Hi Guys

Ganymede, Callisto and Titan are probably the wrong place to start when assessing a mass-ratio between a primary and its satellites. They're all about 50/50 ice/rock, so I'd say Io and Europa are more indicative. They're both about the mass of our moon ~ 1/80 th Earth mass. Hence a simple ratio means any Earth-sized moons need to form around a low-mass star!

But the Mass ratio between a giant and its moons seems to be non-linear - the regular satellites of Saturn and Uranus are roughly in the same size range, while Jupiter's are so much bigger. Titan and Triton are probably captured proto-planets that formed in the same region as their primaries.

Of course Uranus could be what the "ice" core of Saturn looks like without extra H/He.

As for what mass is needed for a habitable moon it needs to be big enough to sustain the geological cycles a habitable planet needs and to generate a magnetic field to shield its atmosphere from decay by the planet's radiation belts. About ~ 0.2 Earth masses might do, since the moon will have a lot of tidal energy to keep things going - eg. Io and Europa.

So the maximum mass of the Jovian for a simple ratio between masses might be ~ 20 Jupiter masses, but if it's non-linear then we're just guessing! Now that we've seen Jovians in the ecospheres of their stars I think habitable moons are a real possibility.

Adam