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 1% of Earth's mass, and only about 6% 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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