There's a comparison to what I'm talking about on a larger scale: Jupiter's got its Trojans and Greeks, and Earth does have them too. Those are more or less loose and sparse groups of asteroids that stick on their parent object's L4 and L5 points. There's no comparison (as far as I'm aware, though I may be uninformed) to it on a smaller, planetary scale. Jupiter's rings are too sparse for it to be noticeable, Saturn's only significant moon, Titan, is far away from the rings. That got me wondering, how would a system like that look. A system without just and only regular, circular rings, but also with a major moon, would add the complexity and uniqueness of the system by transforming some of those rings into its trojans.


If there were a major moon inside, or on the outer layers, of a planetary ring system, would the ring's particles, which would accumulate at the moon's L4 and L5 points, be there for an extended period (talking about hundreds of millions of years) or would they be too unstable and would start accumulating or colliding with the planet or the moon? Or maybe they won't be able to establish themselves there in the first place at all?

  • $\begingroup$ If I understand your question (it's not completely clear), I'm not convinced a moon that's significantly smaller than the planet it orbits could have significant debris in its lagrange points unless it was a very large distance away from the planet to guarantee the planet's gravity didn't disrupt the relationship. Said another way, I believe the moon would have to be a very long way away or very large when compared to the planet. However, what do I mean by "significant?" What's the smallest object you're imagining in the L4/L5 points? $\endgroup$
    – JBH
    Commented Mar 4, 2023 at 23:51
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    $\begingroup$ According to this answer on Quora the lunar L4/L5 points aren't stable because the Sun perturbs them too much. You do get dust clouds up there, but nothing on the scale of ring material. I would expect that such material would get pulled into a regular Earth or Sun orbit in a relatively short time. $\endgroup$
    – Cadence
    Commented Mar 5, 2023 at 5:40
  • $\begingroup$ @JBH The moon here I imagine to be at least around the size of Io, but could very well be bigger, say for example the size of Mars or Venus, as the scenario of it being a captured object would be quite likely in this setting. I'm not sure about the distance, I think it would look the most spectacular if it was just outside the ring system, as its final layer, but further away distances work too. As for what's supposed to be in Lagrange points, I just want material here to at least somewhat look like part of the ring, the thinner the better, though it probably would be hard to achieve $\endgroup$
    – Yulian
    Commented Mar 5, 2023 at 8:53
  • $\begingroup$ @Cadence The planet here that I'm talking about is ~12 AU away from it's Sun-like star and there are just 2 other, inner planets (technically 3, since one is a binary), that are ~11 AUs away at any given time and that don't exceed 6 Earth masses combined, so I don't think their influence would be that significant, especially if the moon itself could have a few moon masses (it would be at least the size of a Galilean Moon, but could very well be a captured planet) $\endgroup$
    – Yulian
    Commented Mar 5, 2023 at 8:57
  • $\begingroup$ How are you going to form a ring with L4 L5 points? Those regions are separated by one sixth of an orbit along the secondary bodies orbit. I guess I am having a failure of imagination on how a ring is constructed from effectively three points. $\endgroup$ Commented Mar 6, 2023 at 19:40

1 Answer 1


A major moon either inside or on the outer layers of a planetary ring system would clean up a huge chunk of the ring system. A moon within the ring system creates a gap, e.g. Pan, Mimas. A major moon would create a major gap.

L4 and L5 are likely to be stable, but there wouldn't be a ring.


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