Title: Quantitative estimates of the surface habitability of Kepler-452b,
Abstract: 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, $h_{050}$, designed to
maximize the potential presence of biosignature-producing activity (Silva et
al.\ 2016). The surface temperature has been computed for a broad range of
climate factors using a climate model designed for terrestrial-type exoplanets
(Vladilo et al.\ 2015). After fixing the planetary data according to the
experimental results (Jenkins et al.\ 2015), we changed the surface gravity,
CO$_2$ 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 $h_{050}>0.2$ only
for CO$_2$ partial pressure $p_\mathrm{CO_2} \lesssim 0.04$\,bar. At this
limiting value of CO$_2$ abundance the planet is still habitable if the total
pressure is $p \lesssim 2$\,bar. In all cases the habitability drops for
eccentricity $e \gtrsim 0.3$. 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 $h_{050}$ resulting from the luminosity evolution of the host star
for a wide range of input parameters. Only a small combination of parameters
yield habitability-weighted lifetimes $\gtrsim 2$\,Gyr, sufficiently long to
develop atmospheric biosignatures still detectable at the present time.