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Any large body orbiting the Sun or another star (see planets of other stars ); comet s, asteroid s, meteoroids (see meteor ), and natural satellite s are excluded.
The word comes from the Greek for "wanderer," because their positions change relative to those of the stars. The nine major planets known to revolve around the Sun, in order of increasing distance, are Mercury , Venus , Earth , Mars , Jupiter , Saturn , Uranus , Neptune , and Pluto . The first four are called terrestrial planets and the next four Jovian planets. Pluto, distinct from either group, resembles a giant comet or one of the icy satellites of the Jovian planets. The terrestrial planets are less than 8,000 mi (13,000 km) in diameter and rocky, with comparatively thin or negligible atmospheres. The Sun's heat is thought to have prevented the abundant gases in the original solar nebula from condensing in them. The Jovian 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. Called gas giants, the Jovian planets appear to be similar in structure; none has an accessible surface. Pluto is by far the smallest body traditionally called a major planet, although many astronomers now consider it the largest known member of the Kuiper belt . Additional bodies rivaling Pluto in size may exist in this region of the solar system; one about half its size was discovered in 2002. The term minor planet is sometimes used (less now than formerly) to refer to any of the asteroids that lie between the terrestrial and Jovian 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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[c mediumvioletred] (as used in expressions)
Planet X
planets of other stars
extrasolar planets