Tidal Heating

Being in the habitable zone is necessary but not sufficient for life. Venus sits near the inner edge and is a lifeless furnace; Mars sits at the outer edge and is a frozen desert. A planet also needs the right atmosphere, a protective magnetic field, geological activity, and possibly a large stabilizing moon like ours.

Tidal heating can create habitable conditions far outside the traditional zone. Jupiter's moon Europa gets almost no sunlight, yet tidal forces from Jupiter's gravity heat its interior enough to keep a liquid ocean beneath the ice. Life there would prove that habitable zones reach far beyond the simple temperature band around a star.