Voyager 2 Neptune and Triton

Neptune (top) and Triton (bottom), 3 days after the Voyager 2 flyby.

Neptune has thirteen known moons. The largest by far is Triton, discovered by William Lassell just seventeen days after the discovery of Neptune itself. It took about one hundred years to discover the second natural satellite, Nereid.

Triton is massive enough to have achieved hydrostatic equilibrium, and would be considered a dwarf planet if it were in direct orbit about the Sun. Triton has a very unusual orbit that is circular but retrograde and inclined. Inward of Triton are six regular satellites, which all have prograde orbits that are not greatly inclined with respect to Neptune's equatorial plane. Some of these orbit among Neptune's rings.

Neptune also has six outer irregular satellites, including Nereid, whose orbits are much farther from Neptune, have high inclinations, and are mixed between prograde and retrograde. Two natural satellites discovered in 2002 and 2003, Psamathe and Neso, 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. Neptune has the largest Hill sphere in the solar system, owing primarily to its large distance from the Sun; this allows it to retain control of such distant moons.

Neptune's moons are named for aquatic personages in Greek and Roman mythology, many of them after Nereids, in keeping with Neptune's position as god of the sea.


File:Tritonian sky.jpg

Triton was discovered by William Lassell in 1846, seventeen days after Neptune was discovered. Nereid was discovered by Gerard P. Kuiper in 1949. In 1981 Larissa was first observed by Harold J. Reitsema, William B. Hubbard, Larry A. Lebofsky and David J. Thole.

No further moons were found until Voyager 2 flew by Neptune in 1989. Voyager 2 recovered Larissa and discovered five new inner moons, bringing the total of known moons of Neptune to eight.

In 2002 and 2003 telescopic surveys found five additional outer moons from Earth observatories bring the total to thirteen. The five new outer moons are Halimede, Sao, Psamathe, Laomedeia, and Neso.

Names Edit

Some asteroids share the same names as moons of Neptune: 74 Galatea, 1162 Larissa. See also Name conflicts of solar system objects.

Note that Triton did not have an official name until the twentieth century. The name "Triton" was suggested by Camille Flammarion in his 1880 book Astronomie Populaire,[1] but it did not come into common use until at least the 1930s.[2] Until this time it was usually simply known as "the satellite of Neptune" (the second satellite, Nereid, was not discovered until 1949).

Characteristics Edit


Main article: Triton (moon)
File:Masses of Neptunian moons.png

Irregular moonsEdit

File:TheIrregulars NEPTUNE.svg

The diagram illustrates 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).

Given the similarity of their orbits, it was suggested that Neso and Psamathe could have a common origin in the break-up of a larger moon.[3]

Triton, the biggest moon following a retrograde but a quasi-circular orbit, also conjectured to be a captured satellite, is not shown. Nereid, which has a prograde but very eccentric orbit, is believed to have been scattered during Triton's capture.[4]


The mass distribution of the Neptunian moons is the most lopsided of any planet. One moon, Triton, makes up nearly all of the mass of the system, with all other moons together comprising only one third of one percent (see diagram). This may be because the capture of Triton destroyed much of the original Neptunian system.

See also: Error: Template must be given at least one article name

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 orbit upon capture would have been highly eccentric, and would have caused chaotic perturbations in the orbits of the original inner Neptunian satellites, causing them to collide and reduce to a disc of rubble. Only after Triton's orbit became circularised did some of the rubble disc re-accrete into the present-day satellites [5].

The mechanism of the Triton’s capture has been the subject of several theories over the years. The most recent as of 2006 postulates that Triton was captured in a three-body encounter. In this scenario, Triton is the surviving member of a binary object[note 1] disrupted by its encounter with Neptune.[6].

Numerical simulations show that a moon discovered in 2002, Halimede, has had a high probability of colliding with Nereid during the lifespan of the system.[7] As both moons appear to have similar (grey) colours, the satellite could be a fragment of Nereid.[8]


</br> Major moon </br> Retrograde moons

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; prograde in light grey and retrograde in dark grey. (Triton is also thought to be captured.)

