Planet Neptune Facts
planet neptune
A Voyager 2 image of Neptune, it is the only spacecraft to have visited the planet
  • Neptune is the furthest planet from the sun and the fourth largest.
  • It has a diameter of 30,775 miles (49,528 km) which is four times larger than Earth.
  • Neptune is 30 times further away from the sun than Earth is.
  • The dwarf planet Pluto moves inside Neptune's orbit every 248 years, for periods of 20 years.
  • Neptune and Uranus are categorized as "ice giants" due to their high levels of water, ammonia and methane.
  • Neptune gets its blue color from clouds of frozen methane in its atmosphere.
  • Winds in Neptune's atmosphere can reach 1300 miles (2,100 km) per hour, the fastest of any planet in the solar system.
  • The dark spot in Neptune's atmosphere was a violent storm similar to the red spot found on Jupiter, it has disappeared since the Voyager 2 visit.
  • Neptune was discovered as a result of mathematical forecasting before it was actually observed.

Neptune's dark spot
Close up of Neptune's Great Dark Spot
Neptune's Atmosphere
The atmosphere of Neptune is very similar in composition to that of its far neighbor Uranus, made up of around 80% hydrogen, 18.5% helium, and 1.5% methane. But rather than being a featurless ball like Uranus Neptune has distinct cloud patterns across its surface with visible bands of cloud formation swirling around the planet.
Neptune's atmosphere also features storms in the form of spots similar to those found on Jupiter, the Great Dark Spot pictured left was a prominent feature when Voyager 2 arrived at Neptune in 1989, it has since disappeared although new storms have emerged. Another surprising aspect of Neptune is the enormous wind speeds the planet generates, regularly reaching over 2,000 kilometers per hour, the fastest wind speeds of any planet in the solar system.
All of this atmospheric activity is thought to be driven by a warm internal heat source, in fact Neptune receives more than twice as much energy from its interior as it does from the sun. Why this is and why Uranus doesn't have this heat source is still largely a mystery.
Neptune's Orbit
Neptune orbits around the sun every 165 years at an average distance of 2.8 billion miles (4.5 billion km). It spins on its own axis every 16 hours compared to 24 hours on Earth.
Neptune's Temperature
Due to its far distance from the sun Neptune has an extremely cold outer atmosphere with temperatures averaging around -220C (-364F). Temperatures at its center rise to around 5000C (9000F).

Neptune moon Triton
Neptune's largest moon Triton
Neptune has 13 known satellites, the largest being Triton (picture right) which has a diameter of around 1,681 miles (2,705 km). It is the only major moon in the solar system which has a retrograde orbit, meaning it orbits in the opposite direction of Neptune's rotation. It is thought that Triton was possibly a comet or dwarf planet that was captured by Neptune's gravity. Unbelievably Triton is geologically active, volcanoes spew crystals of nitrogen ice 6 miles (10 km) above its surface. The frozen surface of Triton possesses the coldest known temperatures in the solar system, -235C (-390F).
Discovery & Origin of Name
Neptune was discovered in 1843 by the young British astronomer John C. Adams. Scientists had previously known that an unknown planet was interfering with the orbit of Uranus, Adams made some unbelievable mathematical predictions to accurately locate the planet. Unfortunately his submitted work was ignored and it wasn't until three years later when the French astronomer J.J. Leverrier made the same predictions which led to the planet being found by an observatory in Berlin. Adams and Leverrier are both credited with its discovery. Leverrier suggested the name Neptune after the Roman god of the sea.

neptune's clouds
Neptune's clouds close up
Planet Neptune Statistics

Diameter: 30,775 miles (49,528 km)
Average Distance from Sun: 2.8 billion miles (4.5 billion km)
Orbital Period: 165 years
Rotation Period: 16.11 hours

Maximum Temperatures: 5000C (9000F) (Core)
Minimum Temperatures: -220C (-364F) (Cloud Tops)

Gravity: 10.71 m/s2 (1.1 x Earth's Gravity)
Density: 1.76 g/cm3 (31.7% Earth's Density)
Mass: 1.0244 x 1026 kg (17 x Earth's Mass)
Volume: 6.2526 x 1013 km3 (58 x Earth's Volume)

Atmosphere: 80% hydrogen, 18.5% helium, and 1.5% methane.