Sciency Words: Ring Arcs

Today’s post is part of a special series that first appeared on Planet Pailly. Every Thursday, we take a look at a new and interesting scientific term to help us all expand our scientific vocabularies together. Today’s term is:

Ring Arcs

Something’s wrong with Neptune’s rings.


Neptune has five rings, all named after astronomers or scientists associated with significant Neptune-related discoveries. They are (in order of increasing distance from Neptune):

  • Galle: named after the guy who discovered Neptune, sort of. He had help from…
  • Le Verrier: named after the guy who calculated Neptune’s exact position, allowing Galle to “discover” it.
  • Lassell: named after the discoverer of Triton, Neptune’s largest moon.
  • Arago: named after the teacher who encouraged Le Verrier in his calculations and helped defend Le Verrier in a dispute with…
  • Adams: named after another person who calculated Neptune’s position before its discovery and started a fuss with Le Verrier over who deserved credit.

There was plenty of drama surrounding the discovery of Neptune, and that has been preserved in the names of the rings that also surround the planet.

Neptune has an unnamed sixth “ring,” if we can be generous enough to call it a ring, located between Arago and Adams. A small moon named Galatea also orbits within that gap. This unnamed ring doesn’t circle all the way around the planet, so it is better described as an “arc.”

Furthermore, a short segment of the outermost ring (Adams) is also broken up into several small arcs. These arcs were originally named Liberty, Equality, and Fraternity (bonus points to anyone who can tell me what the planet Neptune has to do with the French Revolution).

Later, two more arcs were found in the Adams ring, so the list became (in order):

  • Courage: the faintest arc.
  • Liberty: often described as the “leading arc,” even though Courage orbits ahead of it.
  • Equality 1 and Equality 2: the Equalities are so close together that they’re almost a single arc.
  • Fraternity: brings up the rear and is the largest and brightest of Neptune’s arcs.

The existence of these arcs doesn’t make a whole lot of sense. Ring particles should spread out the fill the gaps within a matter of months, yet the arcs have remained stable since their discovery in the 1980’s.

An orbital resonance with Galatea is almost certainly involved, but mathematical models of Galatea and the Adams arcs don’t always match with observations. Neptune may have an additional as-yet-undiscovered moon near its rings, or perhaps some other unknown factor is at work.

Probably aliens.

P.S.: Neptune isn’t the only planet with arcs in its rings. Saturn has them too. So Neptune, you don’t have anything to feel embarrassed about.


Neptune’s Rings and “Ring Arcs” from JPL’s Voyager Mission webpage.

Rings of Neptune from Universe Today.

Stability of Neptune’s Ring Arcs in Question from Letters to Nature.

Written by James Pailly.

To read all the articles in the ‘Sciency Words‘ series, visit the Planet Pailly blog.

  • Paulo R. Mendes

    Poor Neptune! don’t worry – we love you the way you are! 🙂

  • Leonardo Faria

    “(bonus points to anyone who can tell me what the planet Neptune has to do with the French Revolution).”

    The REVOLUTION of the pieces of Adams’s arc around Neptune was studied by a FRENCH astronomer, pissed that the ring had been named after an English astronomer. Allons enfants de la Patrie/Le jour de gloire est arrivé !

    • James Pailly

      Makes sense to me!

      • Is that the reason you had in mind?

        • James Pailly

          I’m not sure I had any idea in mind. Although with all those French astronomers involved in Neptune’s discovery, I’m sure that’s part of it.

  • Sounds like it was Walpole again.

    Still, it does beg the question as to what could cause those Ring Arcs and what could even keep them stale. Good thing NASA’s planning a Flagship mission to those Ice Giants, right? I mean, that’s still a thing?

    • Siebren Kusters

      Always Walpole.