The circumstellar habitable zone is the orbital range in which a specified planet and atmosphere can maintain surface liquid water. Its boundaries depend on stellar spectrum, albedo, greenhouse physics, clouds, planetary mass, and atmospheric inventory.
A runaway greenhouse occurs when outgoing longwave radiation reaches a limiting value while absorbed stellar energy is larger, causing ocean evaporation and rapid surface warming.
At the maximum-greenhouse limit, adding carbon dioxide no longer warms efficiently because condensation and Rayleigh scattering offset its greenhouse effect. This defines a common conservative outer habitable-zone boundary.
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The circumstellar habitable zone (CHZ), often referred to as the "Goldilocks Zone," is the region around a star where conditions might be just right for liquid water to exist on the surface of a planet. This zone is critical for the potential habitability of planets, as liquid water is a key ingredient for life as we know it.