Neptune is the eighth and farthest planet from the Sun in our Solar System. It was the first planet to be located by mathematical prediction: discrepancies in the observed motion of Uranus led Urbain Le Verrier to predict its existence and position; Johann Galle and Heinrich d’Arrest confirmed it within 1° of the predicted location on the night of 23 September 1846.

Like Uranus, Neptune is an ice giant — a dense hot fluid of water, methane, and ammonia surrounds a small rocky core, with a hydrogen-helium atmosphere above. Neptune’s atmosphere, however, is notably more dynamic than Uranus’s: supersonic jets and dark, persistent storms like the “Great Dark Spot” observed by Voyager 2 in 1989 (and later, smaller versions observed by Hubble) mark its visible cloud deck.

Neptune has 16 known moons and 5 rings. Its largest moon, Triton (2,710 km diameter), is unique among large moons in orbiting retrograde — strong evidence that Triton is a captured Kuiper Belt object. Voyager 2 observed nitrogen geysers actively erupting on Triton’s surface in 1989. NASA’s 2023 Planetary Science Decadal Survey identified Neptune/Triton exploration as a high-priority mission concept.