Quantitative estimates of the surface habitability of Kepler-452b

Kepler-452b is currently the best example of an Earth-size planet in the habitable zone of a sun-like star, a type of planet whose number of detections is expected to increase in the future. Searching for biosignatures in the supposedly thin atmospheres of these planets is a challenging goal that requires a careful selection of the targets. Under the assumption of a rocky-dominated nature for Kepler-452b, we considered it as a test case to calculate a temperature-dependent habitability index, h₀₅₀, designed to maximize the potential presence of biosignature-producing activity. The surface temperature has been computed for a broad range of climate factors using a climate model designed for terrestrial-type exoplanets. After fixing the planetary data according to the experimental results, we changed the surface gravity, CO₂ abundance, surface pressure, orbital eccentricity, rotation period, axis obliquity and ocean fraction within the range of validity of our model. For most choices of parameters, we find habitable solutions with h₀₅₀ > 0.2 only for CO₂ partial pressure p_CO_2 ≲ 0.04 bar. At this limiting value of CO₂ abundance, the planet is still habitable if the total pressure is p ≲ 2 bar. In all cases, the habitability drops for eccentricity e ≳ 0.3. Changes of rotation period and obliquity affect the habitability through their impact on the equator-pole temperature difference rather than on the mean global temperature. We calculated the variation of h₀₅₀ resulting from the luminosity evolution of the host star for a wide range of input parameters. Only a small combination of parameters yields habitability-weighted lifetimes ≳2 Gyr, sufficiently long to develop atmospheric biosignatures still detectable at the present time.