There seems to be a fairly good explanation for why the total mass of a gas giant's satellite system is ~1/10000 the mass of the parent planet. This means that habitable moons of gas giant exoplanets are probably very rare, only forming around the largest planets or resulting from capture events.
SwRI researchers offer first explanation for the near constant scale of the gas planet satellite systems
Bad news for fans of habitable moons
Though it means you could get earth-mass planets around brown dwarfs... If it scales up linearly then if you have a BD that is 40 times the mass of jupiter you should have a satellite system that is 40 times the mass of jupiter's system. And 40 times the mass of Ganymede is about 1 earth mass.
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Of course, depending on various specific circumstances (such as the amount of metallicity a system will have, the mass of the parent planet, tidal flexing, etc.), you could still have a Mars-massed moon that could support a biosphere, sparse or rich. Might be a bit more rare than an Earth-massed moon, of course....
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...John...
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Another point - if the moon system formed is more like that of Saturn, with most of the mass concentrated in a single satellite, habitable moons are more likely. This paper gives habitability limits of 0.12 Earth masses (provided tidal interactions can heat the moon enough to support plate tectonics) or 0.23 Earth masses (plate tectonics for 5 Gyr without tidal interactions), and the paper about moon systems has satellites of 0.02-0.03% of the planetary mass forming in some simulations, habitable moons may not be too remote a possibility.
Multiple habitable moons seems unlikely though.
Multiple habitable moons seems unlikely though.
Planetary formation is still a theoretical science and theory is often wrong. Nobody expected to find hot Jupiters, for example. Nobody expected to find multiple moons around Pluto. Some of the KBOs that are being discovered have unexpected orbital parameters. Stellar formation is also fraught with unknowns, especially for multiple star systems.
The sample size for large planets with known moons is four. This is too small to be statistically significant when formulating theories.
Therefore it is unwise to claim that the masses of the moons of all gas giants have an upper limit on their combined masses just because the four giant planets with known moons have such a limit. Why is the theory limited to giant planets? Earth and Pluto. It is implausible to claim that the mechanisms that gave Earth, Pluto and some asteroids giant moons cannot apply to giant planets as well. After all, binary stars exist. Why should objects with large satellites be limited to stars and terrestrial planets? What about the gap in the middle? Why can't a Jupiter-mass planet have a Neptune-mass satellite?
Conversely, it is plausible to suggest that large systems of Earth-mass moons cannot exist in orbit around a Jupiter-mass body. Generally, when a body has a large companion, it usually has only one such companion. Perturbations would tend to disrupt orbits if many bodies of similar masses orbited in close proximity.
The sample size for large planets with known moons is four. This is too small to be statistically significant when formulating theories.
Therefore it is unwise to claim that the masses of the moons of all gas giants have an upper limit on their combined masses just because the four giant planets with known moons have such a limit. Why is the theory limited to giant planets? Earth and Pluto. It is implausible to claim that the mechanisms that gave Earth, Pluto and some asteroids giant moons cannot apply to giant planets as well. After all, binary stars exist. Why should objects with large satellites be limited to stars and terrestrial planets? What about the gap in the middle? Why can't a Jupiter-mass planet have a Neptune-mass satellite?
Conversely, it is plausible to suggest that large systems of Earth-mass moons cannot exist in orbit around a Jupiter-mass body. Generally, when a body has a large companion, it usually has only one such companion. Perturbations would tend to disrupt orbits if many bodies of similar masses orbited in close proximity.