Neptune currently has 16 confirmed natural satellites, which collectively form one of the solar system’s most diverse and complex moon systems. The satellites range from one massive body that dominates the system to many tiny, faint objects. This collection of moons is divided into three distinct groups: the single giant moon, the small inner regular moons, and the distant irregular satellites.
Triton: Neptune’s Largest and Most Unusual Moon
Triton is the largest of Neptune’s satellites, measuring approximately 2,706 kilometers in diameter, comparable in size to Pluto. Its most distinctive feature is its retrograde orbit, meaning it travels around Neptune opposite the planet’s rotation. This backward motion suggests Triton did not form in place around Neptune.
The hypothesis is that Triton was originally a dwarf planet from the Kuiper Belt that was captured by Neptune’s gravity. The capture was likely a violent event that destroyed Neptune’s original moons, reducing them to a debris disc. Triton’s initial, highly elliptical orbit was subsequently circularized by tidal forces, which heated the moon’s interior.
Triton’s surface is remarkably young and displays active geology, including cryovolcanism. The Voyager 2 spacecraft observed geyser-like plumes erupting up to eight kilometers high, spewing nitrogen gas and dark particles. The surface is covered with a highly reflective layer of frozen nitrogen, methane, and carbon monoxide, giving it one of the highest albedos in the solar system. This icy composition and its thin, hazy atmosphere link Triton to other icy objects found in the Kuiper Belt.
The Inner Regular Moons
The inner regular moons form a tight group of seven small satellites that orbit much closer to Neptune than Triton. They follow prograde, nearly circular paths aligned with Neptune’s equator. The moons include:
- Naiad
- Thalassa
- Despina
- Galatea
- Larissa
- Hippocamp
- Proteus
The largest of this group is Proteus, an irregularly shaped body measuring over 400 kilometers across. Most of these inner satellites were discovered by the Voyager 2 spacecraft during its 1989 flyby.
This current inner system is believed to have re-accreted from the rubble disk created when Triton’s capture disrupted the planet’s primordial satellite system. Their close proximity to Neptune’s ring system means they play a role in shaping its structure. The moon Galatea acts as a “shepherd moon,” using its gravitational influence to confine the particles of the narrow Adams ring, which helps keep the ring arcs stable.
The Distant Irregular Satellites
Neptune’s remaining moons are the distant, irregular satellites, which orbit far outside the system of Triton and the inner moons. These small bodies are characterized by highly inclined and eccentric (elliptical) orbits, with some traveling backward in retrograde paths. Their chaotic orbital characteristics suggest that, like Triton, they were captured by Neptune’s gravity rather than forming alongside the planet.
The most extreme example is Neso, which holds one of the longest orbital periods of any known moon in the solar system, taking nearly 27 years to complete a single circuit. At its farthest point, Neso is more distant from Neptune than the planet Mercury is from the Sun. These moons are extremely faint and small, with estimated diameters often less than 60 kilometers. Many of these irregular satellites were discovered in the early 2000s using advanced, ground-based telescopes, with the most recent two confirmed in 2024.