Moons of Neptune

There are 16 known moons of the planet Neptune. Most of these moons are named after water deities and creatures from Greek mythology. The biggest moon is Triton. It was found by William Lassell on October 10, 1846, just 17 days after Neptune was discovered.

Triton is special because it moves around Neptune in the opposite direction from the planet's spin. This suggests Triton did not form with Neptune but was pulled into its orbit from somewhere else in space.

An annotated near-infrared image of some of Neptune's moons as captured by the James Webb Space Telescope in September 2022.

Triton is large enough to have a thin atmosphere and form clouds. It is much bigger than another captured moon, Saturn's moon Phoebe. The capture of Triton was a big event that may have caused Neptune's original moons to crash together.

Inside Triton's orbit are seven smaller moons. These travel around Neptune in the same direction as the planet spins and close to its equator. Some even pass through Neptune's rings.

Beyond Triton, Neptune has eight more distant moons with unusual paths. One of these is Nereid, the largest of the outer moons. It follows a very long and stretched-out route around Neptune. Neptune's farthest known moon, S/2021 N 1, takes about 27 Earth years to orbit Neptune. It is farther away than any other known moon in the Solar System.

History

Triton was discovered by William Lassell in 1846, just seventeen days after the discovery of Neptune. More moons were found later, with Nereid discovered in 1949 and Larissa in 1981 while scientists were looking for rings around Neptune.

In 1989, the spacecraft Voyager 2 found several more moons, including Naiad, Thalassa, Despina, Galatea, and Proteus. Since then, telescopes on Earth and in space have helped discover even more moons, bringing the total to sixteen. These moons are named after water gods and creatures from Greek stories, matching Neptune's role as the god of the sea.

Characteristics

The moons of Neptune are split into two groups: regular and irregular. The regular moons are seven inner moons that travel in circular paths near Neptune's equator. The irregular moons are nine outer moons with more varied and distant paths.

Regular moons include Naiad, Thalassa, Despina, Galatea, Larissa, Hippocamp, and Proteus. Most of these moons are inside Neptune's synchronous orbit and are slowly moving closer to the planet. Proteus is the largest regular moon and has a surface covered in craters. These moons are dark, made mostly of water ice mixed with darker materials.

Orbit diagram of Neptune's inner moons including Triton, with their names and orbit directions indicated

Irregular moons include Triton, Nereid, and several smaller moons. Triton has a nearly circular orbit and a thin atmosphere. Nereid has a very stretched-out orbit and a rough, stretched shape. The other irregular moons have very distant orbits, some of the farthest known in the Solar System.

Main article: Triton (moon)

Main article: Nereid (moon)

List

Orbital diagram of the orbital inclination and orbital distances for Neptune's rings and moon system at various scales. Notable moons and rings are individually labeled. Open the image for full resolution.

The moons of Neptune are listed by how long they take to orbit the planet, from the shortest to the longest time. Some moons, called irregular or captured moons, have orbits that change often because of the pull of planets and the sun. For these moons, the information about their paths is averaged over a long time—30,000 years—so it might look different from other sources. The main moons, including Triton, have their information from a specific date: January 1, 2009. The irregular moons' information is from January 1, 2020. Triton is special because it is the only moon big enough for its surface to become round, and it is shown in bold in the list.

Key
Inner moons (7)	♠ Triton (1)	† Nereid (1)
‡ Halimede (1)	♦ Sao group (3)	♥ Neso group (3)
Orbital period: default is prograde, − is retrograde

Neptunian moons
Label	Name	Pronunciation	Abs. magn.	Diameter (km)	Mass (×10 Eg)	Semi-major axis (km)	Orbital period (±d)	Orbital inclination (°)	Eccentricity	Discovery year	Year announced	Discoverer	Group
III	Naiad	/ˈneɪəd, ˈnaɪæd/	9.6	66 (96 × 60 × 52)	12	48224	0.2943958(2)	5.073	0.0034	1989	1989	Voyager Science Team	inner
IV	Thalassa	/θəˈlæsə/	8.7	82 (108 × 100 × 52)	35	50074	0.31148444(6)	0.137	0.0022	1989	1989	Voyager Science Team	inner
V	Despina	/dəˈspaɪnə/	7.3	150 (180 × 148 × 128)	≈ 110	52526	0.33465551(1)	0.058	0.0005	1989	1989	Voyager Science Team	inner
VI	Galatea	/ˌɡæləˈtiːə/	7.2	176 (204 × 184 × 144)	194	61953	0.42874431(1)	0.023	0.0002	1989	1989	Voyager Science Team	inner
VII	Larissa	/ləˈrɪsə/	6.8	194 (216 × 204 × 168)	≈ 390	73548	0.55465332(1)	0.188	0.0012	1981	1981	Reitsema et al.	inner
XIV	Hippocamp	/ˈhɪpəkæmp/	10.5	34.8±4.0	≈ 2.2	105284	0.950390	0.064	0.0005	2013	2013	Showalter et al.	inner
VIII	Proteus	/ˈproʊtiəs/	5.0	420	≈ 2300	117647	1.12231477(2)	0.048	0.0004	1989	1989	Voyager Science Team	inner
I	Triton^♠	/ˈtraɪtən/	–1.2	2705.2±4.8 (2709 × 2706 × 2705)	2139000	354759	−5.876854	156.904	0.0003	1846	1846	Lassell
II	Nereid^†	/ˈnɪəriəd/	4.4	357 ± 13	≈ 2400	5513900	360.14	5.1	0.751	1949	1949	Kuiper
IX	Halimede^‡	/ˌhæləˈmiːdiː/	9.9	≈ 61	≈ 12	16590500	−1879.08	119.6	0.521	2002	2003	Holman et al.
XI	Sao^♦	/ˈseɪoʊ/	11.0	≈ 40	≈ 3.4	22239900	2912.72	50.2	0.296	2002	2003	Holman et al.	Sao
	S/2002 N 5^♦		11.2	≈ 38	≈ 2.9	23414700	3156.556	46.3	0.433	2002	2024	Holman et al.	Sao
XII	Laomedeia^♦	/ˌleɪəməˈdiːə/	10.8	≈ 40	≈ 3.4	23499900	3171.33	36.9	0.419	2002	2003	Holman et al.	Sao
X	Psamathe^♥	/ˈsæməθiː/	10.8	≈ 38	≈ 2.9	47646600	−9149.514	127.8	0.413	2003	2003	Sheppard et al.	Neso
XIII	Neso^♥	/ˈniːsoʊ/	10.7	≈ 60	≈ 11	49897800	−9794.705	128.4	0.455	2002	2003	Holman et al.	Neso
	S/2021 N 1^♥		12.1	≈ 25	≈ 0.8	50700200	−10036.651	135.2	0.503	2021	2024	Sheppard et al.	Neso

Formation

Neptune's moons have very uneven sizes. One moon, Triton, has almost all of the mass. All the other moons together are only about one-third of one percent of the total mass. This is similar to Saturn's moon system, where Titan has most of the mass. It is different from Jupiter and Uranus, which have more balanced systems.

Scientists think Triton was pulled into Neptune's orbit after the planet's original moons formed. This might have destroyed many of the original moons. Only after Triton's orbit became stable could new moons form from the pieces left behind. One idea is that Triton was captured during an event with three bodies, including a pair of objects from the Kuiper belt. There are also ideas about possible past collisions between moons like Halimede and Nereid, but this is still not certain.

Moons of Neptune

Moons of Neptune

History

Characteristics

List

Formation

Images

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