List of Neptune's moons

Neptune (top) and Triton (bottom).

Neptune has a total of 16 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.

Unusual orbits

Neptune has moons which have unusual orbits. Triton, the largest moon of Neptune, has a retrograde orbit (which means Triton orbits or moves the opposite way from how Neptune spins).

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 executed in space 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.)

Order	Label	Name	Pronunciation (key)	Image	Diameter (km)	Mass (×1016 kg)	Semi-major axis (km)	Orbital period (d)	Inclination (°)	Eccentricity	Discovery year
1	Neptune III	Naiad	ˈneɪ.əd		66 (96 × 60 × 52)	19	48,227	0.294	4.691	0.0003	1989
2	Neptune IV	Thalassa	θəˈlæsə		82 (108 × 100 × 52)	35	50,074	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)	460	73,548	0.555	0.205	0.0014	1981
6	Neptune XIV	Hippocamp	ˈhɪpoʊkæmp		~18	n/a	104,200	0.9362	~ 0.0	~ 0.000	2013
7	Neptune VIII	Proteus	ˈproʊtiəs		420 (436 × 416 × 402)	4,400	117,646	1.122	0.075	0.0005	1989
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
11	Neptune XI	Sao	ˈseɪ.oʊ		44	5.8	22,228,000	2,912.72	53.483	0.1365	2002
12	Neptune XII	Laomedeia	ˌleɪ.ɵmɨˈdiːə		42	5.0	23,567,000	3,171.33	37.874	0.3969	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

Irregular moons

Neptune's irregular satellites.

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).

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.

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.

Naming notes

Some asteroids also have the same names as moons of Neptune: 74 Galatea, 1162 Larissa.

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 16 moons is called Neptune.

Related pages

• Neptune
• Satellite (natural), the word for moons of other planets

Other websites

• Neptune's Moons Archived 2007-06-09 at the Wayback Machine by NASA's Solar System Exploration