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I am creating this planet which is mainly barren and desert around the equator, but towards the poles there are huge mountains with glaciers and frosts. There is not much sea around. The inhabitants are humanoids with very strong animal characteristics, in a society that the jungle law applies (survival of the strongest, tribes, etc.). So, I consider having two moons, one smaller and one quite large, mainly to be used as a metaphor, showcasing the society's inequalities, their sudden and violent mood swings and their unpredictable nature, thinking also that two moons could affect their behaviors in such ways.

So, my first question is if my thinking makes sense from a scientific point of view? The existence of two moons can be aligned with my planet's geography, i.e. a mainly desert planet, with few oceans - that are considered as extremely dangerous and some very few but very big frozen mountain near the poles? Two moons could affect behaviors, right?

My second question is if it is possible (scientifically again), the two moons to share the same ring - in an "infinity/mobius strip" shape maybe? I would really like to use it as another metaphor for the inter-connectiveness of good and evil that exist in everyone and to add a cool effect maybe. :)

Thank you!

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    $\begingroup$ Two moons could affect the behavior, yes, of whatever rockets or asteroids come close. But unfortunately if you are thinking of the behavior of the people living on the planet then no, sorry, there is no link whatsoever between the appearance of the celestial bodies and the social or cultural phenomena. (Unless in this world astrology actually works.) $\endgroup$
    – AlexP
    May 7, 2022 at 21:24
  • $\begingroup$ Hi Arc', the premise is interesting :). Just one thing, can you explain what do you mean by "Two moons could affect behaviors, right?"? Do you mean the behaviour of the planet's geography/climate, or the behavior of its sentient inhabitants? Perhaps both at the same time? $\endgroup$
    – Tortliena
    May 8, 2022 at 0:27
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    $\begingroup$ Are you saying that the moons have the rings, rather than the planet? Because that drags the plausibility waaay down. $\endgroup$
    – jdunlop
    May 8, 2022 at 0:58
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    $\begingroup$ The moons having a shared ring is impossible, at least if those moons are at all mobile, or are far apart, and especially if they are moving at different speeds. (And if the first two don't apply, then gravity will quickly change that) $\endgroup$
    – LWS SWL
    May 8, 2022 at 17:32
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    $\begingroup$ I covered some of this in my answer, but a non-answer problem - generally, spectacular rings (like Saturn's) have a lot of ice in them, and free-floating ice doesn't last inside a star's habitable zone, because... well, if it did, it wouldn't be a habitable zone. $\endgroup$
    – jdunlop
    May 10, 2022 at 2:09

3 Answers 3

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Moons; check. Ring; check. All good!

saturn moons

https://www.nasa.gov/multimedia/imagegallery/image_feature_2065.html

In case you were wondering, all the tax dollars I have ever paid were used to take this picture. I feel good about that! You are all welcome.

SO sweet. Saturn with just 5 of its moons, and 2 of its rings. Showing off, Saturn is. And that big one is just Rhea, not Titan. She is here for her close up.

Saturn is pretty big. But Mars has 2 moons and it does ok so far. I think you can plausibly have 2 moons and a ring on your world. How they affect the denizens of your world can be up to you as the author!

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  • $\begingroup$ And to further complete your answer, there is no known impact to the planet's global climate by having two moons busy buzzing around, is there ^^? $\endgroup$
    – Tortliena
    May 8, 2022 at 0:56
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    $\begingroup$ @Tortliena - our moon is so consistent that it is hard to know what effect it has since it is always having it, often. But what about a moon with a very elliptical orbit that visited infrequently? Could the weird and occasional atmospheric tides (is that a thing?) affect climate? A good question for another OP. $\endgroup$
    – Willk
    May 8, 2022 at 1:09
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    $\begingroup$ @Tortliena That depends on what you mean by "moons". Two of Earth's moons would have a pretty big impact on Earth, vis-a-vis tidal forces. They would also make each other's orbits much less stable. Two of Mars's moons would be no problem. $\endgroup$
    – Cadence
    May 8, 2022 at 1:09
  • $\begingroup$ @Willk thanks man for the awesome pic! Tax money, really well spent in this case! Allow me just to clarify that I was wondering if I could have rings around the moons, not the planet. If not a shared ring between the two moons, then maybe a ring in one of them at least. Thanks! $\endgroup$
    – Arcturus
    May 9, 2022 at 19:30
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No

