Neptune

Neptune is the eighth planet from the Sun. Like Saturn and Uranus it is a gas giant.

Image of Neptune from NASA, L. Sromovsky, and P. Fry (University of Wisconsin-Madison)

It contains a small rocky core surrounded by an icy mantle of frozen water, methane and ammonia. Its diameter is almost four times the Earth's. Neptune has a magnetic field. Unusually for a planet, the magnetic axis is tilted far from the rotation axis. It is thought that the field may originate in the mantle rather than the core. Neptune radiates almost 3 times more energy than it receives from the Sun. The origin of this energy is not understood.

The outer atmosphere is mainly hydrogen and helium, with a little methane. The atmosphere contains clouds and large spots, somewhat like Jupiter's Great Red Spot except they change far quicker. The highest clouds contain crystals of methane ice. Lower ones may contain frozen ammonia or hydrogen sulfide. There is also a haze of hydrocarbons produced by the action of sunlight on methane.

Neptune has 8 moons and probably incomplete rings.