Among the solar system planets,
it is most similar to Uranus,
having a slightly larger mass and a slightly smaller radius.
It has 16 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.