Neptune

December 12, 1998

Neptune is the eighth planet from the Sun, 30 times farther from the Sun than the Earth. It is also the fourth largest planet and is the fourth and farthest of the giant gas planets. Neptune's mass is 17 times Earth's mass. Neptune's diameter is 4 times Earth's diameter. Because of gravity, its gas is so thick it is thicker than water. However, because it is so big, its surface gravity is only a little more than Earth's. 

Neptune takes 165 years to go around the Sun. Neptune has an 16-18 hour day. Neptune has a magnetic field. Its surface temperature is only 50 K. 

Neptune's composition is similar to Uranus' with methane, hydrogen, and helium. Like Uranus, but unlike Jupiter and Saturn, it may not have a distinct internal layering but rather to be more or less uniform in composition. There is most likely a small core (about the mass of the Earth) of rocky material. Its atmosphere is mostly hydrogen and helium with a small amount of methane. 

Neptune's blue color is the result of absorption of red light by methane in the atmosphere.  Like Jupiter and Saturn, Neptune has an internal heat source. It radiates more than twice as much energy as it receives from the Sun.

Like a typical gas planet, Neptune has rapid winds confined to bands of latitude and large storms or vortices. Neptune's winds are the fastest in the solar system, reaching 2000 km/hour. A large dark oval revolves around Neptune every 18 hours. A second dark spot revolves around Neptune every 16 hours. 

Neptune has 8 known moons. Triton is the main moon and is bigger than Earth's moon. 

The gravitational effects of one of the moons define the rings. The Adams Ring is 63,000 km from the center of Neptune, the Leverrier Ring is 53,000 km, and the Galle Ring is 42,000 km, so all are closer to Neptune than the Earth's moon is to Earth.

Neptune was first observed in 1846. Neptune has been visited by only one spacecraft, Voyager 2 on Aug 25 1989.