General Information

Average distance from the Sun

4,497 million km
(2,794 million miles)

Length of year

165 Earth years

Average rotation period

19 hours 6 minutes

Diameter at equator

49,530 km

Approximate surface temperature

- 210 0C


After Uranus was discovered, astronomers were able to follow its motion through the sky, calculate its orbit round the Sun and predict where it would be in the future. Some years later, it was clear that Uranus was not keeping to the path they expected. Two mathematicians, John Couch Adams in England and Urbain Le Verrier in France, independently worked out that the gravity of another, unknown planet could be pulling Uranus off course, and they predicted where this planet might be found in the sky. In september 1846, John Galle and Heinrich D' Arrest, who worked at the Berlin Observatory, turned their telescope towards the predicted position and there was the missing planet, Neptune.

Even with today's powerful telescopes, Neptune appears only as a faint blue disc. In many ways, Neptune is very like Uranus and astronomers think it is likely to have a similar structure. Neptune probably has a rocky core about the size of the Earth. The mantle is a mixture of partly frozen water, ammonia and methane. It is surrounded by a dense atmosphere of hydrogen and helium extending about 9,000 km above the planets icy surface.

Voyager 2 images of Neptune captured in August 1989 reveal bands of different shades of blue. The colour is due to methane in the atmosphere. A large darkoval patch, named the Great Dark Spot, is thought to be an immense circulating storm, rather like the Great Red Spot on Jupiter.


Voyager 2 discovered six previously unknown moons orbiting Neptune. Just two were already known from Earth-based observations: Triton and Nereid. Triton is the largest of Neptune's moons. Its diameter is about 2,700 km and it orbits Neptune at a distance of 355,000 km ever 5-9 days, travelling from east to west.

Triton is thought to be one of the coldest bodies in the Solar System, with a
surface temperature of about    -235 0C.

 Nereid is a much smaller moon than Triton. It has an extraordinary elliptical orbit that brings it within 140,000 km of Neptune, and also takes it almost ten million kilometres away. It takes 360 Earth days for Nereid to complete an orbit of Neptune.

Dark Spots on Neptune

Several dark spots have been seen on Neptune. The largest, about the size of Earth, is called the Great Dark Spot. It may be a gigantic storm, like the Great Red Spot on Jupiter. The Great Dark spot was first seen by the Voyager 2 probe in 1989. But in the 1990s, the Hubble Space Telescope could not find it. Nobody is sure why the Great Dark Spot vanished, or whether it will reappear.