The habitability of Proxima Centauri b II. Possible climates and Observability

Abstract

Radial velocity monitoring has found the signature of a $M\sin i=1.3$~M${}_{\oplus }$ planet located within the Habitable Zone (HZ) of Proxima Centauri \citep{Anglada16}. Despite a hotter past and an active host star the planet Proxima~b could have retained enough volatiles to sustain surface habitability \citep{Ribas2016}. Here we use a 3D Global Climate Model (GCM) to simulate Proxima b's atmosphere and water cycle for its two likely rotation modes (1:1 and 3:2 spin-orbit resonances) while varying the unconstrained surface water inventory and atmospheric greenhouse effect. We find that a broad range of atmospheric compositions allow surface liquid water. On a tidally-locked planet with sufficient surface water inventory, liquid water is always present, at least in the substellar region. With a non-synchronous rotation, this requires a minimum greenhouse warming ($\sim$10~mbar of CO${}_2$ and 1~bar of N${}_2$). If the planet is dryer, $\sim$0.5~bar/1.5~bars of CO${}_2$ (respectively for asynchronous/synchronous rotation) suffice to prevent the trapping of any arbitrary small water inventory into polar/nightside ice caps. We produce reflection/emission spectra and phase curves for the simulated climates. We find that atmospheric characterization will be possible by direct imaging with forthcoming large telescopes. The angular separation of $7\lambda /D$ at 1~$\mu$m (with the E-ELT) and a contrast of $\sim$10${}^{-7}$ will enable high-resolution spectroscopy and the search for molecular signatures, including H${}_2$O, O${}_2$, and CO${}_2$. The observation of thermal phase curves can be attempted with JWST, thanks to a contrast of $2\times 10^{-5}$ at 10~$\mu$m. Proxima~b will also be an exceptional target for future IR interferometers. Within a decade it will be possible to image Proxima~b and possibly determine whether this exoplanet's surface is habitable.

Accepted for publication in Astronomy&Astrophysics, companion paper to Ribas et al

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