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List of Neptune's moons facts for kids

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(Redirected from Moons of Neptune)
Voyager 2 Neptune and Triton
Neptune (top) and Triton (bottom).

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.

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 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.)



Image Diameter
(×1016 kg)
Semi-major axis
Orbital period

1 Neptune III Naiad ˈneɪ.əd
Naiad Voyager.png
(96 × 60 × 52)
19 48,227 0.294 4.691 0.0003 1989
2 Neptune IV Thalassa θəˈlæsə
Naiad Voyager.png
(108 × 100 × 52)
35 50,074 0.311 0.135 0.0002 1989
3 Neptune V Despina dɨsˈpiːnə
(180 × 148 × 128)
210 52,526 0.335 0.068 0.0002 1989
4 Neptune VI Galatea ˌɡæləˈtiːə
Galatea moon.jpg
(204 × 184 × 144)
212 61,953 0.429 0.034 0.0001 1989
5 Neptune VII Larissa ləˈrɪsə
Larissa 1.jpg
(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
Proteus Voyager 2 (big).jpg
(436 × 416 × 402)
4,400 117,646 1.122 0.075 0.0005 1989
8 Neptune I Triton ˈtraɪtən
Triton moon mosaic Voyager 2 (large).jpg
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

TheIrregulars NEPTUNE
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 14 moons is called Neptune.

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