Help on a story idea -- multiple moon systems?

Question

Hi there! So I'm working on a story idea and I had a few thoughts. There are some smart folks in this section (Geoff -- I'm lookin' at you buddy...) and I'd like to get some input on the physics of my proposed home world system.

I'm envisioning a system with a large (>Jupiter-sized) planet with at least two moons (probably more). The two primary moons are a binary system (i.e., orbit each other) that has been captured and now orbit retrograde to the planet similar to Triton/Neptune. I was thinking one would be a little smaller than Earth (maybe Mars-sized?) while the second is a little larger than our Moon. The planet orbits just outside the habitable zone of it's star, but the tidal forces of the moons' orbits generates heat. The larger planet is substantially water and it has enough gravity to maintain some degree of atmosphere. The second is too small to support a permanent atmosphere, but I'm thinking it might have a seasonal one. Any thoughts about moon composition? It's at least carbon based, has some silicates, but has to have a surface of mostly, if not all, water. The smaller could have an ice crust like Europa; do think the larger one would likely be iced over or could it have some open seas as well? Maybe some greenhouse gases to keep it habitable?

So thoughts? This would be a complex, dynamic system. What might keep it stable? I assume another moon; perhaps in a resonant orbit? Anything else? Might there be a combination of orbit eccentricity and resonance between the moons that would keep it stable? If so, any ideas of what that might be specifically? What about radiation from the planet's magnetic field? I suppose the larger of the two could have a molten core that might generate enough of a protective magnetic field; any other possible sources of radiation protection that might exist naturally?

Any flights of imagination, inspiration, or insight you can offer would be most welcome. Other ideas and details are also appreciated. Thanks!!

Paula · Accepted Answer

It is your story.
So you tell it.
The simplest way of doing it is to "invent physics"
In your solar system physics operates differently -- maybe because it is in a parallel universe . Then you can have anything happening.

But in our solar system :
A planet the size of Mars lacks sufficient gravity and can't hold an atmosphere. --- Thus any water present, in gas, liquid or solid state will evaporate/sublimate and be lost to space.

The dynamics of a planet capturing a moon just don't work out. It is just possible that a single moon could be captured -- and most likely into a retrograde orbit. But the initial pass would need to be just above the cloud tops. The moon would be likely fragmented by tidal forces. A binary system could not possibly be captured intact.

mephistowaltz · Answer

I agree that a binary moon system is not outside the realms of possibility, (I especially like your idea of a resonant moon keeping the system stable) but i would suggest that the 2 moons would have to be quite far away from their parent planet. If they were too close, the tidal forces from the parent planet would 'override' the forces of the binary system. 

There's nothing to stop you having a planet resembling earth (i.e., big land masses, h20 in ice or liquid form on the surface and a nitrogen/oxygen atmosphere) and using the tidal forces of the parent planet to generate heat by friction , you could have liquid water. But this would, of course, create massive quakes on the planet. 
As you mention though, being outside the habitatable zone would most likely mean the planet is iced over!

Greenhouse gasses are a great way to heat up a planet!

The 2 small planets would also, most likely, fall inside the parent's magnetic field (just like Jupiter)

Steve B · Answer

It would have thought it's beyond the realms of mathematics for a pair of minor planets in a binary orbit to be 'captured' together, especially if they differ in mass ...

Seems to me more likley that an existing moon would be impacted by a 'minor planet' resulting in both the binary orbit and retrograde motion .... this let's you make one 'metallic core' and the other 'rocky' by some mild hand waving of how the debris from the collision 'collapsed' afterwards to form the two bodies ..

John W · Answer

I guess we'll have to leave this to Geoff but keep in mind that the largest moon in our solar system is Ganymede at 2.5% Earth mass, our Moon is 1.2% Earth mass and Mars is 12% Earth mass.   You may wish to reconsider the size of your moons.

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Help on a story idea -- multiple moon systems?

4 Answers