Fourth largest planet and one of the four gas giants in our solar system. At a mean distance of 4.5 billion km from the Sun Neptune has a volume 72 times that of the Earth and a mass that is 17 times greater than that of the Earth. Its atmosphere is made up mainly of hyrdrogen and helium with a small percentage of methane which gives the planet its blue colour. There are eight moons known to be in orbit around Neptune, the largest being Triton which is only slightly smaller than our own Moon.
NASA's Voyager spacecraft have to date been the only probes to visit Neptune.
Related entries: Deimos
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