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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.

The location between pure-silicate and pure-water planetary mass-radius curves does not determine a unique composition. Possibilities include a rocky iron-silicate interior with a deep water layer, a rock-water mixture, or a mostly rocky planet whose small hydrogen-helium envelope inflates its radius. The atmosphere could accordingly be hydrogen-helium, steam-rich, carbon-dioxide-rich, or secondary gas released from the interior. Residence in the circumstellar habitable zone constrains stellar flux but does not by itself distinguish these models or guarantee surface liquid water.

For an atmosphere spanning fixed base and photospheric pressures,Thus two compositions haveAt equal temperature, a hydrogen-helium atmosphere with $\mu \simeq 2.3$ is abouttimes thicker than a steam atmosphere with $\mu \simeq 18$. This large difference is one reason atmospheric composition contributes strongly to the exoplanet interior-composition degeneracy.

Potential surface habitability requires a stable liquid-water interface, a temperature and pressure outside runaway-greenhouse and high-pressure-ice regimes, and an ${\mathrm{H}}_2$ inventory large enough for collision-induced greenhouse warming but not so deep that pressure and temperature sterilize the ocean. Long-term atmospheric retention, tolerable stellar ultraviolet and particle flux, nutrient cycling, and chemical disequilibrium compatible with metabolism are also needed. A location in the circumstellar habitable zone alone is insufficient.

For $M_p=5M_{\oplus }$ and $R_p=2R_{\oplus }$,Taking $T=300\mathrm{K}$ and hydrogen-rich mean molecular weight $\mu =2.3$ gives the atmospheric scale heightIf a strong feature spans $N_H=5$ scale heights and $R_s=0.5R_{\odot }$, the atmospheric spectral-feature amplitude isClouds, hazes, refraction, and smaller molecular abundance reduce this estimate.

Assume a spherical blackbody planet, uniform reradiation, constant Bond albedo $A_B$, and negligible internal luminosity. Radiative equilibrium givesWith $L_s=K{M}_s^{\beta }$,The inner and outer circumstellar habitable zone boundaries are obtained by setting $T=T_{\max }$ and $T=T_{\min }$ respectively:Real boundaries require wavelength-dependent albedo, clouds, greenhouse effects, and the stellar spectrum, which this equilibrium-temperature model neglects.