Neptune

Neptune is the outer-most known planet in the solar system. Characteristics:

• 17 Earth masses.
• 3.86 Earth radii.
• 16 hours - solar day.
• 165 years - orbital period.
• 30 AU - orbital radius (semi-major axis).
• 0.008678 - orbital eccentricity.

Among the solar system planets, it is most similar to Uranus, having a slightly larger mass and a slightly smaller radius. It has 14 known moons, of which one, Triton, is substantial. Triton has a retrograde orbit, suggesting it is a captured minor planet. Pluto and Neptune have a 3:2 orbital resonance. Reaching Neptune by spacecraft is a challenge (suitable launch windows are very infrequent) and the only mission to approach it was a flyby by Voyager 2 in 1989. Among its observations was a spot on the surface analogous to Jupiter's, Neptune's called the Great Dark Spot (GDS or GDS-89), a storm in Neptune's atmosphere. Subsequent HST observation has shown the spot disappeared and later has shown another such spot.

The term Neptune is also used for Neptune-like extra-solar planets, specifically significantly larger than Earth and smaller than Jupiter or Saturn, and typically a gas planet, perhaps with other Neptune-like characteristics.