All you wanted to know about Neptune in a 12,000 work eBook Short Read

Neptune was discovered in 1846 after two mathematicians - John Couch Adams and Urbain Le Verrier - independently predicted that only the existence of an undiscovered planet beyond the orbit of Uranus could explain the latter's puzzling behaviour.

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Neptune and Uranus were traditionally classed as gas giants along with Jupiter and Saturn, albeit the latter pair are much larger. In common with those of Jupiter and Saturn, the atmosphere of Neptune is primarily composed of hydrogen and helium, but it also contains ices including water, ammonia, and methane. Accordingly, many astronomers now refer to Neptune and Uranus as 'ice giants' rather than gas giants.

Illustrations in 1960s books on astronomy invariably depict Uranus and Neptune as identical grass-green worlds. As revealed by Voyager 2, Uranus is pale cyan, whereas Neptune is predominantly blue. The cyan colour results from the absorption of red light by methane in the atmosphere of Uranus. The deeper azure hue of Neptune is thought to be due to the presence of an additional and as yet unidentified atmospheric component.

Neptune is the only one of the Solar System's major planets that never becomes bright enough to be visible to the naked eye.

Pluto was once popularly thought to be an escaped moon of Neptune. One proposal was that interactions with the Triton caused Pluto to be flung out of the Neptunian system. The process also reversed the direction of Triton's orbit around Neptune. The theory assumed that Pluto was significantly larger than Triton, whereas it is slightly smaller. It was the discovery that Pluto had a large moon, Charon, that finally ruled out the theory.

Neptunium, the 93rd element in the periodic table was first produced in the laboratory in 1940. As the 92nd element had been named uranium after Uranus, it was decided to name the new element after Neptune. Element 94, produced in the laboratory a few months later, was given the name plutonium after Pluto. Neither neptunium nor plutonium exists in nature.

Neptune has 14 known moons, far fewer than Jupiter or Saturn, or even Uranus. However, unlike the latter, Neptune has one large moon, Triton. The Solar System's seventh-largest moon, Triton is midway in size between the Moon and Pluto.

Although it is not uncommon for moons to orbit their primaries in a retrograde orbit, Triton is the only large moon in the Solar System to do so. The Saturnian moon Phoebe, with a diameter one-eighth that of Triton, is the largest-known retrograde moon, and most are far smaller.

Triton's retrograde orbit means that it is gradually spiralling in towards Neptune. It is already closer to Neptune than the Moon is to Earth and eventually, it will enter Roche limit and be pulled apart by tidal forces. The result will be a ring system many times brighter than the one Saturn currently has. However, this will not happen for around 3.6 billion years.

To date, Neptune has been visited by just one space probe, Voyager 2, in 1989. There have been various proposals for further missions to Neptune, though none have reached the development stage. Trident was a proposed NASA Discovery-class mission, which would have involved a flyby of Neptune and Triton. The relatively low-cost mission would have launched in 2025 and reached the Neptune system in 2038 after a Jupiter gravity assist. But it was passed over in favour of a pair of probes to be sent to Venus. Even if they do eventually fly, spacecraft currently being considered by NASA and the Chinese National Space Administration are unlikely to reach Neptune until around the mid-century.

Since its discovery in 1846, Neptune has completed just over one circuit of the Sun. It completed its first orbit since discovery at 22:27 GMT on 12 July 2011. The second will not be completed until 2176.

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