Moons of Neptune

There are 16 known moons of the planet Neptune, fourteen of which are named after water deities and creatures in Greek mythology. The largest is Triton, discovered by William Lassell on 10 October 1846, 17 days after the discovery of Neptune itself. Over a century passed before the discovery of the second natural satellite, Nereid, in 1949, and another 40 years before Proteus, Neptune's second-largest moon, was discovered in 1989.

Triton is unique among moons of planetary mass in that its orbit is retrograde to Neptune's rotation and inclined relative to Neptune's equator, which suggests that it did not form in orbit around Neptune, but was instead gravitationally captured by it. The next-largest satellite in the Solar System suspected to be captured, Saturn's moon Phoebe, has only 0.03% of Triton's mass. The capture of Triton, probably occurring some time after Neptune formed a satellite system, was a catastrophic event for Neptune's original satellites, disrupting their orbits so that they collided to form a rubble disc. Triton is massive enough to have achieved hydrostatic equilibrium and to retain a thin atmosphere capable of forming clouds and hazes.

Inward of Triton are seven small inner moons, Neptune's only regular satellites, all with prograde orbits close to the planet's equatorial plane; some orbit among Neptune's rings. Including the largest, Proteus, they were re-accreted from the rubble disc formed after Triton's orbit became circular. Beyond Triton, Neptune has eight outer irregular satellites: four retrograde and four prograde. Among the prograde satellites is Nereid, the largest of the eight outer moons, which orbits much farther from Neptune at a high inclination. Its orbit is unusually close and highly eccentric for an irregular satellite, suggesting it may once have been a regular satellite significantly perturbed when Triton was captured. The outermost moons of Neptune are the furthest moons from their primaries in the Solar System.