Neptune is the eighth planet of the Solar System. It travels around the Sun once every 164.8 years at an average distance of 4 billion 486 million kilometers (2 billion 790 million miles). Neptune is so faint that it cannot be seen with the unaided eye. When viewed through a telescope of moderate power, Neptune appears as a small disk with a greenish colour. The disk becomes noticeably darker towards its edges; an effect, which is called limb darkening by astronomers.
Named after the Roman god of the sea - equivalent to the Greek god Poseidon - Neptune is a giant planet, like Jupiter, Saturn, and Uranus. Neptune is a world completely hostile to life, as we know it. It is 17 times as massive as the Earth and consists probably of a rocky core about 20,000 kilometers (12,400 miles) in diameter shrouded in a dense atmosphere 15,000 kilometers (9,300 miles) thick and containing mostly hydrogen with a little helium.
Neptune's existence became known long before it was actually discovered. Its orbit was worked out mathematically from the gravitational effects it had on Uranus.
Facts About Neptune
Average Distance from the Sun: 4 billion 486 million km (2 billion 790 million miles)
Length of Year: 164.8 Earth years
Period of Rotation: 15.8 hours
Diameter: 50,950 km (31,600 miles) including atmosphere
Mass: 17.2 (Earth=1)
Density: 1.66 (water=1)
Average Temperature of Outer Atmosphere: -200°C (-330°F)
Number of Known Satellites: 2
© 1988 Encyclopaedia Britannica, Inc.