Moons of Neptune

There are 16 known moons of the planet Neptune, fourteen of which are named after water deities and creatures in Greek mythology. The largest of these moons is Triton, discovered by William Lassell on October 10, 1846, just 17 days after Neptune itself was found. Triton is very special because it orbits Neptune in the opposite direction of the planet's rotation and at an angle to Neptune's equator. This suggests that Triton did not form around Neptune but was captured by Neptune's gravity from somewhere else in space.

Triton is so large that it has a thin atmosphere and can form clouds. It is much more massive than another captured moon, Saturn's moon Phoebe. The capture of Triton was a big event that likely caused Neptune's original moons to crash into each other, forming a cloud of debris. Inside Triton's orbit are seven smaller moons that circle Neptune in the same direction as the planet's rotation and close to its equator. Some of these small moons even travel through Neptune's rings.

Beyond Triton, Neptune has eight more distant moons with unusual orbits. One of these is Nereid, the largest of the outer moons, which travels a very long and stretched-out path around Neptune. Neptune's farthest known moon, S/2021 N 1, takes about 27 Earth years to complete one orbit around Neptune, farther away than any other known moon in the Solar System.

History

Triton was discovered by William Lassell in 1846, just seventeen days after the discovery of Neptune. More moons were found later, with Nereid discovered in 1949 and Larissa in 1981 while scientists were looking for rings around Neptune.

In 1989, the spacecraft Voyager 2 found several more moons, including Naiad, Thalassa, Despina, Galatea, and Proteus. Since then, telescopes on Earth and in space have helped discover even more moons, bringing the total to sixteen. These moons are named after water gods and creatures from Greek stories, matching Neptune's role as the god of the sea.

Characteristics

The moons of Neptune are divided into two groups: regular and irregular. The regular moons are seven inner moons that follow circular paths close to Neptune's equator. The irregular moons are nine outer moons with more varied and distant orbits.

Regular moons include Naiad, Thalassa, Despina, Galatea, Larissa, Hippocamp, and Proteus. Most of these moons are within Neptune's synchronous orbit and are gradually moving closer to the planet. Proteus is the largest regular moon and has a heavily cratered surface. These moons are dark, made mostly of water ice mixed with darker materials.

Irregular moons include Triton, Nereid, and several smaller moons. Triton has a nearly circular orbit and a thin atmosphere. Nereid has a very elongated orbit and a rough, elongated shape. The other irregular moons have highly distant orbits, some of the farthest known in the Solar System.

Main article: Triton (moon)

Main article: Nereid (moon)

List

The moons of Neptune are arranged by how long it takes them to orbit the planet, from the shortest to the longest time. Some moons, called irregular or captured moons, have orbits that change often due to the pull of planets and the sun. For these moons, the information about their paths is averaged over a long time—30,000 years—so it may look different from other sources. The main moons, including Triton, have their information based on a specific date: January 1, 2009. The irregular moons' information is based on January 1, 2020. Triton is special because it is the only moon big enough for its surface to become round, and it is shown in bold in the list.

Formation

The moons of Neptune have a very uneven mass distribution, with one moon, Triton, making up almost all of the mass. All the other moons together only add up to about one-third of one percent of the total mass. This is similar to Saturn's moon system, where Titan holds most of the mass, but different from Jupiter and Uranus, which have more balanced systems.

Scientists believe Triton was captured by Neptune after the planet’s original moons had formed, and this event may have destroyed many of the original moons. Only after Triton’s orbit settled down could new moons form from the leftover pieces. One theory suggests Triton was captured during a complex interaction involving three bodies, including a pair of objects from the Kuiper belt. There are also ideas about possible past collisions between moons like Halimede and Nereid, though this is still uncertain.