A narrow orbital slot around the Sun keeps Earth in a regime where water can stay liquid at the surface. Shift the planet slightly inward and oceans trend toward runaway evaporation; move it outward and global ice cover becomes the dominant state.
This balance is not set by distance alone. Surface temperature emerges from the interplay of solar irradiance, atmospheric pressure and the greenhouse effect, all constrained by radiative equilibrium. If insolation increased just enough to drive persistent ocean loss, water vapor would amplify warming, pushing the climate toward a runaway greenhouse. A comparable decrease in incoming energy would favor ice–albedo feedback, where expanding ice reflects more sunlight and locks the planet into deep freeze.
Astrobiology frames this orbital band as the habitable zone, but Earth occupies only a limited slice where plate tectonics, the carbon cycle and the basic metabolic rate of life can co-exist with stable liquid oceans. Small deviations in orbital radius, axial tilt or atmospheric composition would shift the energy budget outside that window, erasing the surface conditions that currently define home.