First, two moons can't share an orbit unless they orbit a common barycentre. So they would be a pair moving through the sky, not independent movement. But it's the ring that's the real impossibility.

For the ring to independently wrap each moon (for your figure-8/mobius strip), the two moons would have to be within each other's Roche limit or they'd lose the ring to the gravity of their primary... and if they were inside each other's Roche limit, they'd tear each other apart. (Or, more plausibly, the more massive moon would tear the smaller moon apart.)

It is possible to have a temporary ring form, from a catastrophic collision between planetary bodies within the orbit of the moons, but it would only last for a few thousand years. All of this is also dependent on the moons being both massive enough and far enough from their primary to support a ring system. For comparison, our satellite's Hill sphere is only 60000 km across, so even given an event that placed enough debris in it to form a ring, most of the mass of the ring (barring a spectacular coincidence) would be lost to Earth's orbit. And if it were outside the moon's Roche limit, it would coalesce into its own small moon over time.

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  • $\begingroup$ As an addendum, there's also the issue that if your planet is habitable, there's going to be some pretty strong solar wind near it, and so icy ring systems would get ablated away very quickly, and dust could be blown out of its orbit. It's difficult to conceive of a material/orbit combination that could result in rings that lasted any substantial time at all. I guess some aliens could come by with a space-freighter full of sand every couple of decades? $\endgroup$
    – jdunlop
    May 10, 2022 at 2:07
  • $\begingroup$ Two moons absolutely can share an orbit--see Janus & Epimetheus in our solar system. $\endgroup$
    – Logan R. Kearsley
    May 18, 2022 at 19:56
  • $\begingroup$ @LoganR.Kearsley - a horseshoe orbit where neither gets anywhere near the other is not remotely what the OP is asking for. $\endgroup$
    – jdunlop
    May 18, 2022 at 20:18
  • $\begingroup$ Thank you @judunlop for the very comprehensive response. I understand that two moons could share an orbit under certain circumstances (i.e. common barycentre) put it is almost impossible to share a ring. Although, I must admit that I am totally intrigued by what you mentioned about the temporary ring form, even for few thousand years would fit perfectly in my story arc. So, what kind of catastrophe are we talking about? An asteroid hitting a third independent moon maybe? How could such a catastrophe impact the way of living in the planet? $\endgroup$
    – Arcturus
    May 26, 2022 at 17:09
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It's possible for a moon to have a ring system, as shown by Rhea, so each moon could theoretically have its own ring system. If they were close enough to have a shared ring system, though, they would be inside each other's Roche limit and rip each other apart. Your idea of a shared ring system is impossible, but it could be modified so that each moon has its own rings.

If both ring systems are similar, they could still be a symbol of unity/symmetry. Despite how different both moons might be, they both have rings, so they do share something. Perhaps the moons (and therefore the good and evil they represent) have more in common than the people realize.

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  • $\begingroup$ Thank you! What you describe is my fall-back plan to be honest. But I think, in order to highlight the inequalities more dramatically, to have a giant moon without any ring and another dwarf moon with a ring on it, moving each one on different orbits and every few year (I suppose this makes sense) their orbits to "collide" and temporarily look like they are moving together side by side. I suppose this is feasible, right? $\endgroup$
    – Arcturus
    May 26, 2022 at 17:27
  • $\begingroup$ @Arcturus Yes, as long as it's just their orbits matching up and not the actual moons coming closer to each other. $\endgroup$
    – Aelan
    May 29, 2022 at 2:56

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