Pluto, in astronomy, a dwarf planet and the first Kuiper belt, or transneptunian, object (see comet) to be discovered (1930) by astronomers. Pluto has an elliptical orbit usually lying beyond that of Neptune. Although Pluto was long regarded as a planet, since the discovery (beginning in 1992) of other Kuiper belt objects, including one with a diameter larger than that of Pluto, astronomers have recognized the need to reclassify Pluto, and in 2006 the International Astronomical Union (IAU) ended official recognition of Pluto as a planet.

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 an orbit that is more elliptical and tilted than those of the planets (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 beyond the orbit of Neptune was first proposed by Percival Lowell on the basis of observed perturbations of the orbits of Uranus and Neptune. He began searching for such a 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 ninth 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 transneptunian planet.

In 1978, American astronomers James Christy and Robert Harrington discovered the moon Charon, and two smaller, more distant moons, Hydra and Nix, were reported in 2005 by American astronomers Hal Weaver and S. Alan Stern. Pluto's diameter is c.1,400 mi (2,300 km), Charon's is c.748 mi (1,203 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. Hydra and Nix have diameters of less than 100 mi (160 km). Pluto and Charon 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 it and its moons are too distant for precise telescopic observation, so little is known for certain about their size, composition, surface, and other aspects.

An increasing number of Kuiper belt objects were discovered after 1992, many astronomers came to believe that Pluto, rather than being a planet, was really an unusually large and close Kuiper belt object. In 1999, however, the International Astronomical Union (IAU) reaffirmed that Pluto was a planet because of its size and its satellite, something no other transneptunian object was then known to have, but subsequent discoveries brought Pluto's status into question once again. One Kuiper belt object, now named Eris (and originally nicknamed Xena), whose orbit extends to roughly three times the distance of Pluto's, has an estimated diameter (1,500 mi/2,400 km) slightly larger than that of Pluto and also has a moon. It was the discovery of Eris in particular that ultimately led to Pluto's classification (2006), along with Eris and Ceres, as a dwarf planet; transneptunian dwarf planets are now classified as plutoids.

Pluto, in Greek religion and mythology, god of the underworld, son of Kronos and Rhea; also called Hades. After the fall of the Titans, Pluto and his brothers Zeus and Poseidon divided the universe, and Pluto was awarded everything underground. There, with Persephone as his queen, he ruled over Hades. Not only a god of the dead, he is identified as a god of the earth's fertility. The Romans derived their god of the dead—Orcus, Dis, or Dis Pater—from Pluto.

Solar system body, regarded as the ninth planet from the Sun until struck from the list of planets and reclassified as a dwarf planet in August 2006. It was discovered in 1930 by Clyde W. Tombaugh (1906–97) and named after the Greek god of the underworld. Its average distance from the Sun is about 3.7 billion mi (5.9 billion km)—it is located within the Kuiper belt—but its eccentric orbit brings it closer to the Sun than Neptune for 22 years during its 248-year orbit. Its axis is tipped 120°, so it rotates nearly on its side and “backward” (see retrograde motion) once every 6.39 days, locked synchronously with the orbit of its largest moon, Charon, discovered in 1978. Two additional, small moons were discovered in 2005. Pluto has a diameter of about 1,455 mi (2,340 km), less than 1percnt of Earth's mass, and only about 6percnt of Earth's surface gravity. Its estimated average surface temperature is near −390 °F (−235 °C). Its thin atmosphere contains nitrogen, methane, and perhaps other heavier gases. Pluto is thought to be made of frozen gases with a significant fraction of rocky material. Its size, composition, and orbital location in the Kuiper belt sparked a long debate over its classification as a major planet, which culminated in a decision by the International Astronomical Union to drop it from the planetary ranks.

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