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Pluto (planet), ninth planet from the Sun and outermost known member of the solar system. Pluto was discovered as the result of a telescopic search inaugurated in 1905 by American astronomer Percival Lowell, who postulated the existence of a distant planet beyond Neptune as the cause of slight perturbations (see Orbit) in the motions of Uranus. Continued by members of the Lowell Observatory staff, the search ended successfully in 1930, when American astronomer Clyde William Tombaugh found Pluto near the position Lowell had predicted.

Pluto revolves about the Sun once in 247.7 Earth years at an average distance of 5.9 billion km (3.67 billion mi) from the Sun. 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 more than 17.2° to the plane of the ecliptic and never actually crosses Neptune's path.

Visible only through large telescopes, Pluto appears to have a yellowish color. 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 19,000 km (about 12,000 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. The Hubble Space Telescope allowed astronomers to determine the sizes of Pluto and Charon even more accurately in 1994. Pluto is about 2,320 km (1,440 mi) in diameter, about two-thirds the size of Earth's moon. Charon is about 1,270 km (790 mi) in diameter, making Pluto and Charon the planet-satellite pair closest in size in the solar system. Pluto was also found to have a thin atmosphere, probably of methane, exerting a pressure on the planet's surface that is about 100,000 times weaker than Earth's atmospheric pressure at sea level.

In 1994 the Hubble Space Telescope imaged 85 percent of Pluto's surface, revealing bright and dark areas of startling contrast. Astronomers believe that the bright areas are shifting fields of nitrogen ice and the dark areas are fields of methane ice colored by interaction with sunlight. Some of the dark areas may also be valleys or fresh impact craters. These images support the theory that extensive ice caps form on Pluto's poles, especially when the planet is farthest from the Sun. Observations made in 1999 from the Subaru telescope at the Mauna Kea Observatory in Hawaii confirmed earlier findings about Pluto’s atmosphere and also revealed the presence of ethane ice on Pluto.

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 chemical combinations that took place during the formation of the planet under cold temperatures and low pressure. Many astronomers think Pluto may be a former satellite of Neptune, knocked into a separate orbit during the early days of the solar system. Charon could be an accumulation of the lighter materials resulting from the collision. Other astronomers believe that Pluto and Charon are just the most visible objects in an area of the solar system called the Kuiper belt. The Kuiper belt is a ring of material orbiting the Sun beyond the planet Neptune that contains millions of rocky, icy objects like Pluto and Charon. No matter what Pluto’s origins are, it will probably always retain its recognition as a planet for historical and traditional reasons.