= Circumstellar habitable zone
{title2=HZ}
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= Habitable zone
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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 $a\propto\sqrt{L_\star/S_{\rm eff}}$, where $L_\star$ is stellar <luminosity> and $S_{\rm eff}$ 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.<Habitable Zones Around Main-Sequence Stars: New Estimates by Ravi Kumar Kopparapu and others>[Abstract: updated absorption data, dependence on stellar effective temperature and cloud-free assumptions]{ref} The zone is therefore a conditional climate calculation, rather than a guarantee that each <planet> within it is habitable.
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