Neptune is about 49,500 kilometres across. This planet is about 17 times as massive as the Earth. It is the farthest of the gas giants from the Sun, orbiting at 4.4 billion kilometres from the Sun, making a complete journey once every 165 years, although it turns on its axis once every 16 hrs.

Like Uranus, its atmosphere is made of hydrogen and helium and, because it has a large magnetic field, it is thought the planet has a molten rocky core. The surface is very cold, -220 degrees Celsius, yet even this is warmer than would be expected if there was not a source of heat from the core.

A system of dust rings surrounds the planet.

The gas in Neptune's atmosphere moves so fast that it is the windiest place in the Solar System. The strongest winds blow at over 2,200 kilometres an hour.