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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.