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:
- The incident stellar energy and spectrum, including orbital distance and long-term stellar evolution.
- Atmospheric pressure and composition, greenhouse effect, Bond albedo and clouds, which set the relation between absorbed light and surface temperature.
- The water and volatile inventory, together with planetary mass and the ability to retain or replenish an atmosphere.
- Internal and surface evolution, including plate tectonics, outgassing and the carbonate-silicate cycle, which can regulate climate over geological time.
For a Sun-like present-day star and an Earth-like planet, a useful conservative range is approximately –. 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 . Empirical optimistic limits based on past Venus and Mars are about –. These are model conventions rather than exact universal boundaries.
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