Neptune and Triton [image: Voyager 2]

Neptune, named for the Roman sea god, is the last planet out from the Sun, lying at the inner boundary of the Kuiper Belt. It has fourteen known moons and they're a mixed bag. One of them – Triton – represents over 99% of the total mass of Neptune moons. Thirteen little moons share what's left.

Triton is almost certainly an object captured from the Kuiper Belt, a vast region beyond Neptune containing millions of icy objects. The moon has a retrograde orbit, i.e., it orbits in the opposite direction to Neptune's rotation. Objects that form together in a system move in the same direction unless they've been severely disrupted by collisions. (You can read more about Triton by clicking on the link at the end of this article.)

Outer moons – gotcha!
Triton isn't Neptune's only captive moon. The planet also probably captured its outermost five satellites, Halimede, Sao, Laomedeia, Psamathe and Neso. They're all named for Nereids, sea nymphs in Greek mythology. These moons are small, ranging from 40-62 km (25-38 mi) across, and irregular in shape. Their orbits are notably eccentric, meaning they're not round, but rather stretched-out circles.

Two of these little moons have retrograde orbits, and three have prograde orbits, i.e., they orbit in the same direction as Neptune's rotation. Psamathe and Neso are farther away from their planet than any other known moons in the Solar System. They take over a quarter of a century to orbit Neptune. Their orbits also suggest they may be two pieces thrown out from a collision event in the Kuiper Belt.

Inner moons – crash, bang, reform
When Triton joined the Neptunian system it caused gravitational chaos, disturbing the orbits of the existing moons. They crashed into each other and ended up as a disk of rubble around Neptune's equator. After Triton's orbit stabilized, eventually new moons formed from the rubble. We don't know what the original moons were like, but now there are seven small moons closer to Neptune than Triton, some of them inside Neptune's rings.

Voyager 2 discovered five of the inner moons in 1989. In addition, Voyager imaged Larissa which a ground-based team had detected in 1981 using a special technique to search for rings.

In 2013 a seventh inner moon was discovered in 2004 Hubble Space Telescope data. It orbits between Larissa and Proteus, and in February 2019, it was officially named Hippocamp after a mythical sea monster. Although the other inner moons range in size from 50-400 km (30-250 mi), the new one is much smaller, only about 18 km (11 mi) across. No wonder nobody had noticed it before.

Proteus – named for an ancient sea god and shape changer – is the second largest of all Neptune's moons. At around 400 km (250 mi), Proteus might have been spherical, but it didn't quite make it. Triton is the only Neptunian moon with enough mass for its gravity to pull it into a sphere. Of the inner moons only Larissa and Proteus were even large enough for Voyager photos to show surface features. They are both heavily cratered. Proteus's biggest crater Pharos is over 150 km (95 mi) across.

The seven inner moons orbit Neptune's equator in circular prograde orbits. However, because of their closeness to Neptune, their orbits are decaying. This means that they're slowing down, and in the distant future tidal forces will either break them up to form new rings or send them crashing into Neptune.

Found from the ground
Neptune was discovered in 1846, and Triton a few weeks later. Not surprisingly, considering the distance and small size of the other moons, a century passed before a second moon was discovered. The discoverer was Gerard Kuiper after whom the Kuiper Belt was named. He discovered Nereid in 1949.

Since Proteus is bigger than Nereid, you'd have expected it to be discovered first. However all of the inner planets are covered in a dark, reddish material that may be some kind of organic compound. They're also close to Neptune and lost in the glare of its reflected sunlight. The distance from Proteus to Neptune is less than a third of the Earth-Moon distance. Its discovery had to await a visiting space probe.

Nereid has a prograde orbit, but a highly eccentric one. The red line on the diagram shows an orbit like Nereid's. Its distance from Neptune varies from about 1,400,000 km (850,000 mi) to nearly 10 million kilometers (6 million miles). The strange orbit suggests that Nereid's a captured object. However there is also evidence that suggests it could have been an inner moon whose orbit was disrupted during the capture of Triton.

It wasn't until the 21st century that ground-based telescopes had advanced enough to discover the five tiny outer moons.

Naming the moons
Neptune's biggest moon was nameless for about a century. In 1880 French astronomer Camille Flammarion called it Triton after a son of Poseidon (Neptune), but people pretty much ignored it. It was still just Neptune's satellite. This was satisfactory until Kuiper discovered a second one which he named Nereid.

It was then almost another half century before the total number of moons starting shooting up. They were all given designations that identified them. But in addition, the International Astronomical Union (IAU) decided they should stick with the theme of classical water deities in their naming.