Neptune is often seen as Uranus' twin planet. It is similar in size and colour, and has a similar chemical make-up. It was discovered in 1846 by Johann Gotfried Galle when scientists realised that it was difficult to predict Uranus' orbit. It became obvious that there must be something further away in space pulling on Uranus. This object turned out to be Neptune.
The giant blue planet, slightly smaller than Uranus at 49,500 kilometres wide (Uranus is 52,000 kilometres wide) takes 165 years to orbit the Sun, meaning that it never completes a complete journey around the Sun during the lifetime of anybody on Earth, and since its discovery, it has only orbited the Sun once. The completion of its first orbit was as recent as July 2011. It may surprise you to know that Earth, is the next biggest planet in the Solar System, but it could fit into Neptune 60 times!
Voyager 2, a space craft designed to visit the four Gas Giants (Jupiter, Saturn, Uranus and Neptune) arrived at Uranus in 1986, and revealed a bland, boring world. Scientists half-expected Neptune to be similar. However, pictures from Voyager, when it arrived at Neptune on 24th August 1989, (12 years after the Voyager probes were launched) showed Neptune to be a deep blue-coloured planet with white, whispy clouds in its atmosphere, the strongest winds in the Solar System, and a dark spot, similar to Jupiter's Great Red Spot, which was the size of Earth. Incomplete rings, looking like arcs, orbited the planet, suggesting that they haven't had time to completely form yet, and one of Neptune's moons (Triton) showed that, with its eruptions of nitrogen gas, even at the furthest reaches of the Solar System, geological activity still takes place.
Neptune, like Jupiter, Saturn and Uranus, has a thick gassy atmosphere full of mainly hydrogen and helium. Below this are huge amounts of frozen material, possibly water and liquid ammonia. The planet also contains Methane which gives the planet a blue colour, like Uranus. Winds on Neptune can blow at speeds up to 2,000 kilometres an hour!
Surrounding Neptune are thirteen known moons. These moons are all very small, apart from Triton, an active moon. Between the years 1930 and 2006, Neptune was the eighth of nine planets in the Solar System. During this time, Pluto was the ninth planet and most distant. In 2006, astronomers decided to classify Pluto as a Dwarf Planet and Neptune regained its status as the Solar System's most distant planet, and therefore the last of the planets. Even while Pluto was a regarded as a planet, there were times when it came closer to the Sun than Neptune and temporarily made Neptune the most distant.