[6equj5]

>A thought has been lately put forward, which may or may not have truth in it. What if — instead of Jupiter being a world, inhabited by animals and people, as is often supposed, with a small distant sun and five dim moons to give them light — what if Jupiter is himself in some sort a second sun to his moons, and what if those "moons" are really inhabited planets? It may be so. That is all we can say. The idea is not an impossible one. (p. 207)

And that's still all we can say about at least one of them[0]. :)

0: https://en.wikipedia.org/wiki/Europa_(moon)#Habitability_pot...