Jan. 13, 2021
1 p.m. - 2 p.m.
Low mass stars such as our nearest neighbour Proxima Centauri, will still be shining a trillion years from now, when the Universe is dilute and dark. Does that mean that planets with habitable atmospheres about such stars have all that time for complex life to evolve? The conventional definition of the outer edge of the habitable zone for planets with a CO2-H2O supported greenhouse assumes that the only constraints on atmospheric CO2 are determined by radiative and thermodynamics limits. In reality, geochemical processes determining the balance between outgassing sources and weathering sinks of CO2 provide important additional constraints. The implications of these constraints for the outer edge of the habitable zone, and for the lifetime of habitability, and for the prospects for emergence of complex multicellular life are explored. The constraints are particularly important of planets orbiting stars somewhat less massive than the Sun, which are correspondingly more long-lived. A key novel feature of the results presented here is the use of the weathering formulation based on work by Maher and Chamberlain, which yields substantially different behaviour from the commonly used WHAK formulation.