How Many Moons Does Neptune Have?

by Fraser Cain on March 12, 2012

Neptune and Moons

Neptune and Moons

Did you want to know how many moons Neptune has? At the current count, Neptune has 13 moons.

The largest of Neptune’s moons is Triton, which was discovered by William Lassell just 17 days after Neptune itself was discovered. Triton measures 2700 km across, and it’s the 7th largest moon in the Solar System. Triton is unusual because it orbits Neptune in the opposite direction of the planet’s rotation. It’s the only large moon to do this, and astronomers think that Triton was a Kuiper Belt Object captured by Neptune’s gravity millions of years ago.

The second of Neptunes moons is Nereid, discovered in 1949 by Gerard Kuiper. It’s named after one of the sea-nymphs of Greek mythology. It has a diameter of only 340 km, and this is actually Neptune’s third largest moon.

The third moon to be discovered around Neptune was Larissa, found in 1981 by a team of astronomers using ground-based equipment. Shortly after finding Larissa, they lost it again. And it wasn’t rediscovered until Voyager 2 arrived at Neptune in 1989.

Astronomers lost track of Larissa until NASA’s Voyager 2 spacecraft flew past Neptune in 1989. Voyager 2 found Larissa again, and discovered 5 additional inner moons: Naiad, Thalassa, Despina, Galatea and Proteus. After Voyager 2’s flyby was complete, the total for Neptune’s moons rose to 8.

And then in 2002 and 2003, astronomers uncovered 5 additional moons circling Neptune using Earth observatories. The newly discovered moons were named Halimede, Sao, Psamathe, Laomedeia, and Neso.

And that’s how we reached the current total of 13 moons for Neptune.

About those strange names; Neptune is the Roman god of the sea, right? And Triton the Greek god of the sea, so Kuiper named his discovery Nereid, after the Nereids, who are (in Greek mythology) the fifty sea nymph daughters of Nereus and Doris, who accompanied Poseidon (the Greek god of the sea; Triton? His son).

And who are some of the Nereids? Why Galatea, Halimede, Laomedeia, Neso, Psamathe, and Sao!

Which leaves Naiad (a river nymph, in Greek mythology), Thalassa (daughter of Aether and Hemera, in Greek – you get it – but also Greek for ‘sea’), Despina (‘Despoina’ in the original; yet another nymph, daughter of Poseidon and Demeter), Larissa (another nymph; Poseidon’s lover), and Proteus (yet another Greek sea god, son of Poseidon … or maybe of Nereus and Doris – so he’d be the Nereids’ brother – or of Oceanus and a Naiad, or…).

Of course, if you ask the question again in a few years, it might be higher, since astronomers are working to uncover new moons around the planets all the time.

Neptune’s Moon and Rings


Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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