Ninth planet from the Sun.
It was discovered in 1930 by Clyde W. Tombaugh (1906–97) and named after the Greek god of the underworld. It is usually the outermost of the known planets, averaging about 3.7 billion mi (5.9 billion km) from the Sun (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 120k, so it rotates nearly on its side and "backward" (see retrograde motion ) once every 6.39 days, locked synchronously with the orbit of its single moon, Charon, discovered in 1978. Pluto has a diameter of about 1,455 mi (2,340 km), roughly two-thirds that of the Moon, and has less than 1% of Earth's mass and only about 6% of its surface gravity. Its estimated average surface temperature is near -390 kF (-235 kC). 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 have sparked debate over its classification as a major planet.