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So my story takes place on a moon orbiting a gas giant about the size of Jupiter and I was wondering if it was at all possible for a planetary ring to exist beyond the planet's Roche limit. I understand that there are some tiny bands of dust that exist beyond the Roche limit of planets that could be considered "rings" but I was wondering about a large and easily visible ring.

The idea is that this "ring" would exist between the orbit of two of the moons in my planetary system. I guess the origin of the ring would be some kind of collision that blew a moon apart into a whole bunch of tiny pieces. I thought maybe since the moons in this system are rather big, (one is the size of earth, one the size of Mars and there are many smaller moons) that these large moons would create orbital resonances with the ring material that would prevent it from accreting into a new moon.

I was thinking this could work in a similar manner to the way that Jupiter prevents the asteroid belt from accreting. I am also wondering if these resonances could cause small bits of ring material to deorbit and cause meteor showers.

I know, a tough question for sure. Maybe I should have asked it in the astrophysics SE or something but I know there are some pretty smart people in this SE so I hope I'll get some answers. Thanks in advance! (:

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3 Answers 3

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The answer is, in short, Yes.

Saturn, for example, has several ring moonlets and shepherd moons.

Additionally, the Roche Limit varies depending on the size and composition of the satellite itself. One satellite can tolerate a bit more or a bit less than another one.

So, theoretically, you can have a ring between two moons. As far as the specifics of sizes go, that's well beyond what I'm capable of figuring out, but the basic premise seems sound.

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  • $\begingroup$ The moons have to be tiny, though. The moons will punch holes in the ring, the bigger the moon the bigger the hole. $\endgroup$ Commented Aug 28, 2017 at 3:56
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    $\begingroup$ An once in a blue moon, the ring turns into mobius strip during moons alignment... I think $\endgroup$
    – user6760
    Commented Aug 28, 2017 at 4:37
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Yes, but it would be unstable beyond about an order of magnitude greater than the roche limit. The outer edge of Saturn's rings is 8.1 roche limits distant, for example.

The reason being is as soon as tidal forces wanting to spread a planet apart into a ring become negligible, a ring of matter would do the reverse and tend to gravitate into spherical bodies. Any small instability would eventually grow and sweep up the rest of the matter in the ring. Notice that Saturn's rings have a pretty well defined outer border as well as inner.

So yes it is possible, but to a limit.

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Yes.But note the Roche limit applies to celestial bodies held together only by their own gravity. As small objects such a sand grains and boulders that make up a ring are not held together by their own gravity the Roche limit does not apply to them so a ring of such material could theoretically orbit at any distance subject to other restrictions such as atmospheric drag.

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