planet, Any large natural body that orbits the Sun or another star (see planets of other stars) and that is not radiating energy from internal nuclear fusion; dwarf planets, comets, asteroids, meteoroids (see meteor), and natural satellites are excluded. The word planet comes from the Greek for “wanderer,” because the planets’ positions change relative to those of the stars. The eight (formerly nine) recognized planets that orbit the Sun are, in order of increasing distance, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The first four are called terrestrial planets and the next four giant, or Jovian, planets. The terrestrial planets (of which Earth is the largest) are rocky with comparatively thin or negligible atmospheres. The Sun’s heat is thought to have prevented the abundant volatile substances in the solar nebula from condensing in them as they formed. The giant planets formed farther out, where the gases were cool enough to condense, so the planets grew very massive and accumulated huge atmospheres of light gases, mainly hydrogen and helium. None of the giant planets has an accessible surface; their gaseous atmospheres gradually merge with their liquid interiors. Pluto, although it was recognized as a planet for decades, is distinct from either group, being much smaller than any of the eight planets and resembling a giant comet nucleus. In 2006 the International Astronomical Union demoted Pluto to the category of dwarf planet, which reflected astronomers’ conclusion that it is a very large member of the Kuiper belt. Additional bodies rivaling or exceeding Pluto in size (e.g., Eris) exist in the outer region of the solar system. The term minor planet is sometimes used to refer to any of the asteroids that lie mostly between the terrestrial and the giant planets. In astrology great importance is placed on the planets’ positions in the 12 constellations of the zodiac. See also planetesimal; solar system.
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