Neptune is the planet in the solar system that is farthest from the Sun; it has 14 known moons. The largest by far is Triton, which is nearly the size of Earth’s Moon. Triton is the only large moon of the solar system that travels around its planet in retrograde fashion; that is, it orbits Neptune in the opposite direction of the planet’s rotation. Triton is likely a captured Kuiper belt object and is similar in size, density, and surface composition to the dwarf planet Pluto. The other satellites of Neptune are much smaller bodies.
Triton was discovered shortly after Neptune in 1846. Nereid was discovered more than a century later in 1949. Voyager 2 discovered six moons during its 1989 flyby of Neptune. The rest of Neptune’s moons were discovered by telescopes on Earth. The moons of Neptune are named after characters from Greek or Roman mythology associated with Poseidon or his Roman counterpart, Neptune. The moons of Neptune are listed in order of their distance from the planet, with their year of discovery, orbital period in days, and diameter in kilometers.
| name | year discovered | distance from Neptune (km) | orbital period (days)* | diameter (km) |
|---|---|---|---|---|
| Naiad | 1989 | 48,224 | 0.29 | 96 × 60 × 52 |
| Thalassa | 1989 | 50,074 | 0.31 | 108 × 100 × 52 |
| Despina | 1989 | 52,526 | 0.34 | 180 × 148 × 128 |
| Galatea | 1989 | 61,953 | 0.43 | 204 × 184 × 144 |
| Larissa | 1989 | 73,548 | 0.56 | 216 × 204 × 168 |
| Hippocamp | 2013 | 105,284 | 0.95 | 18.00 |
| Proteus | 1989 | 117,646 | 1.12 | 440 × 416 × 404 |
| Triton | 1846 | 354,759 | 5.88 R | 2,706.80 |
| Nereid | 1949 | 5,513,818 | 360.13 | 340.00 |
| Halimede | 2002 | 16,681,000 | 1,879.33 R | 62.00 |
| Sao | 2002 | 22,619,000 | 2,919.16 | 44.00 |
| Laomedeia | 2002 | 23,613,000 | 3,175.62 | 42.00 |
| Psamathe | 2003 | 46,705,000 | 9,128.74 R | 40.00 |
| Neso | 2002 | 50,258,000 | 9,880.63 R | 60.00 |
| *The “R” following the orbital period means the orbit of that satellite is retrograde. | ||||