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Neptune has 9 known and 4 suspected moons. The largest by far is Triton, discovered by William Lassell just 17 days after the discovery of Neptune itself. Two natural satellites discovered in 2002 and 2003, Psamathe and S/2002 N 4, have the largest orbits of any natural satellites discovered in the Solar system to date. They take 25 years to orbit Neptune at an average of 125 times the distance between Earth and the Moon.
It is likely that Neptune's inner satellites are not the original bodies that formed with Neptune but accreted rubble from the havoc that was wreaked after Triton's capture. Triton's original captured orbit would have been highly eccentric, and caused chaotic perturbations in the orbits of the original inner Neptunian satellites, caussing them to collide and become reduced to a rubble disc. Only after Triton's orbit became circularised did some of the rubble disc re-accrete into the present-day satellites .
The natural satellites
The Neptunian moons are listed here by orbital period, from shortest to longest. Triton, which is not only massive enough for its surface to have collapsed into a spheroid, but is comparable in size to our own moon, is highlighted in purple. Irregular (captured) moons are shown in grey. (Triton is also thought to be captured.)
| Name (spheroidal moon in bold) | (Pronunciation respelling key)
Diameter (km) | Mass (1016 kg) | Mean orbital radius (km) | Orbital period** (d) | ||
|---|---|---|---|---|---|---|
| Neptune III | Naiad | nye'-ad | 67 (96×60×52) | ~19 | 48,227 | 0.294 |
| Neptune IV | Thalassa | thə-las'-ə | 83 (108×100×52) | ~35 | 50,075 | 0.311 |
| Neptune V | Despina | des-pee'-nə | 152 (180×150×130) | ~210 | 52,526 | 0.335 |
| Neptune VI | Galatea | gal'-ə-tee'-ə | 175 (204×184×144) | 212 | 61,953 | 0.429 |
| Neptune VII | Larissa | lə-ris'-ə | 195 (216×204×168) | ~490 | 73,548 | 0.555 |
| Neptune VIII | Proteus | proe'-tee-əs | 418 (436 × 416 × 402) | ~5,000 | 117,647 | 1.122 |
| Neptune I | Triton | trye'-tən | 2707 | 2,140,000 | 354,800 | -5.877 |
| Neptune II | Nereid | neer'-ee-id | 340 | ~3,100 | 5,513,400 | 360.14 |
| S/2002 N 1* | 60 | ~9 | 15,728,000 | -1879.71 | ||
| S/2002 N 2* | 38 | ~9 | 22,422,000 | 2914.07 | ||
| S/2002 N 3* | 38 | ~9 | 23,571,000 | 3167.85 | ||
| Neptune X | Psamathe | sam'-ə-thee | 28 | ~1.5 | 46,695,000 | -9115.91 |
| S/2002 N 4* | 60 | ~9 | 48,387,000 (0.32 AU) | -9373.99 | ||
* Awaiting confirmation and naming.
** Negative orbital periods indicate a retrograde orbit around Neptune (opposite to the planet's rotation)
Naming notes
Some asteroids share the same names as moons of Neptune: 74 Galatea, 1162 Larissa. Note that Triton did not have an official name until the 20th century. Although the name was suggested in 1880 by Camille Flammarion, it did not come into common use until at least the 1930s. Usually, it was simply known as "the satellite of Neptune" (the second satellite, Nereid, was not discovered until 1949).
See also
- Jupiter's natural satellites
- Saturn's natural satellites
- Uranus' natural satellites
- Pluto's natural satellites
- Timeline of discovery of Solar System planets and their natural satellites
- Naming of natural satellites
References
Neptunovi prirodni sateliti | Естествени спътници на Нептун | Satèl·lits de Neptú | Neptunovy měsíce | Neptuns måner | Liste der Neptunmonde | Satellites naturels de Neptune | Neptunovi prirodni sateliti | Satelliti naturali di Nettuno | Lista księżyców Neptuna | Спутники Нептуна | Mesiace Neptúna | Neptunus naturliga satelliter | 海王星的卫星
"Neptune's natural satellites"