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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.

The circumstellar habitable zone is the range of orbital distances where a terrestrial planet with a specified atmospheric inventory can retain liquid water at its surface. It is a conditional climate criterion, not a guarantee of life or a requirement for every possible subsurface habitat.

Four influential factors are:

For a Sun-like present-day star and an Earth-like planet, a useful conservative range is approximately $0.95$–$1.7\mathrm{AU}$. The inner limit depends on the adopted moist or runaway greenhouse condition, and the outer limit on the maximum greenhouse outer habitable-zone limit. More restrictive water-loss choices put the inner edge near $0.99\mathrm{AU}$. Empirical optimistic limits based on past Venus and Mars are about $0.75$–$1.8\mathrm{AU}$. These are model conventions rather than exact universal boundaries.

For an Earth-mass planet with a nitrogen-carbon-dioxide-water atmosphere, commonly quoted conservative Solar boundaries are approximatelyThe inner edge is governed by moist-greenhouse water loss and ultimately the runaway greenhouse. The outer edge is the maximum greenhouse outer habitable-zone limit, where additional carbon dioxide condenses or increases reflected light faster than it increases infrared trapping. Optimistic empirical boundaries, based on recent Venus and early Mars, are broader, roughly $0.75$ to $1.77\mathrm{AU}$. The values depend on clouds, planetary mass, atmospheric inventory, and climate assumptions.