From climate models to planetary habitability: temperature constraints for complex life

Abstract

In an effort to derive temperature based criteria of habitability for multicellular life, we investigated the thermal limits of terrestrial poikilotherms, i.e. organisms whose body temperature and the functioning of all vital processes is directly affected by the ambient temperature. Multicellular poikilotherms are the most common and evolutionarily ancient form of complex life on earth. The thermal limits for the active metabolism and reproduction of multicellular poikilotherms on earth are approximately bracketed by the temperature interval 0 o C  T  50 o C. The same interval applies to the photosynthetic production of oxygen, an essential ingredient of complex life, and for the generation of atmospheric biosignatures observable in exoplanets. Analysis of the main mechanisms responsible for the thermal thresholds of terrestrial life suggests that the same mechanisms would apply to other forms of chemical life. We therefore propose a habitability index for complex life, h 050 , representing the mean orbital fraction of planetary surface that satisfies the temperature limits 0 o C  T  50 o C. With the aid of a climate model tailored for the calculation of the surface temperature of Earth-like planets (ESTM), we calculated h 050 as a function of planet insolation, S, and atmospheric columnar mass, N atm , for a few earth-like atmospheric compositions with trace levels of CO 2 . By displaying h 050 as a function of S and N atm , we built up an atmospheric mass habitable zone (AMHZ) for complex life. At variance with the classic habitable zone, the inner edge of the complex life habitable zone is not affected by the uncertainties inherent to the calculation of the runaway greenhouse limit. The complex life habitable zone is significantly narrower than the habitable zone of dry planets. Our calculations illustrate how changes in ambient conditions dependent on S and N atm , such as temperature excursions and surface dose of secondary particles of cosmic rays, may influence the type of life potentially present at different epochs of planetary evolution inside the AMHZ.

Quantitative estimates of the surface habitability of Kepler-452b

“…For these planets, it is important to highlight the most promising targets in terms of potential presence of atmospheric biosignatures. To this aim, we have explored the surface fractional habitability of Kepler-452b with a temperature-dependent index designed to maximize the potential presence of biosignatureproducing biological activity (Silva et al 2016). We have computed the surface temperature with our climate model (ESTM, Vladilo et al 2015), as a function of the known planeray parameters as measured by Jenkins et al (2015), and of the yet unknown parameters.…”

“…, typical of multicellular poikilotherms with active metabolism and capability of reproduction (Precht et al 1973, Clarke 2014, can be used to calculate an index h 050 based on the thermal response of life (Silva et al 2016). The index h 050 offers two advantages with respect to the liquid water index h lw .…”

“…For the above reasons, in the rest of this work we adopt the index h 050 . More details on the thermal limits of life and examples of habitable zones calculated with the h lw and h 050 indices can be found in Silva et al (2016).…”

“…The minimum and maximum insolation values within which h 050 > 0 provide a maximum habitability time span, ∆t h = t (S max ) − t (S min ). Since within this time interval the habitable surface fraction of the planet is not constant, we also calculate an effective habitability time obtained by weighting each time step by the corresponding value of h 050 : τ h = h j δt j , where δt j is the stellar age increment for a given insolation increment (see Silva et al 2016). In practice, by providing larger weight to stellar evolutionary phases corresponding to larger surface habitality fractions, τ h provides a timespan more representative from the habitability point of view.…”

“…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).…”