Sciency Words: Ring Arcs

Sciency Words PHYS copy

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us all expand our scientific vocabularies together. Today’s word is:


Something’s wrong with Neptune’s rings.

Nv11 Neptune's Arcs

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.

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Today’s post is part of Neptune month for the 2015 Mission to the Solar System. Click here to learn more about this series.

4 thoughts on “Sciency Words: Ring Arcs

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