Exploring the Planets

Neptune Facts

Discoverer: Galle, Challis, Adams, and Le Verrier (1846)
Spacecraft Encounter(s): Voyager 2
Mean distance from the Sun: 30.06 AU
(4.497 billion km/2.794 billion mi)
Length of Year: 165 years
Rotation Period: 16.11 hours
Mean Orbital Velocity: 5.43 km/s (3.3 mi/s)
Inclination of Axis: 29.6°
Diameter: 49,528 km/30,775 mi
Number of observed satellites: 8


Comparisons With Earth:
Diameter: 3.883 x Earth's
Average Distance from the Sun: 30.06 x Earth's
Mass: 17.14 x Earth's
Density: 0.31 x Earth's

  • Neptune is the outermost of the four gas giants: Jupiter, Saturn, Uranus, Neptune.
  • Because of its distance from the Sun, Neptune's atmosphere is a frigid -225° C (-373° F).
  • The blue-green color of the planet is due to the presence of methane in the atmosphere. The atmosphere consists mostly of hydrogen, helium, and methane.
  • Until the Voyager encounter in 1989, the rings surrounding Neptune were thought to be arcs. We now know that the rings completely circle the planet, but the thickness of each ring varies along its length.