Captured Earth-Like Moons around Gas Giants
Is there a limit (and if so, what is it) to the size (mass) of terrestrial planet that could be "captured" as a moon by a migrating gas giant?
I'm writing a novel where a colony ship crashes on such a moon. My research tells me that a moon around a gas-giant is not likely to be larger than 1:10,000th of the mass of its parent. If this is true, an Earth-like moon is unlikely to form around anything with less than 30 Jupiter masses, which puts the parent planet in the brown dwarf range. Not what I want.
So, can I get round this by having the gas giant migrate into the inner system, snaring a rocky world approximately the size of Earth, as it goes?
I'm not too concerned about other issues - I'm happy to fudge tidal locking, and have the gas giant only have a couple of other moons to avoid tidal heating, and stick the Earth-like moon 10ish million kms out to avoid the worst of the radiation. But I feel like I can't fudge the mass of the related bodies.
Any help, or related thoughts, would be greatly appreciated.
SOURCES:
http://phl.upr.edu/library/notes/themassandradiusofpotentialexomoons
https://www.reddit.com/r/askscience/comments/32ixle/is_there_a_clear_maximum_gas_giant_moon_size/
This is indicative of similar sites and forums where I got the figures. I've struggled to find anything concrete anywhere else; language is vague, but supports the notion that there is a mass limit for moons forming around gas giants, and that an Earth-mass moon would require a gas giant of several multiples of Jupiter. One site I read suggested that Earth-sized ice moons might form past 10 Jupiter masses, but that any terrestrial moon would be considerably smaller. Obviously, I'm looking for an Earth-mass, terrestrial planet, not a slushie world, and I want to avoid small terrestrial worlds because of the low-gravity.
I'm grateful for the response re the Roche Limit. I did wonder about two planets orbiting a mutual gravitational center, but I don't know how to figure out what kind of effects that might have on the Earth-like planet, and whether I could keep it far enough outside the gas giant's radiation belts.
Thanks for all the replies so far.
This post was sourced from https://worldbuilding.stackexchange.com/q/82415. It is licensed under CC BY-SA 3.0.
1 answer
I'm not sure where you are looking, but first look here: https://www.reddit.com/r/askscience/comments/23a96x/could_an_earth_sized_object_orbit_jupiter/
The answer is Yes; any two objects can orbit each other, include Earth and Jupiter.
You need to be concerned about the Roche Limit, which tells you how far apart they must be in order to do so.
And be aware that gravity works both ways, even for smaller objects: The Earth is pulled into a rotation by our Moon just as much as our Moon is pulled into a rotation by Earth: It isn't just the tides moved by the Moon, but the center of Earth is moving in small circles due to the moon.
So planets of equal mass would circle each other. But Jupiter is 318 x the mass of Earth, and the most massive known planet in the Universe is about 30 x the mass of Jupiter. (FWIW our Earth is 81 x our Moon).
Look up the Roche Limit; that should also tell you what your minimum orbit should be around your big planet (but actual orbit can be thousands of times bigger).
Roche Limit says Earth cannot be any closer than about 67,000 miles to Jupiter without breaking up. However, your planet can be quite a bit further, our Moon is about 40x its rigid body Roche Limit from Earth. But what this means is you can put it about where you like; it does not have to be extremely far from the gas giant. If you want tidal heating of your planet (and lots of earthquakes) put it close; if your planet is warmed otherwise and you want it calmer; I'd keep it at least twenty Roche units away, say 1.4 million miles from Jupiter.
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