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A circumstellar habitable zone is an orbital region in which a specified planetary and atmospheric model can maintain surface liquid water. A simple radiative scaling gives distance $a\propto \sqrt{L_{\star }/S_{\mathrm{eff}}}$, where $L_{\star }$ is stellar luminosity and $S_{\mathrm{eff}}$ the incident irradiance at a chosen climate boundary. The required irradiance depends on the stellar spectrum and atmospheric opacity.

Model inner limits involve water loss or a runaway greenhouse; a maximum greenhouse outer habitable-zone limit is a common outer criterion. Published one-dimensional estimates explicitly depend on water and carbon dioxide absorption and on their treatment of clouds.^[ref] The zone is therefore a conditional climate calculation, rather than a guarantee that each planet within it is habitable.

Exoplanet habitability asks whether a world's physical conditions could support life, commonly using sustained liquid water as an operational target. This requires more information than mass, radius or orbital distance alone. Atmospheric escape, the volatile inventory, the greenhouse effect and heat transport can make bodies receiving similar stellar power very different.

A circumstellar habitable zone is a useful model-dependent filter for possible surface liquid water. Calculations must specify atmospheric composition and climate assumptions; incident stellar power alone is not a detection of an ocean.^[ref] An exoplanet biosignature asks a separate observational question about evidence for biology, and must be interpreted in this planetary context.

Ravi Kumar Kopparapu coauthored estimates of stellar habitable zones using radiative-convective climate models. The boundaries depend on stellar radiation and atmospheric assumptions; being inside a modeled zone is a climatic possibility criterion rather than evidence that a planet is inhabited.^[ref]