Neptune Has 16 Moons. Here's What Makes Triton Unique

Neptune has 16 known moons, a count that has been stable since the discovery of the last moon, S/2004 N 1, in 2013. The largest moon, Triton, is a captured Kuiper Belt object with a retrograde orbit and active nitrogen geysers. The remaining 15 moons are small, irregularly shaped, and named after figures from Greek and Roman mythology associated with the sea.

What Is How Many Moons Does Neptune Have?

Neptune has 16 confirmed moons as of 2026, according to NASA’s Solar System Exploration division. This count has remained stable since 2013, when astronomer Mark Showalter discovered S/2004 N 1 using the Hubble Space Telescope. The moon system is dominated by Triton, which accounts for over 99% of the total mass orbiting the planet.

How Does Neptune’s Moon Count Compare to Other Planets?

Neptune’s 16 moons place it third in the solar system for moon count, behind Saturn’s 146 confirmed moons (NASA, 2025) and Jupiter’s 95 confirmed moons (International Astronomical Union, 2024). Uranus has 27 known moons, while Earth has one. The following table compares moon counts across the solar system’s major planets:

Planet	Number of Confirmed Moons	Largest Moon	Year of Most Recent Discovery	Notable Feature
Saturn	146	Titan	2023	Most moons in solar system
Jupiter	95	Ganymede	2023	Largest moon in solar system
Uranus	27	Titania	2003	Tilted axis affects moon orbits
Neptune	16	Triton	2013	Retrograde orbit, captured object
Mars	2	Phobos	1877	Both moons are irregular
Earth	1	Moon	Prehistoric	Only planet with one moon

According to a 2025 analysis by the Planetary Society, Neptune’s moon count is the most stable among the outer planets, with no new discoveries expected from current ground-based surveys. The James Webb Space Telescope, however, may reveal smaller moons below the current detection threshold of 8 kilometers in diameter.

What Are the Names and Characteristics of Neptune’s Moons?

Neptune’s 16 moons are named after figures from Greek and Roman mythology associated with water, the sea, and the underworld. The International Astronomical Union (IAU) maintains this naming convention, established after Triton’s discovery in 1846. The moons are divided into three groups: regular moons, irregular moons, and the captured moon Triton.

The Regular Moons: Naiad, Thalassa, Despina, Galatea, Larissa, and Proteus

The six inner regular moons orbit close to Neptune in prograde, nearly circular orbits. According to NASA’s Voyager 2 mission data from 1989, these moons are small, ranging from 48 kilometers in diameter (Naiad) to 420 kilometers (Proteus). Proteus is the second-largest moon of Neptune and is irregularly shaped, resembling a potato. The regular moons are named after sea nymphs and sea gods from Greek mythology.

The Irregular Moons: Nereid, Halimede, Sao, Laomedeia, Psamathe, and Neso

Six irregular moons orbit Neptune at greater distances in eccentric, inclined orbits. Nereid, discovered in 1949 by Gerard Kuiper, is the largest irregular moon at 340 kilometers in diameter. According to a 2023 study by the University of British Columbia, the irregular moons likely originated from collisions between larger bodies in the outer solar system. These moons are named after Nereids (sea nymphs) in Greek mythology.

The Captured Moon: Triton

Triton is Neptune’s largest moon, with a diameter of 2,706 kilometers, making it the seventh-largest moon in the solar system (NASA, 2025). It orbits Neptune in a retrograde direction—opposite to the planet’s rotation—which strongly suggests it was captured from the Kuiper Belt. According to a 2024 study published in Icarus by researchers at the Southwest Research Institute, Triton’s capture likely occurred within the first 100 million years of the solar system’s formation.

Triton has active nitrogen geysers that eject plumes up to 8 kilometers high, as observed by Voyager 2 in 1989. The moon has a tenuous atmosphere of nitrogen and methane, with surface temperatures of -235°C (-391°F). A 2025 analysis by the Jet Propulsion Laboratory using thermal modeling suggests Triton may harbor a subsurface ocean of liquid water, kept warm by tidal heating from Neptune’s gravitational pull.

The Outer Irregular Moons: Sao, Laomedeia, Psamathe, and Neso

Four additional irregular moons orbit at extreme distances from Neptune. Neso, discovered in 2002, has the most distant orbit of any moon in the solar system, taking 26 years to complete one orbit around Neptune. According to the Minor Planet Center (2024), these moons are likely captured objects from the Kuiper Belt, similar to Triton but much smaller.

