什麼是冥王星

太陽系中第九大行星。1930年由湯博（Clyde W. Tombaugh）發現，以希臘神話中地獄之神命名。通常認爲冥王星是已知大行星中最外圍的一個，其到太陽的平均距離約爲59億公里。但是由於其偏心軌道的作用，在其248年的軌道週期中有22年它比海王星更加靠近太陽。由於距離和星體大小的緣故，冥王星通常顯得相對黯淡，即使是透過天文望遠鏡觀察也是如此。冥王星的自轉軸心與其軌道的傾角爲122，這使它的自傳方向近似於反向；自轉週期爲6.387天，與其唯一的一個衛星冥衛一的軌道同步鎖定。冥衛一於1978年被發現。冥王星直徑約爲2,400公里，約是月球大小的三分之二，質量不足地球的1%，表面重力約爲地球的6%。紅外輻射觀測（參閱infrared astronomy）表明冥王星極冠的成分是甲烷冰，這種冰態冠有時會延伸到距冥王星赤道半程處。據估計，冥王星的平均表面溫度爲零下205攝氏度。它稀薄的大氣中包含甲烷和很可能一些較重的氣體，可能是氮。

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.