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Old 11-13-2018, 10:44 PM   #1
scc
 
Join Date: Mar 2013
Default [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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Old 11-13-2018, 10:57 PM   #2
Anthony
 
Join Date: Feb 2005
Location: Berkeley, CA
Default Re: [Space] How Big Can Gas Giant Moons Get?

We don't know exactly.
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Old 11-13-2018, 11:35 PM   #3
Crystalline_Entity
 
Join Date: Jul 2015
Location: England
Default 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).
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Old 11-14-2018, 12:53 AM   #4
Sunrunners_Fire
 
Join Date: Mar 2010
Default Re: [Space] How Big Can Gas Giant Moons Get?

Quote:
Originally Posted by scc View Post
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?
GURPS Space (pages 84 ~ 87) should have the information you are looking for.

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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Old 11-14-2018, 01:34 AM   #5
scc
 
Join Date: Mar 2013
Default Re: [Space] How Big Can Gas Giant Moons Get?

Quote:
Originally Posted by Crystalline_Entity View Post
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).
I'm assuming that panspermia is in effect, or rather something like it. Life begin on one of the moons (The one that humanity evolved on) and was then spread to the others by meter impacts ejecting rocks contain life.

Quote:
Originally Posted by Sunrunners_Fire View Post
GURPS Space (pages 84 ~ 87) should have the information you are looking for.

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.
This is about size of moons in relation to the planets they orbit.
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Old 11-14-2018, 05:39 AM   #6
AlexanderHowl
 
Join Date: Feb 2016
Default 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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Old 11-14-2018, 09:57 AM   #7
Fred Brackin
 
Join Date: Aug 2007
Default Re: [Space] How Big Can Gas Giant Moons Get?

Quote:
Originally Posted by Anthony View Post
We don't know exactly.
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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Old 11-14-2018, 11:21 AM   #8
AlexanderHowl
 
Join Date: Feb 2016
Default 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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Old 11-14-2018, 08:41 PM   #9
Sunrunners_Fire
 
Join Date: Mar 2010
Default Re: [Space] How Big Can Gas Giant Moons Get?

Quote:
Originally Posted by scc View Post
This is about size of moons in relation to the planets they orbit.
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.
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Old 11-14-2018, 11:06 PM   #10
AlexanderHowl
 
Join Date: Feb 2016
Default 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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