Voyager 2 Neptune and Triton

Neptune (top) and Triton (bottom), three 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, Nereid.

Unusual orbits

Triton orbits Neptune on a circular but retrograde orbit. While retrograde orbits are common among distant irregular satellites, Triton is a unique case of retrograde moon so close to its planet.

The third largest moon of Neptune, Nereid follows a prograde but the most eccentric orbit among the moons of the solar system, being at its apocenter more than seven times further from the planet than at its pericenter.

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.

Theories of origin

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 [1].

The mechanism of the Triton’s capture have been the subject of a few theories over the years. The most recent postulates that Triton was captured in a three body encounter. In this scenario, Triton is the surviving member of a binary object1 disrupted by the encounter with Neptune.[2].

Numerical simulations show that another moon discovered in 2002, Halimede has had a high probability of collision with Nereid during the lifespan of the system.[3] As both moons appear to have similar (grey) colours, the satellite could be a fragment of Nereid.[4]

1Binary objects, gravitational association of two objects, are quite common among Trans-Neptunian Objects (>10%; the most known is Pluto -Charon) and less so among the asteroids (e.g. Ida and Dactyl).

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

Order Name (spheroidal moon in bold)

(Pronunciation key)

Image Diameter (km) Mass
(1016 kg)
Semi-major axis (km) Orbital period (d)
1 Neptune III Naiad 'ne?.?d or 'na?.æd 67 (96×60×52) ~19 48,227 0.294
2 Neptune IV Thalassa ??'læ.s? 83 (108×100×52) ~35 50,075 0.311
3 Neptune V Despina d?'spa?.n? 152 (180×150×130) ~210 52,526 0.335
4 Neptune VI Galatea ?gæ.l?'t?i.? 175 (204×184×144) 212 61,953 0.429
5 Neptune VII Larissa l?'??.s? 195 (216×204×164) ~420 73,548 0.555
6 Neptune VIII Proteus 'p??o?.ti.?s 418 (436 × 416 × 402) ~5,000 117,647 1.122
7 Neptune I Triton 't??a?.t?n 2707 2,140,000 354,800 -5.877
8 Neptune II Nereid 'n?.?i.?d 340 ~3,100 5,513,400 360.14
9 Neptune IX Halimede ?hæ.l?'mi.di 60 ~9 15,728,000 -1879.71
10 Neptune XI Sao 'se?.o? 38 ~9 22,422,000 2914.07
11 Neptune XII Laomedeia ?le?.?.m?'di.?</br>or ?le?.?.m?'da?.? 38 ~9 23,571,000 3167.85
12 Neptune X Psamathe 'sæ.m?.?i 28 ~1.5 46,695,000 -9115.91
13 Neptune XIII Neso ' 60 ~9 48,387,000
(0.32 AU)

Negative orbital periods indicate a retrograde orbit around Neptune (opposite to the planet's rotation)

Irregular satellites

File:TheIrregulars NEPTUNE.svg

The diagram illustrates the orbits of Neptune’s irregular satellites 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.[5]

Triton, the biggest moon following a retrograde but a quasi-circular orbit, also conjectured 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.[6]

Naming notes

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

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 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).


  1. D. Banfield and N. Murray (1992). "A dynamical history of the inner Neptunian satellites". Icarus 99: 390.
  2. 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)
  3. 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)
  4. T.Grav, M.Holman and W.Fraser, Photometry of Irregular Satellites of Uranus and Neptune, The Astrophysical Journal, 613 (2004), pp.L77–L80 (preprint)
  5. Scott S. Sheppard, David C. Jewitt, Jan Kleyna, A Survey for "Normal" Irregular Satellites Around Neptune: Limits to Completeness (preprint)
  6. Goldreich, P.; Murray, N.; Longaretti, P. Y.; Banfield, D. Neptune's story, Science, 245, (1989), p. 500-504.

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