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

TL;DR

This study uses climate modeling to explore possible climates of Proxima Centauri b, assessing habitability and observability with upcoming telescopes, considering different rotation states and atmospheric compositions.

Contribution

It provides the first detailed 3D climate simulations of Proxima b for various rotation modes and atmospheric conditions, predicting potential habitability and observational signatures.

Findings

A broad range of atmospheres can sustain surface liquid water.

Liquid water persists in the substellar region for tidally-locked planets.

Future telescopes can detect atmospheric molecules like H2O, O2, and CO2.

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 $2\times10^{-5}$ 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.