什么是行星

环绕太阳或其他恒星的大型物体，通常大於彗星、流星体（参阅meteor）或卫星。Planet一词来自希腊文，意思是流浪汉，因为行星相对於恒星的位置不断变动着。已知绕行太阳有九大行星，距离太阳由近而远分别是水星、金星、地球、火星、木星、土星、天王星、海王星与冥王星。前四个称为类地行星，再来四个是类木行星。冥王星不属於这两类，而像是类木行星的冰质卫星。类地行星的直径小於1万3000公里，由岩石组成，大气较为稀薄或微不足道。一般认为是太阳的热使得原始太阳星云的气体无法在这些行星大量凝结。类木行星形成的位置较远，温度低，气体能够凝结，因此这些行星发育得极为巨大，并保有大量以轻质气体组成的大气，主要是氢和氦。类木行星又称气体巨星，结构似乎都很类似，都没有可以触及的表面。冥王星是九大行星最小的一个，外头可能还有行星（如X行星）。内行星和外行星由小行星带分隔开来，这个带由数万个小行星组成。西洋占星术重要的步骤就是将行星放入黄道十二宫之内。亦请参阅planetesimal、solar system。

planet

Any large body orbiting the sun or another star (see planets of other stars), usually comets, meteoroids (see meteor), or satellites of a larger body. 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 distance from it, 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 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 retained 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 major planet. There may be planets beyond it (e.g., Planet X). The inner and outer planets are separated by tens of thousands of minor planets, making up the asteroid belt. In astrology, great importance is placed on the planets' positions in the 12 constellations of the zodiac. See also planetesimal, solar system.