Jack of Harts

Many people wonder how moons of gas giants so far outside the Goldilocks Zone of a star can be habitable themselves.  The secret is that gas giants are truly massive, comparable more to stars than to their lesser, terrestrial brethren.  With that mass comes a major gravitational well that pulls and pushes at the moons trapped in their miniature, substellar systems.  That friction heats those moons from the inside out, and that heat makes all the difference.