Neptune is the 8th planet in the solar system. It lies beyond the orbit of Uranus, at over 4 ½ billion km from the Sun.Neptune is named after the Roman God of the sea. In Greek he is known as Poseidon.It is the fourth largest planet in the solar system. One day on Neptune lasts for 17 hours. It is a very cold place, the average temperature is -220ºC.Neptune takes nearly 165 earth years to travel once around the Sun. It is too far away and too faint to ever be seen with the naked eye.There was great debate about who actually discovered Neptune. In 1843, British mathematician, John Couch Adams calculated the orbit of an eighth planet, but it was German astronomer Johann Gottfried Galle who first spotted the planet in 1846.Neptune is a gas giant composed mainly of hydrogen and helium gas, with a tiny molten rocky core. Methane in the upper atmosphere give it a vivid blue colour.It has an active atmoshere that produces dark spots in the upper clouds, anticyclone storms similar to Jupiter's great red spot.There is a very faint ring system. Probably the result of a minor moon that broke up.Neptune has 13 moons but most are mere pieces of rock. Only one, Triton can really be considered a true moon.Neptune has only been visited by one space craft, Voyager 2 which flew past in 1989. Almost everything we know of Neptune comes from this one mission.