Neptune

Neptune orbits the Sun at a distance of almost five thousand million kilometres. At such a great distance it takes the planet 165 years to travel around the Sun. It has not yet made a complete orbit since its discovery in 1846. Although the most distant of all the so-called "gas giants", Neptune shares many properties with its warmer relatives, particularly Uranus. Both planets are of very similar sizes, and spin at similar rates. The interiors of the two planets are thought to be similar, a central rocky core surrounded by a liquid mantle of hydrogen, helium, methane and ammonia. The outer gaseous atmosphere that we observe then surrounds this.

While the discovery of Uranus was made by accident, Neptune's location was carefully calculated using Newton's law of gravitation. It was noticed that the orbit of Uranus deviated slightly from the position predicted by theory. This convinced astronomers that another large planet outside the orbit of Uranus could be tugging Uranus with its gravity. On the first night of the search the planet was found just one degree away from the predicted location. This was seen as a tremendous triumph for the power of Newtonian science.

Neptune’s most obvious feature is its Great Dark Spot, named because it resembles Jupiter’s Great Red Spot. The size of the Earth, this marking is thought to be a region of high pressure that lacks the typical atmospheric methane. Although similar in many ways, the Great Dark Spot seems to have been formed much deeper in the atmosphere than Jupiter’s blemish. The Dark Spot was thought to be a long-term feature, but observations by Voyager 2 and the Hubble space telescope showed the marking disappear rapidly and then reappear elsewhere. Apparently extremely rapid large-scale changes are common on Neptune.

Neptune has two large and six small moons, most of which are extremely dark in colour. Many of the moons have long elongated orbits, and one of the larger moons, Triton actually rotates backwards around Neptune. The temperature on Triton is a chilling -220°C, so cold that even nitrogen is frozen solid. Although Neptune has a smaller mean distance from the Sun than Pluto does, it has been the most distance object in our solar system between January 1979 and March 1999. This is because Pluto follows a long elongated path, which periodically loops inside Neptune’s orbit.