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List of Neptune's moons
Neptune has a total of 14 known moons.  The largest moon is Triton which was discovered by William Lassell just seventeen days after Neptune was found. One hundred years later the second moon, Nereid was found. The Hubble Telescope found the 14th moon in 2013.
Most of Neptune's moons are named after sea nymphs.
Two moons, Psamathe (discovered in 2002) and Neso (discovered in 2003), are unusual because they have the largest orbits for any moons discovered in the solar system so far. They take about 25 years to make one orbit and are 125 times more distant from Neptune than our Moon is from Earth.
How Neptune's moons formed
Many astronomers and scientists think that Neptune's inner moons were not original bodies that formed with Neptune, but were remains of debris from Triton that slowly came together. Other scientists think that when Triton was captured by Neptune, Triton's mass and odd orbit caused Neptune's original moons to be destroyed by gravity, only to be reformed after Triton formed a stable orbit.
While many astronomers agree that Triton was not an original moon of Neptune, some think that Triton was captured in a "three-body-encounter". What must have happened was that Triton was part of a binary pair with another unknown celestial body. The pair was then captured but Triton survived and the other object being affected by Neptune's gravity.
Halimede, discovered in 2002, was perhaps a piece of Nereid when the moon was hit with another object. Both moons seemed to have the same gray colors. Also Halimede was also calculated that the moon had a high chance of colliding with Nereid in the past.
List of moons
The Neptunian moons are listed here by its orbital period, from shortest to longest. Triton, which is not only massive enough for its surface to have collapsed into a sphere, but is comparable in size to our own moon, is highlighted in purple. Irregular (captured) moons are shown in grey; prograde in light grey and retrograde in dark grey. (Triton is also thought to be captured.)
(km) [note 3]
(×1016 kg)[note 4]
| Semi-major axis
| Orbital period
|1||Neptune III||Naiad||ˈneɪ.əd|| 66
(96 × 60 × 52)
|2||Neptune IV||Thalassa||θəˈlæsə|| 82
(108 × 100 × 52)
|3||Neptune V||Despina||dɨsˈpiːnə|| 150
(180 × 148 × 128)
|4||Neptune VI||Galatea||ˌɡæləˈtiːə|| 176
(204 × 184 × 144)
|5||Neptune VII||Larissa||ləˈrɪsə|| 194
(216 × 204 × 168)
|6||Neptune XIV||Hippocamp||ˈhɪpoʊkæmp||~18||n/a||104,200[note 6]||0.9362||~ 0.0||~ 0.000||2013|
|7||Neptune VIII||Proteus||ˈproʊtiəs|| 420
(436 × 416 × 402)
|8||Neptune I||Triton ♠||ˈtraɪtən|| 2,705.2 ± 4.8
(2,709 × 2,706 × 2,705)
|2,140,800 ± 5200||354,759||5.877||156.865||0.0||1846|
|9||Neptune II||Nereid||ˈnɪəriː.ɪd||340 ± 50||2,700||5,513,818||360.13||7.090||0.7507||1949|
|10||Neptune IX||Halimede ♠||ˌhælɨˈmiːdiː||62||16||16,611,000||1,879.08||112.712||0.2646||2002|
|13||Neptune X||Psamathe ♠||ˈsæməθiː||40||4.4||48,096,000||9,074.30||126.312||0.3809||2003|
|14||Neptune XIII||Neso ♠||ˈniːsoʊ||60||15||49,285,000||9,740.73||136.439||0.5714||2002|
The picture at the right shows the orbits of Neptune's irregular moons discovered so far. The eccentricity of the orbits is represented by the yellow segments (extending from the pericentre to the apocentre) with the inclination represented on Y axis. The satellites above the X axis are prograde, the satellites beneath are retrograde. The X axis is labelled in Gm (million km) and the fraction of the Hill sphere's (gravitational influence) radius (~116 Gm for Neptune).
Triton, the biggest moon following a retrograde but a quasi-circular orbit, also thought to be a captured satellite, is not shown. Nereid, on a prograde but very eccentric orbit is believed to be scattered during Triton's capture.
Note that Triton did not have an official name until the twentieth century. Although the name was suggested in 1880 by Camille Flammarion, it did not come into regular 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). One of the 14 moons is called Neptune.
- Order refers to the position among other moons with respect to their average distance from Neptune.
- Label refers to the Roman numeral attributed to each moon in order of their discovery.
- Diameters with multiple entries such as "60×40×34" are for bodies that are not spherical.
- Mass of all moons of Neptune except Triton were calculated assuming a density of 1.3 g/cm³. The mass of Triton is from Jacobson, 2009.
- Each moon's inclination is given relative to its local Laplace plane. Inclinations greater than 90° indicate retrograde orbits (in the direction opposite to the planet's rotation).
- Based on the value of the period given in the Sky & Telescope reference, which is not consistent with the semi-major axis value from the same source.
- "Nasa's Hubble telescope discovers new Neptune moon". bbc.co.uk. 15 July 2013. https://www.bbc.co.uk/news/science-environment-23318301.
- "How Many Moons?". NASA. https://spaceplace.nasa.gov/how-many-moons/en/#.
- D. Banfield and N. Murray (1992). "A dynamical history of the inner Neptunian satellites". Icarus 99: 390. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1992Icar...99..390B&db_key=AST&data_type=HTML&format=&high=444b66a47d03051.
- C.B. Agnor & D.P. Hamilton Neptune's capture of its moon Triton in a binary-planet gravitational encounter, Nature, 441 (2006), pp. 192. (pdf)
- T.Grav, M.Holman and W.Fraser, Photometry of Irregular Satellites of Uranus and Neptune, The Astrophysical Journal, 613 (2004), pp.L77–L80 (preprint)
- M.Holman, JJ Kavelaars, B.Gladman, T.Grav, W.Fraser, D.Milisavljevic, P.Nicholson, J.Burns, V.Carruba, J-M.Petit, P.Rousselot, O.Mousis, B.Marsden, R.Jacobson Discovery of five irregular moons of Neptune, Nature, 430 (2004), pp. 865-867. Final preprint(pdf)
- "Planet and Satellite Names and Discoverers". Gazetteer of Planetary Nomenclature. USGS Astrogeology. July 21, 2006. http://planetarynames.wr.usgs.gov/append7.html.
- Jacobson, R.A. (2008) NEP078 - JPL satellite ephemeris
- Kelly Beatty (15 July 2013). "Neptune's Newest Moon". Sky & Telescope. http://www.skyandtelescope.com/news/Neptunes-Newest-Moon-215535121.html. Retrieved 15 July 2013.
- Scott S. Sheppard, David C. Jewitt, Jan Kleyna, A Survey for "Normal" Irregular Satellites Around Neptune: Limits to Completeness (preprint)
- goldreich, P.; Murray, N.; Longaretti, P. Y.; Banfield, D. Neptune's story, Science, 245, (1989), p. 500-504.