Neptune is the eighth and last planet in the Solar System. It is smaller than Uranus and is distinctly blue in colour.

Neptune as seen by Voyager 2 in 1998 Credit: NASA

Neptune is an ice giant, similar to Uranus, with its atmosphere containing mainly hydrogen, helium and some methane. It orbits the Sun once every 165 years. This means that it has only completed one full orbit since it was discovered in 1846. A day on Neptune (the time it takes to spin once) is 16 hours on Earth.

Neptune is 4.5 billion kilometres from the Sun. This is a huge distance, when you consider the distance from the Earth to the Sun is about 152 million kilometres. At this distance the Sun would be small and faint. Neptune is a very dark and cold planet. Large storms regularly form on Neptune and wind speeds can be up to 2000 km/h, the fastest in the Solar System.

Neptune has a ring system containing 5 rings, but it isn’t as grand as Saturns rings. There is one area of interest in Neptune’s rings called arcs. These are areas where there are clumps of material that forms an arc. Normally, one would expect these clumps to even out across the rings but scientists think the clumps are caused by the gravitational interaction of the moons, of which there are 14. The planet it named after the Roman god of the sea and the moons after lesser sea gods. The largest moon is called Triton. It is a very cold moon with geysers that spray icy material into space and a small atmosphere, which was discovered by Voyager.

Neptune is the only planet that is not visible to the human eye. Therefore it was not known to ancient people the same way that Mercury, Venus, Mars, Jupiter and Saturn were. Neptune had been spotted a number of times by various astronomers but mistaken for a star. It appears that some of the first sightings were recorded by Galileo, when he plotted a star on his charts that matched the position of Neptune on 28th December 1612 and 27th January 1613. Interestingly, Neptune was the first planet discovered using mathematics.

After Uranus was discovered, astronomical tables of its orbit were published by a French astronomer called Alexis Bouvard. Subsequent follow up observations showed that Uranus didn’t follow his predictions. One explanation for this was that another, as of yet unknown planet, was pulling on Uranus. British Astronomer, John Couch Adams and French astronomer, Urbain Le Verrier, independently began to try and calculate the position of the undiscovered planet. Initially, neither scientist received much support in their effort.

However, in 1845, Le Verrier published some of his calculations. When the Astronomer Royal, Sir George Airy, realised how similar his calculations were to Adams, he immediately asked James Challis, the Director of Cambridge Observatory to look for the planet and his search began in August 1846. Meanwhile, Le Verrier sent a letter to Johann Gottfried Galle in the Berlin Observatory, asking him to check his latest prediction for the location of the planet. He received the letter on 23rd September 1846, and that night discovered Neptune only a small distance from where Le Verrier had predicted.

It later turned out James Challis, had seen it twice in August of the same year while searching but mistook if for a star due to out of date start charts! Following the discovery, there was a heated debate between France and Britain as to who deserved credit for the discovery. Eventually consensus was reached that both Adams and Le Verrier deserved equal credit.

Due to its distance and apparent size from Earth, it is hard to study Neptune with Earth based telescopes. Telescopes with adaptive optics and the Hubble Space Telescope are now used to study Neptune. The only spacecraft to have visited Neptune is Voyager 2, in 1989. So far there have been no approved missions to study Neptune but it is hoped that one will be launched in the 2020s or 2030s, However, none have been selected so far.

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