冥王星(Pluto)
太阳系中第九大行星。1930年由汤博（Clyde W. Tombaugh）发现，以希腊神话中地狱之神命名。通常认为冥王星是已知大行星中最外围的一个，其到太阳的平均距离约为59亿公里。但是由於其偏心轨道的作用，在其248年的轨道周期中有22年它比海王星更加靠近太阳。由於距离和星体大小的缘故，冥王星通常显得相对黯淡，即使是透过天文望远镜观察也是如此。冥王星的自转轴心与其轨道的倾角为122，这使它的自传方向近似於反向；自转周期为6.387天，与其唯一的一个卫星冥卫一的轨道同步锁定。冥卫一於1978年被发现。冥王星直径约为2,400公里，约是月球大小的三分之二，质量不足地球的1%，表面重力约为地球的6%。红外辐射观测（参阅infrared astronomy）表明冥王星极冠的成分是甲烷冰，这种冰态冠有时会延伸到距冥王星赤道半程处。据估计，冥王星的平均表面温度为零下205摄氏度。它稀薄的大气中包含甲烷和很可能一些较重的气体，可能是氮。
English version：
Pluto
Ninth planet from the sun. It was discovered in 1930 by Clyde W. Tombaugh (1906-1997) and named after the Greek god of the underworld. It is usually the outermost of the known planets, averaging about 3.7 billion mi (5.9 billion km) from the sun, but its eccentric orbit brings it closer to the sun than Neptune for 22 years during its 248-year orbit. Because of its distance and size, Pluto always appears relatively faint, even through a telescope. Its axis is tipped 122°, so it rotates nearly on its side and “backwards,” once every 6.387 days, locked synchronously with the orbit of its single moon, Charon, discovered in 1978. Pluto has a diameter of about 1,500 mi (2,400 km), roughly two-thirds that of the moon, and has less than 1% of earth's mass and only about 6% of its surface gravity. Infrared observations (see infrared astronomy) suggest that Pluto has methane ice polar caps, which sometimes appear to extend halfway to its equator. Its estimated average surface temperature is −380°F (−205°C). Its thin atmosphere contains methane and probably some heavier gases, possibly nitrogen.
