How many moons does Neptune have? Triton and the other 15

Neptune has 16 known moons, NASA says. One of them is big: Triton, a frozen world a little wider than Pluto that goes around Neptune the "wrong" way. The other 15 are much smaller; the second largest, Proteus, is only about 416 km across. The quick facts:

How many moons does Neptune have?

Sixteen, according to NASA's Neptune pages updated in 2026. Older sources give smaller totals, because faint moons kept being found. The two newest on NASA's list still have catalogue codes instead of names, S/2002 N5 and S/2021 N1, and NASA cites 2024 announcements for both.

The named moons are Triton, Nereid, Naiad, Thalassa, Despina, Galatea, Larissa, Proteus, Halimede, Psamathe, Sao, Laomedeia, Neso and Hippocamp. NASA explains the theme: Neptune is named after the Roman god of the sea, so its moons are named after lesser sea gods and nymphs from Greek myths.

Only one of them is large. Using NASA JPL's measured sizes, Triton is about 2,705 km across, about 78% of our Moon's width and a little wider than Pluto (our calculation). The next largest, Proteus, is only about 416 km across. Compare that with the many moons of Jupiter and Saturn, or the two tiny moons of Mars.

Triton: the moon that goes backwards

Triton was discovered on 10 October 1846 by the British astronomer William Lassell, just 17 days after Neptune itself was found, NASA says. It is the only large moon in our solar system that orbits in the opposite direction to its planet's spin.

We checked that with NASA JPL's Horizons system. Seen from above Neptune's north pole, Neptune spins anticlockwise and its moon Proteus goes round anticlockwise too, once every 27 hours. Triton goes clockwise, once every 5.88 days, about 355 thousand km from Neptune's centre. Its orbit is tilted 156.8° to Neptune's equator; anything over 90° means going round backwards (our calculation).

In the animation below, seen from above Neptune's north pole, Triton stays about 355,000 km from Neptune's centre and Proteus about 118,000 km (our calculation). The positions are NASA JPL Horizons predictions for 15 to 27 November 2026, in Neptune's own equator frame. Neptune and the orbits are drawn to scale, but the moon dots are drawn bigger so you can see them. Triton's orbit looks slightly oval because it is tilted and we look at it from above.

Why backwards? NASA says scientists think Triton is a Kuiper Belt object, an icy world from beyond Neptune, that was captured by Neptune's gravity millions of years ago. A captured moon does not have to follow the direction of the planet's spin.

What is Triton like?

Very cold and surprisingly active. NASA gives a surface temperature of about -235 °C (-391 °F). Triton has a crust of frozen nitrogen over an icy mantle, probably around a core of rock and metal, and a thin atmosphere made mainly of nitrogen with a little methane.

When Voyager 2 flew past in 1989, it found active geysers on Triton, throwing icy material more than 8 km (5 miles) up, according to NASA. Voyager's pictures of the south polar region show about 50 dark streaks. NASA says gas may have burst out of vents and carried dark particles with it; winds in Triton's thin atmosphere then spread those particles across the ice. Voyager 2 photographed about two-thirds of Triton, so part of it has never been seen up close.

Could people survive on Triton without a spacesuit or shelter? No. Its surface is extremely cold, about -235 °C, and its thin nitrogen atmosphere has no breathable air.

Proteus, Nereid and Neptune's smaller moons

Proteus is Neptune's second-largest moon, about 416 km across (NASA JPL). Voyager 2 discovered it in 1989. NASA describes it as having an odd box-like shape: with just a little more mass, its own gravity would have pulled it into a sphere. It goes around Neptune in about 27 hours.

Nereid was discovered on 1 May 1949 by Gerard Kuiper with a telescope on the ground. NASA says it has one of the most stretched, oval orbits of any moon in the solar system, taking 360 days to go round once. In NASA JPL Horizons data for November 2026 to January 2028 we found that Nereid comes within about 1.4 million km of Neptune and moves out to about 9.6 million km, almost seven times farther (our calculation).

The rest are small. Naiad, Thalassa, Despina, Galatea, Larissa and Hippocamp orbit close to Neptune, well inside Triton's orbit, and each goes round in less than a day, according to NASA JPL's orbit table. Halimede, Psamathe, Sao, Laomedeia, Neso and the two unnamed moons are far out, on huge, stretched, tilted orbits. Half of each orbit's longest width (its semi-major axis) is about 17 to 51 million km, and one trip round takes about 5.1 to 27.5 years (our conversion of NASA JPL's orbit times). NASA gives some of them sizes: Hippocamp, found in 2013, is only about 34 km across, and Halimede, Sao and Laomedeia are each about 30 to 40 km. NASA gives no size for Psamathe, Neso and the two unnamed moons.

Does Neptune have rings?

Yes. NASA says Neptune has at least five main rings, named Galle, Leverrier, Lassell, Arago and Adams, and four prominent ring arcs. The arcs, called Liberté, Egalité, Fraternité and Courage, are clumps of ring material in the outermost ring, the Adams ring. Voyager 2 photographed the rings up close in August 1989.

Neptune moons FAQ

Does Neptune have 14 or 16 moons? 16, according to NASA in 2026. Fourteen have names; the two newest are still known by their codes, S/2002 N5 and S/2021 N1. NASA does not list 28 moons for Neptune.

How many big moons does Neptune have? Just one: Triton, about 2,700 km across. The second largest, Proteus, is only about 416 km across and not round.

Is Triton a moon of Neptune? Yes, Triton is Neptune's largest moon. Scientists think it started out as an icy object from the Kuiper Belt that Neptune's gravity captured.

What is Triton made of? A crust of frozen nitrogen over an icy mantle, probably around a core of rock and metal, NASA says.

What is Neptune's largest moon? Triton. It is a little wider than Pluto and about 78% of the width of our Moon (our comparison of Triton's NASA JPL diameter with NASA's diameters for the Moon and Pluto).

What is Neptune's smallest moon? NASA does not name one. The smallest sizes NASA gives are about 30 to 40 km, for Hippocamp (about 34 km) and for Halimede, Sao and Laomedeia. Psamathe, Neso and the two unnamed moons have no size on NASA's pages. The sources we used do not establish which of Neptune's 16 moons is smallest.

What is the Nereid moon? Neptune's third-largest moon by NASA JPL's sizes, found in 1949, with one of the most stretched orbits of any moon: it takes 360 days to go round Neptune.

What is Proteus? Neptune's second-largest moon, a box-shaped moon found by Voyager 2 in 1989. It goes round Neptune in about 27 hours.

How far is Neptune from the Sun? About 30 astronomical units (au), NASA says: roughly 4.5 billion km, 30 times as far as Earth (our conversion). One Neptune year lasts about 165 Earth years.

Has any spacecraft visited Neptune? Yes. Voyager 2 flew about 4,800 km over Neptune's cloud tops on 25 August 1989, NASA says. You can read about the other Voyager in our Voyager 1 article.

How we checked

The number of moons, their names and most facts come from NASA's Neptune pages, which we checked on 12 November 2026; where NASA's older moon pages give a smaller number, we followed the newest pages. Moon sizes come from NASA JPL's table of satellite measurements (twice the mean radius) and NASA's moon pages; the Moon's and Pluto's diameters come from NASA. Orbit sizes and times of the small moons come from NASA JPL's table of satellite orbits. Predicted positions of Triton, Proteus and Nereid come from NASA JPL Horizons, in Neptune's own equator frame, every 30 minutes for 12 days and every day for Nereid over 14 months. From them we calculated the orbit tilts and directions, orbit times and distances. Photos are Voyager 2 images from NASA, listed under Sources.

None of the images or videos is AI-generated.