The planet Neptune has a planetary ring system that is not very bright. It is made up of a few rings and some "ring arcs". The "ring arcs" are part of the outer ring called the Adams ring and are not to be seen anywhere else. The ring particles (small parts) are weirdly dark colored, and have large amounts of microscopic dust. Neptune has thirteen moons. Four of them orbit the planet inside the outermost rings.
Discovery and observations
The rings around Neptune could not be seen from Earth with strong telescopes. The evidence that they were there was first seen when scientists were watching the planet as it passed between the Earth and a star. (When this happens, the light of the star is blocked. This is called "stellar occulation".) Scientists watched this happen 50 times, and on five of those times in the early 1980s they saw the star give a little "blink" just before or after the planet covered it. This meant that rings around the planet were blocking the light, but the rings were not complete.
In 1989 the unmanned spacecraft Voyager 2 flew by Neptune. Voyager 2 sent back pictures that gave most of the information that scientists now have about the rings of Neptune. The pictures also help scientists to know why the rings looked incomplete. Not only did the pictures give a closer view, but they also showed the rings in a way that cannot ever be seen from earth. From earth the rings are always seen with the light of the sun shining towards them. When the spacecraft went past the planet, it took pictures with the sun shining behind the rings, as well. Big particles in the rings looked dark against the sunlight, but all tiny dust particles (microscopic dust) shone brightly when the sun was behind it. This allowed the scientists to work out what the rings were made of, and how all the particles were arranged.
As better types of telescopes have been developed, scientists can now see some of the rings from earth. The brightness of Neptune makes it hard to see the rings, which are much dimmer and the faintest rings still cannot be seen, even with vey powerful telescopes.
Adams ring and the Ring Arcs
The most famous of Neptune's rings is the thin outer Adams ring, although by comparison with the rings of the planets Saturn or Uranus, it is very hard to see. It was originally named as 1989 N1R. The reason that the Adams ring is famous is that it has a feature that has not been seen on any other planetary rings. These are three long narrow "arcs" which are much brighter and look more solid than the rest of the ring system. Each one spans from 4-10° of the whole ring. All the arcs are close to each other, spanning under 40° of longitude, only about 1/8th of the edge of the ring. It has been a puzzle to understand why the stuff in these arcs does not spread out evenly through the whole ring.
The three arcs have been named with French names Liberté, Égalité, and Fraternité. These names came from the famous saying of the French Revolution, which translates into English as "Liberty, Equality and Brotherhood!". These names were suggested by the scientists who discovered them during stellar occultations in 1984 and 1985.
Four of Neptune's 13 moons are inside the ring system. The one that is nearest to the Adam's ring is called Galatea. It is a lumpy mass of rock that orbits around the planet at about the same speed as the planet is rotating. It is about 1000 kilometers inside the ring.
Many scientists now think that it is the effect of the moon Galatea that is causing the three arcs in the Adam's ring. The ring also has 42 wiggles, which are like waves 30 kilometers high. Scientists are able to use this information to work out the mass of the moon Galatea. .
The best photos that came back from Voyager 2 showed that there are clumps of particles in the rings, but no-one can tell whether they are made of fine dust, or if there may be bigger particles in the clumps as well. All of Neptune's rings have a lot of microscopic dust but particularly the arcs of the Adam's ring. Scientists know this because they looked very bright in the photos that Voyager 2 took with the sun shining behind them. There is much more dust in the rings of Neptune than there is in the rings of Saturn.
The Adams ring is very red. The background particles that make up the ring look twice as bright near the three arcs as they do on the side of the ring that is opposite the arcs.
Arc dynamics
With the arrival of the Hubble Space Telescope and adaptive optics ground-based telescopes the ring arcs have been re-examined several times, starting in July 1998.