海王星(Neptune)
从太阳算起的第八颗行星，1846年发现，用罗马海神命名。离太阳的平均距离为45亿公里，绕太阳一圈需165年，每16.11小时自转一周。海王星的质量比地球质量的17倍还多，体积是地球的58倍，在它大气层顶部的引力比地球的强12%。它的赤道直径为49,528公里。海王星主要由氢和氦组成。它没有明显的固体表面，但可能有一个冻结的坚固的核心。它的大气中包含了大量的甲烷气体，甲烷吸收红光，使海王星看上去是深蓝色的。1989年发射的「旅行者号2号」太空探测器发现了速度超过每秒700公尺的风，是任何一个太阳行星上最快的。还发现一些暗斑，可能是与木星的大红斑类似的风暴。海王星只能接收到很少的太阳辐射，但测得的温度约为-214，暗示其内部可能有热源。海王星的弱磁场使太阳风和宇宙线陷在围绕该行星的一条环带上。海王星至少有四个环，主要由尘埃大小的粒子组成。已知海王星有八颗卫星；最大的是特里同，几乎与地球的月球一样大。
English version：
Neptune
Eighth planet from the sun, discovered in 1846 and named after the Roman god of the sea. It has an average distance of 2.8 billion mi (4.5 billion km) from the sun, taking 165 years to complete one orbit and rotating every 16.11 hours. Neptune has more than 17 times earth's mass, 58 times its volume, and 12% stronger gravity at the top of its atmosphere. It has an equatorial diameter of 30,775 mi (49,528 km). Neptune consists largely of hydrogen and helium. It has no apparent solid surface but may have a frozen, rocky core. Its atmosphere contains substantial amounts of methane gas, whose absorption of red light causes Neptune's deep blue color. The Voyager 2 space probe in 1989 discovered winds of over 700 meters per second, the fastest of any of the sun's planets, and dark spots that may be storms similar to Jupiter's Great Red Spot. Neptune receives little solar radiation, but temperature measurements of about −353°F (−214°C) suggest an internal heat source. Neptune's weak magnetic field traps solar wind and cosmic rays in a belt around the planet. Neptune has at least four rings, made largely of dust-sized particles, and eight known satellites; the largest is Triton, almost as big as earth's moon.