Pluto

Pluto, in astronomy, ninth planet of the solar system, orbiting the sun at a mean distance of 3.67 billion mi (5.9 billion km) once every 248.4 years. Pluto was discovered in 1930 following observations of perturbations in Neptune's orbit. Little is known of Pluto's composition, atmosphere, or mass. Its diameter is estimated to be between 1,500 mi (2,400 km) and 2,400 mi (3,800 km). Its orbit is very eccentric, occasionally bringing it closer to the sun than Neptune. It may have originated as a satellite of Neptune. In 1978 the U.S. astronomer James Christy discovered that Pluto had a satellite, named Charon, with a diameter perhaps as large as a third of Pluto's itself.

See also: Planet; Solar System.