Neptune, the most distant giant planet in our solar system, hosts a remarkable and complex system of natural satellites. This vast collection of orbiting bodies includes a mix of small, dark rocks and one immense, unique world, shaped by a turbulent history of gravitational interactions. While only two moons were known before the late 20th century, the 1989 flyby of the Voyager 2 spacecraft dramatically increased the known population. Today, the confirmed count of moons orbiting the ice giant stands at 14, each named for a Greek or Roman mythological figure associated with water or the sea.
The Giant and Unique Satellite: Triton
The largest and most scientifically compelling moon in the Neptunian system is Triton, which was discovered in 1846. Triton is the only moon in the system massive enough to be pulled into a spherical shape by its own gravity. Its orbit is highly unusual because it is retrograde, meaning it travels in the direction opposite to Neptune’s rotation, a feature that suggests it did not form with the planet.
This backward path indicates that Triton was likely a dwarf planet gravitationally captured from the Kuiper Belt, a vast ring of icy bodies beyond Neptune. The capture event is thought to have violently disrupted Neptune’s original satellite system, leading to the formation of the current, smaller inner moons from the resulting debris. Voyager 2’s visit revealed a geologically active world with a thin nitrogen atmosphere and evidence of cryovolcanism, manifesting as plumes of nitrogen gas and dark material erupting from the surface.
The Regular Inner Moon System
The seven inner moons are classified as regular satellites because they follow prograde, nearly circular orbits with very low inclinations, traveling close to Neptune’s equatorial plane. These satellites are small, dark, and irregularly shaped, with the exception of Proteus, which is the largest of the group. The seven inner moons are:
- Naiad
- Thalassa
- Despina
- Galatea
- Larissa
- Hippocamp
- Proteus
The inner moons interact strongly with Neptune’s faint ring system. Galatea acts as a shepherd moon for the most prominent feature, the Adams Ring, using its gravity to confine the ring particles into narrow arcs. Hippocamp, the most recently discovered moon, was confirmed in 2013 from old Hubble Space Telescope images, and it is the smallest known satellite in the system. As they orbit close to the planet, the inner moons are subject to tidal forces that are slowly causing their orbits to decay, meaning they will eventually spiral into Neptune’s atmosphere or break apart to form new rings.
The Irregular Outer Moons
The remaining six moons are classified as irregular satellites, defined by their distant, highly inclined, and often eccentric orbits. Their highly varied orbital paths and greater distance from the planet suggest they are captured objects, much like Triton, but they are significantly smaller and less massive. The six irregular moons are:
- Nereid
- Halimede
- Sao
- Laomedeia
- Psamathe
- Neso
Nereid is the largest of this distant group and is unique even among the irregular moons due to its extremely eccentric orbit. This elongated path causes Nereid’s distance from Neptune to change drastically throughout its orbit, a possible consequence of the massive gravitational disruption caused by Triton’s capture. The remaining five moons, which were largely discovered in the early 2000s, are believed to be fragments of a few larger bodies that were captured and then fragmented by collisions. Neso and Psamathe are notable for having the largest known orbits of any moon in the solar system, taking over 25 years to complete a single, sweeping revolution around Neptune.