Find out how many moons does Neptune have, what are their main characteristics, and why they are divided into the inner and outer moons of this icy giant.
Neptune, an icy gas giant situated as the eighth planet from our Sun, was unearthed in 1846 by astronomers Urbain Le Verrier and Johann Halle. Following conventional planetary naming conventions, Neptune bears the title of the Roman god of the sea, akin to the Greek deity Poseidon. Just seventeen days following its discovery, astronomers discerned the presence of a moon system orbiting the planet.
How many moons does Neptune have?
Initially, only Triton, Neptune’s most substantial moon, was visible. However, as the mid-20th century approached and beyond, advancements in ground-based telescopes and the advent of robotic space probes led to the discovery of numerous additional moons.
Presently, Neptune boasts 14 acknowledged moons, all christened after minor water deities from Greek mythology as a tribute to their parent planet.
Discovery and naming of Neptune’s moons
Triton, Neptune’s largest and most massive moon, was the inaugural discovery among its satellites. William Lascelles first sighted it on October 10, 1846, a mere seventeen days following Neptune’s identification. Nearly a century elapsed before subsequent moons came to light.
Nereid, Neptune’s second most sizable and massive moon, was unveiled on May 1, 1949, by Gerard P. Kuiper, utilizing photographic plates from the MacDonald Observatory in Fort Davis, Texas, after whom the Kuiper Belt is named.
The third satellite, later named Larissa, was fortuitously observed by Harold Reitsema, William B. Hubbard, Larry A. Lebofsky, and David Tholen on May 24, 1981. Its discovery was an incidental outcome of a protracted quest for rings akin to those encircling Uranus, initiated four years prior. The revelation of this moon was prompted by a subtle reduction in the star’s luminosity during its closest proximity to Neptune, signifying the presence of a satellite rather than a ring.
Subsequent to Voyager 2’s 1989 voyage past Neptune, which rediscovered Larissa and disclosed five additional inner satellites – Naiad, Thalassa, Despina, Galatea, and Proteus – no further satellites emerged until 2001. Two surveys employing sizable ground-based telescopes – the Cerro Tololo Inter-American Observatory and the Canada-France-Hawaii telescopes – unearthed five more outer satellites, elevating the total count to thirteen. In subsequent years, both teams re-observed all five of these satellites: Galimede, San, Psamate, Laomede, and Neso.
On July 15, 2013, astronomers led by Mark R. Showalter of the SETI Institute announced the discovery of a hitherto unknown fourteenth satellite in images captured by the Hubble Space Telescope between 2004 and 2009. This unnamed satellite, provisionally labeled as S/2004 N 1, measures no more than 16-20 km in diameter.
According to astronomical tradition, all of Neptune’s moons are taken from Greek and Roman mythology. In this case, they are all named after the gods of the sea or the children of Poseidon (which include Triton, Proteus, Depsina, and Thalassa), minor Greek water diets (Naiad and Nereid), or nereids – water nymphs of Greek mythology (Galimede, Galatea, Nesso, Sao, Laomedeus, and Psamatea).
However, many satellites did not receive official names until the 20th century. The name Triton was originally proposed by Camille Flammarion in his 1880 book Popular Astronomy, but did not come into common use until at least the 1930s.
Neptune’s inner (regular) moons
We have learned how many moons Neptune has, but what is their classification?
Neptune’s regular moons occupy proximate orbits to the planet, following circular paths within its equatorial plane. Arranged by their distance from Neptune, they include Naiad (48,227 km), Thalassa (50,074 km), Despina (52,526 km), Galatea (61,953 km), Larissa (73,548 km), S/2004 N 1 (105,300 ± 50 km), and Proteus (117,646 km).
With the exception of two outer satellites, all others are in synchronous orbit with Neptune, meaning their orbital periods are longer than Neptune’s rotational period (0.6713 days), subjecting them to tidal deceleration.
The inner moons share a close association with Neptune’s system of narrow rings. Among them, Naiad and Thalassa orbit between the Halle and Leverier rings, while Despina orbits just within the Leverier ring. Galatea, the subsequent satellite, orbits within the most prominent Adams ring, aiding in its stability by gravitational interaction that supports the ring’s particles.
Based on observational data and density assumptions, Nayada measures 96 × 60 × 52 km and weighs approximately 1.9 × 1017 kg. Meanwhile, Thalassa has dimensions of 108 x 100 x 52 km and weighs 3.5 x 1017 kg; Despina has dimensions of 180 x 148 x 128 and weighs 21 x 1017 kg; Galatea has dimensions of 204 x 184 x 144 and weighs 37.5 x 1017 kg; Larissa – 216 x 204 x 168 and weighs 49.5 x 1017 kg; S/2004 N1 – 16-20 km in diameter and weighs 0.5 ± 0.4 x 1017 kg; and Proteus – 436 x 416 x 402 and weighs 50.35 x 1017 kg.
Only the two most significant regular satellites have been imaged with adequate resolution to discern their shapes and surface characteristics. However, except for Larissa and Proteus, which exhibit predominantly spherical shapes, all of Neptune’s moons are thought to be elongated. Furthermore, these moons are uniformly dark entities, featuring geometric albedos ranging from 7 to 10%.
Analysis of their spectra suggests that they are composed of water ice tainted with traces of exceedingly dark material, likely organic compounds. In this regard, Neptune’s inner moons share similarities with Uranus’ inner moons.
