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The fourth-largest planet in the solar system, 30,540 miles (49,160 kilometers) across. Neptune is the eighth planet in distance from the sun. Neptune orbits the sun every 165 years at an average distance of about 2,794 million miles (4,498 million kilometers) and rotates about once every 16 hours. Much of what we now know about Neptune was discovered by the Voyager 2 flyby in 1989 which passed within a few thousand miles of Neptune. Neptune has a surface of liquid hydrogen. Its atmosphere is composed of ammonia, helium, and methane. The methane in the upper atmosphere freezes and forms an ice cloud which casts a shadow on the clouds below. Neptune has large, dark ovals on its surface which astronomers believe are hurricane-like storms. Neptune has a ring system consisting of four rings; two thin and two thick. The rings are composed of dark particles which vary in size. Neptune has thirteen known natural satellites, four of which orbit within the rings. The largest satellite is Triton. Triton is thought to be a combination of rock and ice. Neptune was discovered because Uranus (the planet closest to Neptune) did not follow the orbital path it was predicted to follow. Astronomers thought that the gravitational pull of another object was affecting the orbit of Uranus. This led them to look for an object influencing Uranus's path and they discovered Neptune.