Neptune

Neptune, fourth-largest planet in the solar system and the eighth from the sun, with a mean distance of 2.7941 billion mi (4.4966 billion km). Its diameter is about 30,760 mi (49,500 km), and it has 2 known moons. Its atmosphere contains hydrogen, helium, methane, and ammonia. Neptune was first discovered in 1846 by Johann Galle using computations by Urbain Leverrier based on the irregularities of Uranus's orbit. Neptune circles the sun once for every 164.8 earth years, and its day (one rotation on its axis) is 15.8 hours.

See also: Planet; Solar System.