Neptune has 16 known moons, with Triton being by far the largest and most famous. The others are small, irregularly shaped worlds, most of which were discovered after the Voyager 2 flyby in 1989 or by the Hubble Space Telescope and large ground-based telescopes in the 21st century.
Discovery and Naming History
Neptune’s first moon, Triton, was found in 1846 by British astronomer William Lassell just 17 days after the planet itself was discovered. Lassell also discovered Neptune’s second moon, Nereid, in 1949. For over a century, these were the only two known moons. The next moon, Larissa, was found in 1981 during a stellar occultation (when the moon passed in front of a star), but it was confirmed by Voyager 2 in 1989. The Voyager 2 flyby in 1989 added six more moons: Naiad, Thalassa, Despina, Galatea, Proteus, and the confirmation of Larissa. The remaining moons were discovered between 2002 and 2013 using powerful Earth-based telescopes, bringing the total to 16.
The Giant Moon Triton
Triton is Neptune’s largest moon, with a diameter of about 2,700 kilometers (1,680 miles), making it the seventh-largest moon in the solar system. It is unique among large moons because it orbits Neptune in a retrograde direction—opposite to Neptune’s rotation. This is strong evidence that Triton did not form with Neptune but was captured from the Kuiper Belt, a region of icy bodies beyond Neptune. Triton has a thin atmosphere of nitrogen and methane, and its surface is covered with nitrogen ice, water ice, and frozen carbon dioxide. It is one of the coldest known places in the solar system, with surface temperatures around -235°C (-391°F). Despite the cold, Triton is geologically active: the Voyager 2 spacecraft observed geysers erupting nitrogen gas and dark particles from its surface. Triton is also notable for its “cantaloupe terrain,” a wrinkled landscape of pits and ridges that is unlike anything else in the solar system.
The Inner Regular Moons
Neptune has seven small inner moons that orbit close to the planet in the same direction as its rotation. These are Naiad, Thalassa, Despina, Galatea, Larissa, Hippocamp, and Proteus. They are all irregularly shaped and are thought to be fragments of a larger moon that was disrupted by an ancient collision. The largest of these is Proteus, which is about 420 kilometers (260 miles) across and has a boxy shape. These inner moons orbit within Neptune’s ring system and interact gravitationally with the rings. For example, Galatea acts as a shepherd moon for the Adams ring, helping to keep it narrow. Hippocamp, discovered in 2013, is the smallest of these inner moons, with a diameter of only about 34 kilometers (21 miles). It orbits close to Proteus and may have broken off from it after a collision.
The Irregular Outer Moons
Beyond Triton, Neptune has seven irregular moons that orbit at great distances and have inclined or eccentric orbits. These are Nereid, Halimede, Sao, Laomedeia, Psamathe, Neso, and S/2021 N 1 (a provisional name for a moon announced in 2024). Nereid is the largest of these, with a diameter of about 340 kilometers (210 miles), and has one of the most eccentric orbits of any moon in the solar system. It is thought to have been a regular moon that was disturbed when Triton was captured. The other irregular moons are much smaller, ranging from about 40 to 60 kilometers (25 to 37 miles) across. They are likely captured Kuiper Belt objects. Neso has the most distant orbit of any moon in the solar system, taking about 27 years to complete one orbit around Neptune.
How Neptune’s Moons Were Formed
The formation of Neptune’s moons is a story of capture and collision. Triton is almost certainly a captured Kuiper Belt object because of its retrograde orbit and its composition, which is similar to Pluto’s. When Triton was captured, it would have disrupted any original moons that existed, causing collisions and ejections. The inner regular moons may be the result of a collision between a captured moon and a pre-existing body, leaving behind a debris disk that coalesced into the small moons we see today. The irregular outer moons are likely also captured objects, but from different regions of the Kuiper Belt. The exact details are still uncertain, but the current picture is that Neptune’s moon system is a chaotic remnant of its violent history.
Observing Neptune’s Moons
Observing Neptune’s moons is a challenge for amateur astronomers. Triton is the only moon that can be seen with a large amateur telescope, and even then it appears as a faint point of light near the planet. To see any of the other moons, you need a professional observatory or a space telescope. The Hubble Space Telescope has been instrumental in discovering and studying the small moons. For example, Hubble observations in 2004 and 2005 led to the discovery of Psamathe and Neso. The James Webb Space Telescope has also begun to study Neptune’s moons, providing detailed spectra that reveal their surface compositions. If you want to see Triton yourself, you’ll need a telescope with an aperture of at least 20 centimeters (8 inches) and a dark sky. The best time to observe is when Neptune is at opposition, which happens about once a year.
Common Misconceptions
One common misconception is that Neptune’s moons are all named after Greek gods or mythological figures. In fact, they are named after water deities from Greek and Roman mythology, following the tradition of naming Neptune’s moons after sea gods and nymphs. For example, Triton is a sea god, Nereid is a sea nymph, and Naiad is a freshwater nymph. Another misconception is that Triton is a captured moon because of its retrograde orbit, but some scientists have proposed that it could have formed in a disk around Neptune and later been flipped by a collision. However, the retrograde orbit is still best explained by capture. Finally, some people think that Neptune’s rings are related to its moons, and indeed they are: several of the inner moons, like Galatea and Despina, gravitationally interact with the rings, helping to shape them.
Frequently Asked Questions
How many moons does Neptune have?
Neptune has 16 known moons. The first was discovered in 1846, and the most recent was announced in 2024 (provisionally named S/2021 N 1). The count may increase with future observations.
What is the largest moon of Neptune?
Triton is the largest, with a diameter of about 2,700 kilometers (1,680 miles). It contains more than 99% of the mass of all Neptune’s moons combined.
Why is Triton’s orbit retrograde?
Triton orbits Neptune in the opposite direction to Neptune’s rotation. This is strong evidence that Triton was not born with Neptune but was captured from the Kuiper Belt. The capture likely happened early in Neptune’s history, and Triton’s retrograde orbit is a relic of that event.
Can I see Neptune’s moons with a backyard telescope?
Only Triton is visible with a large amateur telescope, and it appears as a faint star-like point. The other moons are too small and dim for typical backyard equipment. You would need a professional observatory or a space telescope like Hubble to see them.
Are any of Neptune’s moons habitable?
No. Triton is extremely cold and has a thin atmosphere, but it is not habitable as we know it. The other moons are even less hospitable. No evidence of life exists on any of Neptune’s moons.
What is the most distant moon of Neptune?
Neso is the most distant, orbiting at an average distance of about 49 million kilometers (30 million miles) from Neptune. It takes about 27 years to complete one orbit.
Related
- Mount Meru in Mythology: Axis of the Universe
- Kur in Mythology: Mesopotamian Underworld
- Asgard in Mythology: Realm of the Norse Gods
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