Circumstellar habitable zone (HZ)

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.

A runaway greenhouse is a loss of thermal balance when absorbed stellar power exceeds the maximum outgoing thermal radiation available to a water-rich atmosphere in the relevant climate state. Surface warming increases atmospheric water vapour and hence infrared opacity. The layers emitting to space need not warm as rapidly as the surface, allowing outgoing power to approach a limit rather than increasing indefinitely with surface temperature.

If absorbed power remains above that limit, the climate cannot settle into the same ocean-bearing equilibrium and continued warming drives evaporation. The criterion concerns the whole atmospheric radiative transfer problem; a large ordinary greenhouse effect alone does not establish a runaway. Bond albedo, clouds, stellar spectrum and atmospheric inventory affect its threshold and the resulting circumstellar habitable zone.

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.