Learn About Neptune's Moons

Know Neptune's 14 Moons

Illustration of the gas giant planet Neptune and its largest moon Triton.
Illustration of the gas giant planet Neptune and its largest moon Triton. Stocktrek Images / Getty Images

Neptune has 14 moons, the latest discovered in 2013. Each of the moons is named for a mythological Greek water deity. 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.

The first moon to be discovered was Triton, which is also the largest one. William Lassell discovered Triton on October 10, 1846, only 17 days after Neptune was discovered. Gerard P. Kuiper discovered Nereid in 1949. Larissa was discovered by Harold J. Reitsema, Larry A. Lebofsky, William B. Hubbard, and David J. Tholen on May 24, 1981. No other moons were discovered until the Voyager 2 fly-by of Neptune in 1989. Voyager 2 discovered Naiad, Thalassa, Despine, Galatea, and Proteus. Ground-based telescopes found five more moons in 2001. The 14th moon was announced on July 15, 2013. Tiny S/2004 N1 was discovered from the analysis of old images taken by the Hubble Space Telescope.

The moons can be categorized as regular or irregular. The first seven moons or the inner moons are Neptune's regular moons. These moons have circular prograde orbits along the equatorial plane of Neptune. The other moons are considered irregular, as they have eccentric orbits that are often retrograde and far from Neptune. Triton is the exception. While it is considered an irregular moon because of its inclined, retrograde orbit, that orbit is circular and close to the planet.

Neptune's Regular Moons

Neptune seen from its tiny, distant moon, Nereid. (Artist's conception)
Neptune seen from its tiny, distant moon, Nereid. (Artist's conception). Ron Miller/Stocktrek Images / Getty Images

The regular moons are closely associated with Neptune's five dusty rings. Naiad and Thalassa actually orbit between the Galle and LeVerrier rings, while Despina may be considered a shepherd moon of the LeVerrier ring. Galatea sits just inside the most prominent ring, the Adams ring.

Naiad, Thalassa, Despina, and Galatea are within range of Neptune-synchronous orbit, so they are being tidally decelerated. This means they orbit Neptune more quickly than Neptune rotates and that these moons will eventually either crash into Neptune or else break apart. S/2004 N1 is Neptune's smallest moon, while Proteus is its largest regular moon and second largest moon overall. Proteus is the only regular moon that is roughly spherical. It resembles a slightly faceted polyhedron. All of the other regular moons appear to be elongated, although the smallest ones have not been imaged with much accuracy to date.

The inner moons are dark, with albedo values (reflectivity) ranging from 7% to 10%. From their spectra, it is believed their surfaces are water ice containing a dark substance, most likely a mixture of complex organic compounds. The five inner moons are believed to be regular satellites that formed with Neptune.

Triton and the Irregular Moons of Neptune

Photograph of Triton, the largest moon of planet Neptune.
Photograph of Triton, the largest moon of planet Neptune. Stocktrek Images / Getty Images

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. While the inner moons formed in situ, it's believed all of the irregular moons were captured by Neptune's gravity.

Triton is Neptune's largest moon, with a diameter of 2700 km (1700 mi) and mass of 2.14 x 1022 kg. 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. Scientists believe this may mean Triton is a captured object, rather than a moon that formed with Neptune. 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. Triton is noteworthy for a few other reasons. It has a nitrogen atmosphere, like Earth, although Triton's atmospheric pressure is only about 14 μbar. Triton is a round moon with a nearly circular orbit. It has active geysers and may have a subterranean ocean.

Nereid is Neptune's third-largest moon. It has a highly eccentric orbit that might mean it was once a regular satellite that was disturbed when Triton was captured. Water ice has been detected on its surface.

Sao and Laomedeia have prograde orbits, while Halimede, Psamathe, and Neso have retrograde orbits. The similarity of the orbits of Psamathe and Neso might mean they are remnants of a single moon that broke apart. The two moons take 25 years to orbit Neptune, giving them the largest orbits of any natural satellites.

Historical References

Lassell, W. (1846). "Discovery of supposed ring and satellite of Neptune". Monthly Notices of the Royal Astronomical Society. 7: 157. 

Lassell, W. (1846). "Discovery of supposed ring and satellite of Neptune". Monthly Notices of the Royal Astronomical Society. 7: 157.

Smith, B. A.; Soderblom, L. A.; Banfield, D.; Barnet, C.; Basilevsky, A. T.; Beebe, R. F.; Bollinger, K.; Boyce, J. M.; Brahic, A. (1989). "Voyager 2 at Neptune: Imaging Science Results". Science246 (4936): 1422–1449.