Neptune, the most distant planet in our solar system, is a frigid ice giant accompanied by a retinue of natural satellites. Its remote location presents challenges for observation, yet advancements in telescopic technology and the historic Voyager 2 flyby have unveiled some of its secrets. These moons offer insights into the formation and evolution of the outer solar system.
The Current Count
Neptune currently has 16 known moons. This number has grown significantly since the first discovery, reflecting the ongoing nature of astronomical exploration and continuous improvement in observational capabilities. Early observations identified only a couple of satellites, but subsequent missions and advanced ground-based telescopes have steadily added to the count. The discovery of smaller, fainter moons highlights that our understanding of planetary systems is always evolving.
Naming Neptune’s Lunar Family
Neptune’s moons are named after minor water deities and creatures from Greek mythology, honoring the Roman god of the sea, Neptune (Poseidon in Greek mythology). The currently named moons include:
- Triton
- Nereid
- Naiad
- Thalassa
- Despina
- Galatea
- Larissa
- Proteus
- Halimede
- Sao
- Laomedeia
- Psamathe
- Neso
- Hippocamp
Triton: Neptune’s Largest Moon
Triton is Neptune’s largest and most intriguing moon, accounting for over 99.5% of the mass orbiting Neptune. It is unique among large moons in our solar system due to its retrograde orbit, revolving in the opposite direction to the planet’s rotation. This suggests Triton was gravitationally captured from the Kuiper Belt millions of years ago, rather than forming alongside Neptune. Its composition, density, and size are remarkably similar to Pluto, supporting the capture hypothesis.
Triton’s surface consists of frozen nitrogen and water ice, with temperatures around -235 degrees Celsius (-391 degrees Fahrenheit), making it one of the coldest places in the solar system. Triton is geologically active, exhibiting a young surface with few impact craters. Voyager 2, the only spacecraft to visit Neptune and Triton, observed active cryovolcanism, with plumes of nitrogen gas and dust erupting from beneath the surface, reaching heights of up to 8 kilometers. This activity suggests a subsurface ocean of liquid water beneath its icy crust, heated by tidal forces.
Triton has a thin, hazy atmosphere composed primarily of nitrogen, with trace amounts of methane and carbon monoxide, originating from the sublimation of nitrogen ice on its surface. Its high albedo, reflecting about 70% of sunlight, contributes to its cold temperatures. Triton’s retrograde orbit means it is slowly spiraling inward towards Neptune. In billions of years, it is predicted to be torn apart by the planet’s tidal forces, potentially forming a new ring system.
The Remaining Moons
Beyond Triton, Neptune’s other moons are smaller and fall into two main categories: inner regular moons and outer irregular moons. The seven inner regular moons—Naiad, Thalassa, Despina, Galatea, Larissa, Hippocamp, and Proteus—orbit close to Neptune in prograde, nearly circular paths aligned with the planet’s equatorial plane. These moons likely re-formed from the debris of Neptune’s original satellite system, disrupted during Triton’s capture. Many inner moons are dark and heavily cratered; some, like Despina and Galatea, act as shepherd moons, influencing Neptune’s narrow ring system.
Proteus is the second-largest moon of Neptune and the largest inner satellite, measuring over 400 kilometers across. Despite its size, Proteus is irregularly shaped, resembling a “box-like” polyhedron rather than a sphere. It is one of the darkest objects in the solar system, reflecting only about 6% of sunlight. Its heavily cratered surface shows no signs of geological modification. Hippocamp, a smaller inner moon, is thought to be a fragment that broke off from Proteus due to comet bombardment, making it a third-generation satellite.
Neptune’s outer irregular moons include Nereid, Halimede, Sao, S/2002 N 5, Laomedeia, Psamathe, Neso, and S/2021 N 1. These satellites have highly inclined, eccentric, and often retrograde orbits far from Neptune, suggesting they were gravitationally captured. Nereid, the third-largest moon, has the most eccentric orbit of any moon in the solar system, with its distance from Neptune varying by a factor of nearly seven. The outermost moons, such as Psamathe and Neso, have exceptionally large orbits, taking about 25 Earth years to complete a single revolution around Neptune.