The habitable zone is the region where conditions are such that life can survive. It is the “Goldilocks” region that is just right to allow for stable liquid water. Too far away from the major heat source—the Sun—and water will be ice. Too close in and the water won’t condense onto a planet’s surface in the first place. Water might exist below the surface of a planet too, though that is unlikely to house the diversity of life that a large ocean can promote.
