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, following the discovery (beginning in 1992) of other Kuiper belt objects, including one with a diameter larger than that of Pluto, astronomers considered reclassifying 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.

Pluto's surface, as imaged by the New Horizons space probe during its flyby in 2015, is complex, with cratered areas and smooth icy areas as well as ice mountains, possible ice volcanoes, and evidence of glacial activity, indicating the planet is geologically active the surface consists largely of frozen nitrogen. Pluto is thought to have a rocky, silicate core surrounded by water ice the thin atmosphere contains nitrogen, carbon monoxide, and methane, and has discrete layers of haze. The surface temperature is estimated to be about −360°C (−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. Together, Pluto and Charon may be considered to form a double dwarf planet system. 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 about 12,180 mi (19,600 km). Pluto and Charon orbit a common center of mass that lies between them, above the surface of Pluto, completing one orbit in about 6.4 earth days. Both keep the same side facing one another at all times because they rotate synchronously as they orbit.

Two smaller, more distant moons, Nix and Hydra, were reported in 2005 by American astronomers Hal Weaver and S. Alan Stern, and two more small moons, Kerebos and Styx, were reported in 2011 and 2012 by American astronomer Mark Showalter. The smaller moons are irregular shaped. Hydra, the largest, is about 33 mi (54 km) along its longest axis Nix, 27 mi (43 km) Kerebos, 7 mi (12 km) and Styx, 4 mi (7 km). The smaller moons orbit at roughly two to three times the distance of Charon, with Styx being the closest, Hydra the most distant, and Nix and Kerebos between them.

As 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 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) as a dwarf planet transneptunian dwarf planets are now classified as plutoids.

See W. Hoyt, Planets X and Pluto (1980) S. A. Stern and J. Mitton, Pluto and Charon (1999) B. W. Jones, Pluto (2010).

The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.

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