什麼是行星

環繞太陽或其他恆星的大型物體，通常大於彗星、流星體（參閲meteor）或衞星。Planet一詞來自希臘文，意思是流浪漢，因為行星相對於恆星的位置不斷變動着。已知繞行太陽有九大行星，距離太陽由近而遠分別是水星、金星、地球、火星、木星、土星、天王星、海王星與冥王星。前四個稱為類地行星，再來四個是類木行星。冥王星不屬於這兩類，而像是類木行星的冰質衞星。類地行星的直徑小於1萬3000公里，由巖石組成，大氣較為稀薄或微不足道。一般認為是太陽的熱使得原始太陽星雲的氣體無法在這些行星大量凝結。類木行星形成的位置較遠，温度低，氣體能夠凝結，因此這些行星發育得極為巨大，並保有大量以輕質氣體組成的大氣，主要是氫和氦。類木行星又稱氣體巨星，結構似乎都很類似，都沒有可以觸及的表面。冥王星是九大行星最小的一個，外頭可能還有行星（如X行星）。內行星和外行星由小行星帶分隔開來，這個帶由數萬個小行星組成。西洋占星術重要的步驟就是將行星放入黃道十二宮之內。亦請參閲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.