Impact of vegetation albedo on the habitability of Earth-like exoplanets

Abstract

Vegetation can modify the planetary surface albedo via the Charney mechanism, as plants are usually darker than the bare surface of the continents. We updated ESTM ( Earth-like surface temperature model ) to incorporate the presence, distribution and evolution of two dynamically competing v e getation types that resemble grasslands and trees (the latter in the double stages of life: adults and seedlings). The ne wly de veloped model was applied to estimate how the climate-v e getation system reaches equilibrium across different rocky planetary configurations, and to assess its impact on temperature and habitability. With respect to a world with bare granite continents, the effect of v e getation-albedo feedback is to increase the average surface temperature. Since grasses and trees exhibit different albedos, they affect temperature to different degrees. The ultimate impact on climate depends on the outcome of the competition between these v e getation types. The change in albedo due to v e getation e xtends the habitable zone and enhances the o v erall planetary habitability be yond its traditional outer edge. This effect is especially rele v ant for planets that have a larger extension of continents than Earth. For Earth, the semimajor axis d = 1.04 au represents the turning point where v e getation enhances habitability from h = 0.0 to 0.485 (in the grass-dominance case), to h = 0.584 (in the case of coexistence between grasses and trees), and to h = 0.612 (in the tree-dominance case). This illustrates the transition from a snowball state to a planet with intermediate habitability at the outer edge of the circumstellar habitability zone.