[Space] How Big Can Gas Giant Moons Get?

[RyanW]

Originally Posted by Sunrunners_Fire

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.

It may be possible for a moon to be nearly the same diameter as its parent planet, with a dense super Jupiter orbited by a more conventionally fluffy gas giant.