11-13-2018, 10:44 PM | #1 |
Join Date: Mar 2013
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[Space] How Big Can Gas Giant Moons Get?
I'm wanting to have a lower tech sci-fi setting by using the moon system of a gas giant as my setting some of the moons are habitable, with the TL ranging from 5 to 7, and the over all set resembling the scramble for Africa, with the extra moons taking the place of Africa. My question then is how big can I make these moons? Both in Earth mass' and Earth diameter's?
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11-13-2018, 11:35 PM | #3 |
Join Date: Jul 2015
Location: England
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Re: [Space] How Big Can Gas Giant Moons Get?
There was a possible discovery of a Neptune-sized moon recently,https://www.bbc.co.uk/news/science-environment-45707309, so I'm not sure there's any real limit - however the planet there was several Jupiter masses.
If you designed a gas giant with Earth-sized moons, I don't think anyone could claim it was unphysical given what we know at the moment (it might be improbable that they'd all have Earth-like environments without artificial intervention though). |
11-14-2018, 12:53 AM | #4 | |
Join Date: Mar 2010
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Re: [Space] How Big Can Gas Giant Moons Get?
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If you want something more precise than that, I would suggest looking into the factors involved in determining a world's molecular weight retained; this appears to be a relatively simple starting point if you wanted to go down that rabbit hole. |
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11-14-2018, 01:34 AM | #5 | ||
Join Date: Mar 2013
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Re: [Space] How Big Can Gas Giant Moons Get?
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11-14-2018, 05:39 AM | #6 |
Join Date: Feb 2016
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Re: [Space] How Big Can Gas Giant Moons Get?
There is probably no real limit, though any moon that is more than 10% the size of the primary should probably be classified as a binary planet rather than a moon. The rules given in GURPS Space cannot even replicate our system, Jupiter and Saturn have far too many moonlets, so I would suggest going with whatever you want. As long as you do not have major moons sharing the same orbits, you should be fine from a scientific point of view.
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11-14-2018, 09:57 AM | #7 |
Join Date: Aug 2007
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Re: [Space] How Big Can Gas Giant Moons Get?
Yes, "as big as Titan and probably larger" is not exact.
A gas giant far enough in to promote Earth-like temp is a so-so thing. The many "Hot Jupiters" we've discovered may be a transient early sustem phenomenon i.e the Hot Jupiters form beyond the snow line but move inside it but eventually fall into the local sun.. Also, given the importance of the Theian impact on Earth's development a "super-Mars" might be more likely than really Earth-like. That'd be soemwhat lower density but you'd need for it to still have a molten core. Tide-locking to the GG priomary might shut down an innate magnetic field but these moons might be inside the GG's magnetic field which could protect their atmopheres.
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Fred Brackin |
11-14-2018, 11:21 AM | #8 |
Join Date: Feb 2016
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Re: [Space] How Big Can Gas Giant Moons Get?
It depends, as the gas giant could also be a source of radiation. A Jupiter at Earth orbit would generate over 100,000 rads an hour for its closest moons, which would make life difficult (a Neptune at Earth orbit would be fine though).
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11-14-2018, 08:41 PM | #9 |
Join Date: Mar 2010
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Re: [Space] How Big Can Gas Giant Moons Get?
Ah, okay. In that case ...
The maximum total mass of the gas giant's satellites should be no more than 2% of the gas giant's mass. (Warning! This'll produce some freakishly big moons, but, well, Luna is a freakishly big moon.) Or did you want maximum diameter? If so, physics seems to suggest that 80k miles diameter is about the top end for gas giants with any excess mass going to increase density rather than further increasing volume. |
11-14-2018, 11:06 PM | #10 |
Join Date: Feb 2016
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Re: [Space] How Big Can Gas Giant Moons Get?
Actually, the maximum may be much larger than that, but it probably a workable approximation of reality. I would actually have a more dynamic system:
Roll 3d6: 3-10: Major moon has a mass equal to 2d-1/10,000 % of primary mass 11-12: Major moon has a mass equal to 2d-1/1,000 % of primary mass 13-14: Major moon has a mass equal to 2d-1/100 % of primary mass 15-16: Major moon has a mass equal to 2d-1/10 % of primary mass 17-18: Major moon has a mass equal to 2d-1 % of primary mass The results are major moons that range from 0.0001% of primary mass to 11% of primary mass. |
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planet atmosphere, planet generation, space, system generation |
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