Pluto, large, distant member of the Solar System that formerly was regarded as the outermost and smallest planet. It also was considered the most recently discovered planet. In August 2006 the International Astronomical Union (IAU), the organization charged by the scientific community with classifying astronomical objects, voted to remove Pluto from the list of planets and give it the new classification of dwarf planet. The change reflects astronomers’ realization that Pluto is a large member of the Kuiper belt, a collection of debris of ice and rock left over from the formation of the solar system.
Discovered in 1930, it was named for the Roman god of the underworld. Its largest moon, Charon, is almost its equal in size and the two are often considered a double system. Telescopes have not yet been able to get a clear view of this faraway world, but the spacecraft New Horizons is scheduled to arrive there in 2015. At about 5.9 billion km (3.7 billion miles or 39.5 astronomical units) from the Sun, Pluto is so remote that the Sun’s light takes 5 hours to reach it. It is so cold there that gases such as carbon monoxide exist in ice form.
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