[note 2]
[note 3]
Image Diameter
(km)[note 4]
(×1016 kg)[note 5]
Semi-major axis
See also: Error: Template must be given at least one article name
Orbital period
See also: Error: Template must be given at least one article name
(°)[9][note 6]
1 Neptune III Naiad ˈneɪəd 66 (96×60×52) ~19 48 227 0.294 4.691° 0.0003 1989
2 Neptune IV Thalassa θəˈlæsə 82 (108×100×52) ~35 50 075 0.311 0.135° 0.0002 1989
3 Neptune V Despina dɨsˈpiːnə 150 (180×148×128) ~210 52 526 0.335 0.068° 0.0002 1989
4 Neptune VI Galatea ˌɡæləˈtiːə 176 (204×184×144) 212 61 953 0.429 0.034° 0.0001 1989
5 Neptune VII Larissa ləˈrɪsə 194 (216×204×168) ~420 73 548 0.555 0.205° 0.0014 1981
6 Neptune VIII Proteus ˈproʊtiəs 420 (436 × 416 × 402) ~5 000 117 647 1.122 0.075° 0.0005 1989
7 Neptune I ‡♠Triton ˈtraɪtən
Triton moon mosaic Voyager 2 (large)
2707 2 140 000 354 800 −5.877 156.865° 0.0 1846
8 Neptune II Nereid ˈniːriː.ɪd 340 ~3 100 5 513 400 360.14 7.090° 0.7507 1949
9 Neptune IX Halimede ˌhælɨˈmiːdiː 62 ~16 15 728 000 1 879.71 112.712° 0.2646 2002
10 Neptune XI Sao ˈseɪ.oʊ 44 ~5.8 22 422 000 2 914.07 53.483° 0.1365 2002
11 Neptune XII Laomedeia ˌleɪ.ɵmɨˈdiːə 42 ~5.0 23 571 000 3 167.85 37.874° 0.3969 2002
12 Neptune X Psamathe ˈsæməθiː 38 ~3.7 46 695 000 9 115.91 126.312° 0.3809 2003
13 Neptune XIII Neso ˈniːsoʊ 60 ~15 48 387 000
(0.32 AU)
9 373.99 136.439° 0.5714 2002


  1. Binary objects, objects with moons such as the PlutoCharon system, are quite common among the larger Trans-Neptunian objects.
  2. Order refers to the position among other moons with respect to their average distance from Neptune.
  3. Label refers to the Roman numeral attributed to each moon in order of their discovery.
  4. Diameters with multiple entries such as "60×40×34" reflect that the body is not spherical and that each of its dimensions has been measured well enough.
  5. Mass of the small irregular moons (Halimede through Neso) was calculated assuming a density of 1.3 g/cm³. Unless otherwise noted, the uncertainty in the reported masses is not available.
  6. 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).


  1. Flammarion, Camille (1880). "Astronomie populaire, p. 591". Retrieved on 2007-04-10.
  2. "Camile Flammarion". Hellenica. Retrieved on 2008-01-18.
  3. Scott S. Sheppard, David C. Jewitt, Jan Kleyna, A Survey for "Normal" Irregular Satellites Around Neptune: Limits to Completeness (preprint)
  4. Goldreich, P.; Murray, N.; Longaretti, P. Y.; Banfield, D. Neptune's story, Science, 245, (1989), p. 500-504.
  5. D. Banfield and N. Murray (1992). "A dynamical history of the inner Neptunian satellites". Icarus 99: 390. doi:10.1016/0019-1035(92)90155-Z, 
  6. 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)
  7. 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)
  8. T.Grav, M.Holman and W.Fraser, Photometry of Irregular Satellites of Uranus and Neptune, The Astrophysical Journal, 613 (2004), pp.L77–L80 (preprint)
  9. 9.0 9.1 Jacobson, R.A. (2008) NEP078 - JPL satellite ephemeris
  10. 10.0 10.1 "Planet and Satellite Names and Discoverers". Gazetteer of Planetary Nomenclature. USGS Astrogeology (July 21 2006). Retrieved on 2006-08-05.

External linksEdit

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