A PROPOSAL FOR CLIMATE STABILITY ON H-GREENHOUSE PLANETS.

A terrestrial planet in an orbit far outside of the standard habitable zone could maintain surface liquid water as a result of H-H collision-induced absorption by a thick H atmosphere. Without a stabilizing climate feedback, however, habitability would be accidental and likely brief. In this letter I propose stabilizing climate feedbacks for such a planet that require only that biological functions have an optimal temperature and operate less efficiently at other temperatures. For example, on a planet with a net source of H from its interior, H-consuming life (such as methanogens) could establish a stable climate. If a positive perturbation is added to the equilibrium temperature, H consumption by life will increase (cooling the planet) until the equilibrium climate is reestablished. The potential existence of such feedbacks makes H-warmed planets more attractive astrobiological targets.