alt=Shown in this image are Neptune and some of its moons: Triton, Galatea, Naiad, Thalassa, Despina, Proteus, and Larissa|thumb|upright=1.5|An annotated [[near-infrared image of some of Neptune's moons as captured by the James Webb Space Telescope in September 2022.]]
There are 16 known moons of the planet Neptune, fourteen of which are named after water deities and creatures in Greek mythology. The largest of them 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 passed 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. They, including the largest, Proteus, 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. Neptune's outermost moon S/2021 N 1, which has an orbital period of about 27 Earth years, orbits farther from its planet than any other known moon in the Solar System. After deciding on a whim to expand the search area to radii well beyond the rings, he found an unambiguous dot that represented the new moon. He then found it repeatedly in other archival HST images going back to 2004. Voyager 2, which had observed all of Neptune's other inner satellites, did not detect it during its 1989 flyby, due to its dimness. but it did not come into common use until at least the 1930s. Until this time it was usually simply known as "the satellite of Neptune". Other moons of Neptune are also named for Greek water gods, in keeping with Neptune's position as god of the sea: Two asteroids share the same names as moons of Neptune: 74 Galatea and 1162 Larissa.
Characteristics
The moons of Neptune can be divided into two groups: regular and irregular. The first group includes the seven inner moons, which follow circular prograde orbits lying in the equatorial plane of Neptune. The second group consists of all nine other moons including Triton. They generally follow inclined eccentric and often retrograde orbits far from Neptune; the only exception is Triton, which orbits close to the planet following a circular orbit, though retrograde and inclined.) and thus are being tidally decelerated. Naiad, the closest regular moon, is also the second smallest among the inner moons (following the discovery of Hippocamp), whereas Proteus is the largest regular moon and the second largest moon of Neptune. The first five moons orbit much faster than Neptune's rotation itself ranging from 7 hours for Naiad and Thalassa, to 13 hours for Larissa.
The inner moons are closely associated with Neptune's rings. The two innermost satellites, Naiad and Thalassa, orbit between the Galle and LeVerrier rings. Surface features include the large southern polar cap, older cratered planes cross-cut by graben and scarps, as well as youthful features probably formed by endogenic processes like cryovolcanism.
Nereid
Nereid is the third-largest moon of Neptune. It has a prograde but very eccentric orbit and is believed to be a former regular satellite that was scattered to its current orbit through gravitational interactions during Triton's capture.
Spectroscopic observations conducted by the James Webb Space Telescope have shown that Nereid's spectrum differs significantly from those of minor planets in the outer Solar System. Although many irregular satellites share similarities with Kuiper belt objects, Nereid does not correspond to the observed spectral types of Kuiper belt objects, implying that it likely formed around Neptune instead of being captured. If this interpretation is confirmed, Nereid would represent the only intact original moon of Neptune to have survived Triton's destructive arrival.
Normal irregular moons
Among the remaining irregular moons, Sao, S/2002 N 5, and Laomedeia follow prograde orbits, whereas Halimede, Psamathe, Neso and S/2021 N 1 follow retrograde orbits. There are at least two groups of moons that share similar orbits, with the prograde moons Sao, S/2002 N 5, and Laomedeia belonging to the Sao group and the retrograde moons Psamathe, Neso, and S/2021 N 1 belonging to the Neso group. Triton, the only Neptunian moon massive enough for its surface to have collapsed into a spheroid, is emboldened.
{| class="wikitable" style="font-size:90%; text-align:left"
|-
|+ Key
|-
| style="background:#fff;" | Inner moons (7)
| style="background:#ccf;" | ♠ Triton (1)
| style="background:#efefef"| † Nereid (1)
|-
| style="background:#d3d3d3"| ‡ Halimede (1)
| style="background:#f0f0b0"| ♦ Sao group (3)
| style="background:#f4c2c2"| ♥ Neso group (3)
|-
| colspan=3 | Orbital period: default is prograde, − is retrograde
|}
{| class="wikitable sortable sort-under-center sticky-table-row1 sticky-table-col1 col1center col2center col3center col4center col12center col13center col14left col15left" style="font-size:90%;text-align:right;"
|+ style="text-align:center;" | Neptunian moons
|- style="background: #efefef;"
! Label<br />