Neptune is the eighth and final planet in the Solar System. It was discovered in 1856 by German astronomer Johann Gottfried Galle at the Berlin Observatory in Germany.

Diameter: 49,500 km
Mass: 1.02 x 1026 kg
Composition: Hydrogen, helium and ice
Orbit: 4,500,000,000 km from the Sun
Distance from other planets: View matrix

As one of the four gas giant planets, Neptune belongs to a group which are composed in the main of the gases hydrogen and helium. There are three other gas giants, namely Jupiter, Saturn and Uranus. Neptune was the first planet to be found using a system based on a prediction of where other planets would lie in the Solar System. Johann Gottfried Galle discovered Neptune using calculations provided to him by French mathematician Urbain Joseph Le Verrier. Less than three weeks after the discovery of the planet, Neptune’s biggest moon, Triton, was found.

Neptune is named after the Roman god of the sea, which is in keeping with the mythological names of the other planets – with the exception of Earth. Up until the discovery of Pluto, Neptune was the last planet of the Solar System. The addition of Pluto as the ninth planet stripped it of this title, but Pluto’s subsequent reclassification as a dwarf planet now makes Neptune once again the furthest planet from the Sun.

Neptune is the third-heaviest and fourth-biggest planet in the Solar System. Its composition is largely made up of the gases hydrogen and helium. As with Uranus, it also has a high proportion of ice present. This is due to the extreme cold, which in turn is due to its huge distance from the Sun. Its distinctive blue colour is thought to be as a result of large quantities of methane present in the atmosphere. Neptune has six rings which are not thought to have existed for very long. However, they appear to be incomplete and there has been evidence since the Voyager 2 mission of 1989 that they are rapidly disintegrating. On the surface, there is known to be patterns of weather and it is extremely windy.

There are 13 satellites which orbit Neptune. Triton, the largest moon, orbits the planet in a retrograde orbit unlike the other moons. On Triton, the surface temperature is also very cold at about -235 degrees Celsius. Neptune orbits the Sun in 165 years. In 2010, it completed it first orbit since its discovery.

Neptune cannot be seen with the naked eye as it is too far away. In 1989, the only spacecraft to have visited Neptune so far, Voyager 2, travelled close to the planet. It came within 4,800 km. Most observations have been conducted from Earth using powerful telescopes. Five moons were discovered in this way and astronomers have also been studying Neptune’s rings from the Keck Observatory.

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