Uranus and Neptune. He began searching for the planet in 1905, although he did not publish his calculations of its predicted position until 1914. Independent calculations were published by W. H. Pickering and others. In 1929, the search for a new planet was resumed at Lowell Observatory, and on Feb. 18, 1930, using photographic plates and a blink microscope, Clyde W. Tombaugh discovered an object whose motion was consistent with that of a trans-Neptunian planet.
In 1978, American astronomers James Christy and Robert Harrington discovered Pluto's only known moon, Charon. Together, Pluto and Charon form a double planet system that is consistent with theoretical calculations. Pluto's diameter is c.1,400 mi (2,300 km), Charon's is c.728 mi (1,172 km), and the radius of Charon's orbit is 12,200 mi (19,640 km); Charon completes one orbit in about 6.4 earth days. Pluto and Charon are unique in that they both keep the same side facing one another at all times because they rotate synchronously as Charon orbits Pluto. No spacecraft has yet visited Pluto and Charon, and they are too distant for precise telescopic observation, so little is known for certain about their size, composition, atmosphere, or surfaces.
An increasing number of astronomers believe that Pluto, rather than being a planet, is really an unusually large comet whose true family lies within the Kuiper belt. In 1999, however, the International Astronomical Union (IAU) reaffirmed that Pluto was a planet because of its size and its satellite, something no comet was known to have, but recent discoveries have brought Pluto's status into question once again. One Kuiper belt object, designated 1988 W31, has a satellite, and another, 2002 LM60 (Quaoar), is more than half as large as Pluto.
See W. Hoyt, Planets X and Pluto (1980); S. A. Stern and J. Mitton, Pluto and Charon (1999).
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