Title: Opportunities and Obstacles for Life on Proxima Centauri b

Speaker: Rory Barnes (Assistant Professor, Department of Astronomy, University of Washington)

Abstract: I will describe plausible evolutionary scenarios for the planet Proxima Centauri b in order to identify environmental factors that affect its long-term habitability. Relevant processes act on size scales ranging from the galactic to the planet's interior. Dynamical models suggest there is a significant probability that Proxima Centauri has had encounters with its companion stars, Alpha Centauri A and B, that are close enough to destabilize Proxima Centauri's planetary system. If the system has an additional planet, as suggested by the discovery data, then it may perturb planet b's eccentricity and inclination, possibly driving those parameters to non-zero values, even in the presence of strong tidal damping. The internal evolution of the planet depends on composition and tidal heating, which may prevent the generation a magnetic field. If planet b formed in situ, then it experienced ~160 million years in a runaway greenhouse as the star contracted during its formation. This early brightness may have permanently desiccated the planet and/or produced a large abiotic oxygen atmosphere. On the other hand, if Proxima Centauri b formed with a thin hydrogen atmosphere (~1% of the planet's mass), then this envelope could have shielded the water long enough for it to be retained before being blown off itself. The modeling of a wide range of Proxima b's evolutionary pathways reveals that water retention is the biggest obstacle for planet b's habitability.