The most plentiful source of steady energy in our solar system comes from the Sun. In a particular zone around the Sun—not too far away, but not too close—solar radiation is intense enough to melt ice into liquid water, but not so harsh as to vaporize the liquid water into steam. Earth happens to be in the Sun’s habitable zone.

Interestingly, under the surface of many solar system bodies, steady energy may also come from deep within the core. If tidal interaction is present, energy may constantly be flowing throughout the space body; and if mass differentiation is still going on, where dense metallic material sinks slowly through the bodies’ lighter rocky or gaseous layers, the gravitational potential energy released by that process can be both gentle and persistent over very long timescales. Solar system bodies other than Earth where underground energy sources may be enough to support life include Mars, Europa, and Ganymede.