Pluto, 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. Pluto also has three other small moons. 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.
Pluto Article
Pluto summary
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Kuiper belt Summary
Kuiper belt, flat ring of icy small bodies that revolve around the Sun beyond the orbit of the planet Neptune. It was named for the Dutch American astronomer Gerard P. Kuiper and comprises hundreds of millions of objects—presumed to be leftovers from the formation of the outer planets—whose orbits
solar system Summary
Solar system, assemblage consisting of the Sun—an average star in the Milky Way Galaxy—and those bodies orbiting around it: 8 (formerly 9) planets with more than 210 known planetary satellites (moons); many asteroids, some with their own satellites; comets and other icy bodies; and vast reaches of
Sun Summary
Sun, star around which Earth and the other components of the solar system revolve. It is the dominant body of the system, constituting more than 99 percent of its entire mass. The Sun is the source of an enormous amount of energy, a portion of which provides Earth with the light and heat necessary