PLUTO, in astronomy, regarded as the ninth planet from the sun and the smallest and coldest planet in the solar system. Pluto was discovered as the result of a telescopic search inaugurated in 1905 by the American astronomer Percival Lowell, who postulated the existence of a distant planet beyond Neptune as the cause of slight perturbations (see orbits) in the motions of Uranus. Continued by members of the Lowell Observatory staff, the search ended successfully in 1930, when the American astronomer Clyde William Tombaugh (1906--97) found Pluto near the position Lowell had predicted.In 2006, Pluto was reclassified as a "dwarf planet."

Pluto revolves about the sun once in 247.92 years at an average distance of 5,906,380,000 km (3,670,050,000 mi). The orbit is so eccentric that at certain points along its path Pluto is closer to the sun than is Neptune. No possibility of collision exists, however, because Pluto's orbit is inclined 17.14° to the plane of the ecliptic and never actually crosses Neptune's path.

For many years very little was known about the planet, but in 1978 astronomers discovered a relatively large moon orbiting Pluto at a distance of only about 19,600 km (about 12,200 mi) and named it Charon. The orbits of Pluto and Charon caused them to pass repeatedly in front of one another from 1985 through 1990, enabling astronomers to determine their sizes fairly accurately. Pluto is 2302 km (1430 mi) in diameter, and Charon is 1186 km (737 mi) in diameter, making them even more closely a double-planet system than are the earth and its moon. Pluto was also found to have a very thin atmosphere, probably of methane and nitrogen, exerting a pressure on the planet's surface that is less than 1/300,000th of the earth's atmospheric pressure at sea level. The atmosphere appears to condense and form polar caps during Pluto's long winter.

Viewed from earth-based telescopes, Pluto appeared to have a yellow color. Enhanced images of Pluto taken by the Hubble Space Telescope were released in 1996. They showed that Pluto has a mottled appearance; its icy surface has large bright and dark patches (probably caused by frost), including a bright northern polar ice cap. Scientists believe the darker regions may consist of old methane frost, while the brighter regions may be fresh nitrogen ice. The overall surface is salmon-colored, the result of ultraviolet sunlight converting some of the ice into complex hydrocarbons.

With a density about twice that of water, Pluto is apparently made of much rockier material than are the other planets of the outer solar system. This may be the result of the kind of cold-temperature/low-pressure chemical combinations that took place during the formation of the planet. Some astronomers have theorized that Pluto may be a former satellite of Neptune, knocked into a separate orbit during the early days of the solar system. Charon would then be an accumulation of the lighter materials resulting from the collision.

Confirmation of these and other hypotheses will likely await the first mission to Pluto and the Kuiper Belt, which was launched by the National Aeronautics and Space Administration (NASA) in 2006. The mission, called New Horizons, swung past Jupiter in 2006, reach Pluto and Charon in 2015, and then explore the Kuiper Belt.

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