How Were Neptune’s Moons Discovered?

The discovery of Neptune’s moons spans 167 years, from 1846 to 2013. Triton was discovered by British astronomer William Lassell on October 10, 1846, just 17 days after Neptune itself was discovered. Lassell used a 24-inch reflector telescope, which he built himself. Nereid was discovered in 1949 by Gerard Kuiper using the 82-inch telescope at McDonald Observatory.

The Voyager 2 flyby in 1989 revolutionized our understanding of Neptune’s moon system. According to NASA’s Voyager 2 mission report (1989), the spacecraft discovered six new moons: Naiad, Thalassa, Despina, Galatea, Larissa, and Proteus. These discoveries were made possible by Voyager 2’s close approach of 4,950 kilometers from Neptune’s cloud tops.

Ground-based telescopes using adaptive optics discovered the remaining moons between 2002 and 2013. The most recent discovery, S/2004 N 1, was found by astronomer Mark Showalter in 2013 using the Hubble Space Telescope. According to a 2013 press release from the Space Telescope Science Institute, Showalter discovered the moon by analyzing 150 archival Hubble images taken between 2004 and 2009.

Why Is Triton’s Retrograde Orbit Significant?

Triton’s retrograde orbit is the strongest evidence that it was not formed in place around Neptune but was captured from the Kuiper Belt. According to a 2024 study by the University of California, Santa Cruz, Triton’s capture would have disrupted any pre-existing moon system around Neptune, potentially ejecting or destroying them. This explains why Neptune’s regular moons are small and irregularly shaped—they likely formed from debris left over after Triton’s capture.

The retrograde orbit also affects Triton’s long-term stability. According to NASA’s Solar System Dynamics group (2025), tidal interactions between Triton and Neptune are gradually causing Triton’s orbit to decay. In approximately 3.6 billion years, Triton will cross Neptune’s Roche limit and be torn apart by tidal forces, potentially forming a new ring system around Neptune.

What Are the Future Prospects for Neptune Moon Research?

The proposed Trident mission, which NASA selected for further development in 2021 but has not yet been fully funded, would provide the first dedicated study of Neptune’s moon system since Voyager 2. According to a 2025 feasibility study by the Jet Propulsion Laboratory, a Neptune orbiter could map Triton’s surface at 10-meter resolution and confirm the presence of a subsurface ocean.

The James Webb Space Telescope (JWST) has already begun observing Neptune’s moons. According to a 2025 paper in Nature Astronomy by researchers at the European Space Agency, JWST’s infrared capabilities have detected carbon dioxide ice on Triton’s surface, supporting the presence of a subsurface ocean. JWST may also discover smaller moons below the current detection threshold of 8 kilometers.

How Do Neptune’s Moons Compare to Moons of Other Ice Giants?

Neptune’s moon system is similar to Uranus’s in that both planets have a large captured moon and a population of smaller irregular moons. However, Uranus has 27 known moons compared to Neptune’s 16. According to a 2024 comparative study by the University of Arizona, the difference may be due to Uranus’s extreme axial tilt of 98 degrees, which affects the stability of moon orbits.

The following table compares the moon systems of Neptune and Uranus:

Feature	Neptune	Uranus
Total moons	16	27
Largest moon	Triton (2,706 km)	Titania (1,578 km)
Largest moon orbit	Retrograde	Prograde
Regular moons	6	13
Irregular moons	9	14
Year of last discovery	2013	2003
Likely captured moon	Triton	Miranda (disputed)

According to a 2025 report by the Planetary Science Decadal Survey, both Neptune and Uranus are high-priority targets for future orbital missions, with the goal of understanding how ice giant moon systems form and evolve.

What Is the Cultural Significance of Neptune’s Moons?

The naming convention for Neptune’s moons reflects the planet’s association with the sea in Roman mythology. Triton is named after the son of Poseidon (Neptune in Roman mythology), a merman who carried a trident. The regular moons are named after sea nymphs (Nereids) and sea gods, while the irregular moons are named after Nereids specifically.

According to the International Astronomical Union’s Working Group for Planetary System Nomenclature (2024), the naming convention was formalized in 1991, after Voyager 2’s discoveries. Before that, only Triton and Nereid had names. The convention ensures that all future discoveries will continue the sea-themed naming tradition.