Moons in orbit around planets can be heated by the gravitational tug of that planet. Jupiter's Europa and Saturn's Enceladus are posible abodes of life because the giant planets' gravity is constantly pulling on the moons as they move along their orbits. That pull interacts with the orbital speed to stretch and reshape the interior of the moons, creating friction and heat.
Heat radiating from such moons makes those moons visibly brighter-- so much so, in fact, that such moons orbiting exoplanets might be easier to see than the exoplanets themselves. Further, as possibly in the cases of Europa and Enceladus, such heating could create oases of life well outside the so-called habitable zone of a star.