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The last planet

Image of Neptune.
Image of Neptune. / NASA
  • Neptune is the most distant planet from the Sun at almost 4.5 billion km away

When Pluto was downgraded to dwarf planet status in 2006, it put Neptune as being the most distant planet from the Sun at almost 4.5 billion km. It is so far from the Sun that it takes almost 165 earth years to travel once around in its orbit.

Neptune's discovery in 1846 was by pure mathematics. French mathematician Urbain Le Verrier had calculated its position after observing discrepancies in the orbit of Uranus. Le Verrier sent the coordinates to his friend Johann Gottfried Galle at the Berlin observatory asking him to verify. Galle found Neptune on the same night he received Le Verrier's letter, within 1° of the predicted position. The discovery was hailed as one of the most remarkable moments of 19th century science.

At the same time in England, but unknown to Le Verrier, English mathematician John Crouch Adams was also carrying out his own calculations and observations of Uranus and he too came up with the same result. Unfortunately he mailed his results to the Royal Greenwich Observatory two days after Galle had discovered the planet and missed out on becoming the discoverer of a new planet.

Various names were proposed for the new planet such as Janus and Oceanus; Le Verrier wanted to name the planet after himself but this proved very unpopular as France had objected to Uranus being named after its British discoverer, Herschel. Eventually the name of Neptune the Greek God of the sea was adopted, thus keeping up with tradition of naming all of the planets (except Earth) after Greek Gods.

Neptune is the fourth largest planet in the solar system, it is slightly smaller than Uranus and, like Jupiter, Saturn and Uranus , is a gas giant made up of mostly hydrogen, helium and methane. Its atmosphere also contains icy clouds and the fastest wind speeds recorded in the solar system. Particles of icy methane and other gases give Neptune the deep blue colour that distinguishes it from green coloured Uranus. It has at least 14 moons in orbit around it.

Everything we know about Neptune comes from a NASA spacecraft Voyager 2 that flew past the planet in 1989.

You need good binoculars or a small telescope to see Neptune, but on Monday 30 October it will be less than 1º from the moon, helping you to find it easily. Look slightly above and a little to the left of the moon and if you see a tiny blue 'star' then you have found it!