Neptune’s outer (irregular) moons
Neptune’s irregular moons comprise the remaining moons orbiting the planet (including Triton). Typically, they traverse tilted, eccentric, and often retrograde paths, situated far from Neptune. The sole exception is Triton, which orbits in close proximity to the planet on a circular path, albeit retrograde and inclined.
Arranged by their distance from Neptune, the irregular satellites include Triton, Nereid, Halimede, Sao, Laomede, Neso, and Psamate, encompassing both prograde and retrograde entities. Except for Triton and Nereid, Neptune’s irregular moons bear resemblance to those of other gas giants and are thought to have been captured by Neptune’s gravitational pull.
These irregular moons share similar characteristics in terms of size and mass, spanning from approximately 40 km in diameter and 4 x 10^16 kg in mass (such as Psamathus) to 62 km and 16 x 10^16 kg for Halimedes.
Triton and Nereid
Triton and Nereid stand out as distinctive irregular moons of Neptune and are hence treated separately from the other five irregular satellites. Four primary distinctions set them apart from the rest.
Firstly, they rank as the largest among the known irregular moons in the solar system. Triton, in particular, dwarfs all other irregular satellites by nearly an order of magnitude and constitutes over 99.5% of the total mass orbiting Neptune, encompassing the planet’s rings and its thirteen other known moons.
Secondly, both Triton and Nereid exhibit notably diminutive semi-axes, with Triton’s being an order of magnitude smaller than those of other known irregular satellites.
Thirdly, these moons display unusual orbital eccentricities: Nereid boasts one of the most eccentric orbits among known irregular satellites, while Triton’s orbit approximates a nearly perfect circle.
Lastly, Nereid possesses the lowest inclination of any known irregular satellite.
Triton, with an average diameter of approximately 2700 km and a mass of 214080 ± 520 x 10^17 kg, claims the title of Neptune’s largest moon and is the sole entity sizable enough to achieve hydrostatic equilibrium, presenting a spherical shape. Positioned between Neptune’s inner and outer moons at a distance of 354,759 km from the planet, Triton travels along a retrograde quasi-circular orbit. Composed primarily of nitrogen, methane, carbon dioxide, and water ice, Triton boasts a high geometric albedo exceeding 70% and a Bond albedo of up to 90%, rendering it one of the brightest objects in the solar system. Its reddish surface hue results from the interaction of ultraviolet radiation and methane, precipitating the formation of tholins.
Triton ranks among the coldest satellites in the solar system, with a surface temperature hovering around 38 K (-235.2 °C). However, geological activity, including cryovolcanism, and temperature fluctuations causing sublimation, contribute to Triton being one of the two moons in the solar system with a significant atmosphere. This atmosphere, primarily composed of nitrogen with trace amounts of methane and carbon monoxide, exhibits a pressure of about 14 bar. With a density of roughly 2 g/cm^3, Triton suggests a composition comprising approximately two-thirds rock and one-third ice, primarily water ice, possibly accompanied by a subterranean ocean. Surface features include a substantial southern polar cap, ancient crater plains etched with graben and scarps, and youthful features resulting from endogenous resurfacing.
Attributed to its retrograde orbit and relative proximity to Neptune, Triton is classified among the planet’s irregular moons and is believed to be a captured object, conceivably a dwarf planet once situated in the Kuiper belt. Its orbital characteristics predicate eventual tidal deceleration, culminating in Triton spiraling inward to collide with Neptune in roughly 3.6 billion years.
Nereid, the third-largest moon of Neptune, navigates a translational yet highly eccentric orbit, likely stemming from its status as a former regular satellite, subsequently dispersed into its current orbit by gravitational interactions during Triton’s capture. Spectroscopic analysis has confirmed the presence of water ice on its surface. Nereid exhibits significant irregular fluctuations in apparent stellar magnitude, likely attributed to forced precession or chaotic rotation, coupled with an elongated shape and distinct bright or dark spots on its surface.
Given the disproportionate mass distribution of Neptune’s moons, Triton is thought to have been captured post the formation of Neptune’s original satellite system, much of which would have been disintegrated during the capture process. Various theories have been posited concerning Triton’s capture mechanism, with the prevailing one proposing Triton as a remnant of a binary Kuiper Belt object obliterated in collision with Neptune. In this scenario, Triton was ensnared in a three-body collision, adopting a retrograde orbit, while its counterpart was either annihilated or expelled during the event.
Triton’s presumably highly eccentric orbit upon capture would have engendered chaotic perturbations in the orbits of Neptune’s original inner moons, ultimately precipitating their collision and metamorphosis into a debris disk. It was only after Triton’s orbit stabilized into a circular trajectory that certain remnants of the debris re-coalesced into the present-day regular satellites. Consequently, Neptune’s extant inner moons are not original entities that coalesced with Neptune.
Numerical simulations suggest a 0.41 probability of a collision between a satellite of Galileo and Nereid in the past. Although the occurrence of such an event remains uncertain, both satellites share a similar “gray” coloration, hinting at the possibility of Galimedes being a fragment of Nereid.
How Neptune’s moons have been studied
Given the distance from the Sun, the only mission that has ever studied Neptune and its moons up close was the Voyager 2 mission. And while no missions are currently planned, several proposals have been made that would send a robotic probe to the system in the late 2020s or early 2030s.
