Sao and Laomedeia have prograde orbits, while Halimede, Psamathe, and Neso have retrograde orbits. Water ice has been detected on its surface. It has a highly eccentric orbit that might mean it was once a regular satellite that was disturbed when Triton was captured. It has active geysers and may have a subterranean ocean. Triton is a round moon with a nearly circular orbit. It has a nitrogen atmosphere, like Earth, although Triton's atmospheric pressure is only about 14 μbar. Triton is noteworthy for a few other reasons. It is in synchronous rotation with Neptune and the tidal deceleration means the moon is slowly falling toward the planet. It also means Triton is subject to tidal deceleration and (because it is so massive) that it exerts an effect on the rotation of Neptune. Scientists believe this may mean Triton is a captured object, rather than a moon that formed with Neptune. Its immense size puts it an order of magnitude larger than the next-largest irregular moon in the solar system and larger than the dwarf planets Pluto and Eris. Triton is the only large moon in the solar system that has a retrograde orbit, which means it orbits in the opposite direction of Neptune's rotation. This one moon accounts for 99.5% of the mass orbiting Neptune. Triton is Neptune's largest moon, with a diameter of 2700 km (1700 mi) and mass of 2.14 x 10 22 kg. While the inner moons formed in situ, it's believed all of the irregular moons were captured by Neptune's gravity. While all of the moons have names relating to the god Neptune or to the sea, the irregular moons are all named for daughters of Nereus and Doris, the attendants of Neptune.

Photograph of Triton, the largest moon of planet Neptune. The five inner moons are believed to be regular satellites that formed with Neptune. From their spectra, it is believed their surfaces are water ice containing a dark substance, most likely a mixture of complex organic compounds. The inner moons are dark, with albedo values (reflectivity) ranging from 7% to 10%. All of the other regular moons appear to be elongated, although the smallest ones have not been imaged with much accuracy to date. It resembles a slightly faceted polyhedron. Proteus is the only regular moon that is roughly spherical. S/2004 N1 is Neptune's smallest moon, while Proteus is its largest regular moon and second-largest moon overall. This means they orbit Neptune more quickly than Neptune rotates and that these moons will eventually either crash into Neptune or else break apart. Naiad, Thalassa, Despina, and Galatea are within range of Neptune-synchronous orbit, so they are being tidally decelerated. Galatea sits just inside the most prominent ring, the Adams ring. Naiad and Thalassa actually orbit between the Galle and LeVerrier rings, while Despina may be considered a shepherd moon of the LeVerrier ring. The regular moons are closely associated with Neptune's five dusty rings. Ron Miller / Stocktrek Images / Getty Images Neptune seen from its tiny, distant moon, Nereid. While it is considered an irregular moon because of its inclined, retrograde orbit, that orbit is circular and close to the planet. The other moons are considered irregular, as they have eccentric orbits that are often retrograde and far from Neptune. These moons have circular prograde orbits along the equatorial plane of Neptune. The first seven moons or the inner moons are Neptune's regular moons. The moons can be categorized as regular or irregular. Tiny S/2004 N1 was discovered from the analysis of old images taken by the Hubble Space Telescope. The 14th moon was announced on July 15, 2013. Ground-based telescopes found five more moons in 2001. Voyager 2 discovered Naiad, Thalassa, Despine, Galatea, and Proteus. No other moons were discovered until the Voyager 2 flyby of Neptune in 1989. William Lassell discovered Triton on October 10, 1846, only 17 days after Neptune was discovered. The first moon to be discovered was Triton, which is also the largest one. Moving from closest to Neptune to furthest out, their names are Naiad, Thalassa, Despina, Galatea, Larissa, S/2004 N1 (which has yet to receive an official name), Proteus, Triton, Nereid, Halimede, Sao, Laomedeia, Psamathe, and Neso. Each of the moons is named for a mythological Greek water deity.

Neptune has 14 moons, the latest discovered in 2013.

Neso moon