Neptune - Neptune's moons and rings | planet | Britannica.com
Earth
Of Voyager’s six discoveries, all but Proteus orbit Neptune in less time than it takes the planet to rotate. Hence, to an observer positioned near Neptune’s cloud tops, these five would appear to rise in the west and set in the east. Voyager observed two of its discoveries, Proteus and Larissa, closely enough to detect both their size and approximate shape. Both bodies are irregular in shape and appear to have heavily cratered surfaces. The sizes of the other four are estimated from a combination of distant images and their brightnesses, based on the assumption that they reflect about as much light as Proteus and Larissa—about 7 percent. Proteus, with a mean radius of about 208 km (129 miles), is a little larger than Nereid , with a mean radius of about 170 km (106 miles). Because Proteus is so near to Neptune, however, scientists on Earth had been unable to detect it in the planet’s glare. The other five moons are much smaller, each having a mean radius of less than 100 km (60 miles).
Voyager did not observe Nereid at close range, but data from the probe indicate that it has a nearly spherical shape. Voyager detected no large variations in brightness as Nereid rotated. Although the spacecraft was unable to determine a rotation period, the moon’s highly elliptical orbit makes it unlikely that it is in synchronous rotation—i.e., that its rotation and orbital periods are equal. The rotation period of Triton is synchronous, and those of Neptune’s other inner moons are probably synchronous or very nearly so.
Planets: Fact or Fiction?
Triton is similar in size, density, and surface composition to the dwarf planet Pluto . Its highly inclined, retrograde orbit suggests that it is a captured object, which perhaps formed originally, like Pluto, as an independent icy planetesimal in the outer solar system. Its original orbit would have been highly eccentric, but tidal interactions between Triton and Neptune—cyclic deformations in each body caused by the gravitational attraction of the other—eventually would have reshaped its path around Neptune into a circle. The process of Triton’s capture and circularization of its orbit would have severely disrupted any previously existing system of moons that had formed along with Neptune from a disk of protoplanetary material. Nereid’s radical orbit may be one consequence of this process (although the possibility that Nereid too is a captured object has not been ruled out). Moons that were in orbit between Proteus and Nereid would have been ejected from the Neptunian system, thrown into Neptune itself, or absorbed by the molten Triton. Even those moons orbiting closer to Neptune would not have escaped some disruption. The present orbits of Naiad through Proteus  are probably very different from their original orbits, and these moons may be only fragments of the original bodies that formed with Neptune. Subsequent bombardment by Neptune-orbiting debris and by meteoroids from interplanetary space may have further altered their sizes, shapes, and orbits.
The ring system
Ringling Bros. Folds Its Tent
Evidence that Neptune has one or more rings arose in the mid-1980s when stellar occultation studies from Earth occasionally showed a brief dip in the star’s brightness just before or after the planet passed in front of it. Because dips were seen only in some studies and never symmetrically on both sides of the planet, scientists concluded that any rings present do not completely encircle Neptune but instead have the form of partial rings, or ring arcs.
Neptune’s ring system, captured by Voyager 2 in two long-exposure backlit images made a few hours …
Jet Propulsion Laboratory/National Aeronautics and Space Administration
Images from Voyager 2, however, revealed a system of six rings, each of which in fact fully surrounds Neptune. The putative arcs turned out to be bright regions in the outermost ring, named Adams, where the density of ring particles is particularly high. Although rings also encircle each of the other three giant planets, none displays the striking clumpiness of Adams. The arcs are found within a 45° segment of the ring. From leading to trailing, the most prominent are named Courage, Liberté, Egalité 1, Egalité 2, and Fraternité. They range in length from about 1,000 km (600 miles) to more than 10,000 km (6,000 miles). Although the moon Galatea, which orbits just planetward of the inner edge of Adams, may gravitationally interact with the ring to trap ring particles temporarily in such arclike regions, collisions between ring particles should eventually spread the constituent material relatively uniformly around the ring. Consequently, it is suspected that the event that supplied the material for Adams’s enigmatic arcs—perhaps the breakup of a small moon—occurred within the past few thousand years.
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The other five known rings of Neptune— Galle, Le Verrier, Lassell, Arago, and Galatea, in order of increasing distance from the planet—lack the nonuniformity in density exhibited by Adams. Le Verrier, which is about 110 km (70 miles) in radial width, closely resembles the nonarc regions of Adams. Similar to the relationship between the moon Galatea and the ring Adams, the moon Despina orbits Neptune just planetward of the ring Le Verrier. Each moon may gravitationally repel particles near the inner edge of its respective ring, acting as a shepherd moon to keep ring material from spreading inward. (For fuller treatments of shepherding effects, see Saturn: Moons: Orbital and rotational dynamics ; Uranus: The ring system .)
Galle, the innermost ring, is much broader and fainter than either Adams or Le Verrier, possibly owing to the absence of a nearby moon that could provide a strong shepherding effect. Lassell consists of a faint plateau of ring material that extends outward from Le Verrier about halfway to Adams. Arago is the name used to distinguish a narrow, relatively bright region at the outer edge of Lassell. Galatea is the name generally used to refer to a faint ring of material spread all along the orbit of the moon Galatea.
Rings of Neptune