After Uranus was discovered, astronomers were able to follow its motion through the sky, calculate its orbit around 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.
Neptune probably has a rocky core 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 5,600 miles (9,000 km) above the planet's icy surface.
Voyager 2 images of Neptune captured in August 1989 reveal bands of different shades of blue. The color is due to methane in the atmosphere. A large dark oval 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 1,680 miles (2,700 km) and it orbits Neptune at a distance of 221,000 miles (335,000 km) every 5-9 days, traveling from east to west. This makes it rather unusual: all the other larger moons in the Solar System go around the other way.
Triton is thought to be one of the coldest bodies in the Solar System, with a surface temperature of about -455�C
Nereid is a much smaller moon that Triton. It has an extraordinary elliptical orbit that brings it within 87,000 miles (140,000 km) or Neptune, and also takes it almost 6 million miles (about 10 million km) away. It takes 360 Earth days for Nereid to complete an orbit of Neptune.
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