The habitability of Proxima Centauri b

Abstract

Radial velocity monitoring has found the signature of a M sin i = 1.3 M ⊕ planet located within the Habitable Zone (HZ) of Proxima Centauri (Anglada-Escudé et al. 2016). Despite a hotter past and an active host star the planet Proxima b could have retained enough volatiles to sustain surface habitability (Ribas et al. 2016). 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. Any low-obliquity low-eccentricity planet within the HZ of its star should be in one of the climate regimes discussed here. 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 (∼10 mbar of CO 2 and 1 bar of N 2). If the planet is dryer, ∼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 7λ/D at 1 µm (with the E-ELT) and a contrast of ∼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 × 10 −5 at 10 µm. 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.

Temporal Evolution of the High-energy Irradiation and Water Content of TRAPPIST-1 Exoplanets

“…In this section, we study the evolution of water loss from the TRAPPIST-1 planets, in particular during their runaway greenhouse phase (see Barnes & Heller 2013;Luger & Barnes 2015;and Bolmont et al 2017 for generic brown dwarfs and M dwarfs and Ribas et al 2016 andBarnes et al 2016 for the M dwarf planet Proxima-b). The idea is that, once a planet reaches the HZ, its water can recombine and condense.…”

A Panchromatic View of Brown Dwarf Aurorae

“…When making this comparison, however, it may be important to also take into account the planetary system architectures. While a single very close-in planet may suffice to either seed the plasma disk or provide the obstacle for the auroral flux tube, tidal interactions may spin down the brown dwarf host, possibly limiting its ability to power significant aurorae, although this would depend on substantial unknowns, such as the degree of brown dwarf tidal dissipation (Ribas et al 2016;Gillon et al 2017). In contrast, interactions in multiple-satellite systems, such as the one around Jupiter, can prevent the total tidal synchronization of the close-in satellites through resonant orbital interactions and a pumping of the orbital eccentricity (see Peale 1999 and references therein).…”

Formation and composition of planets around very low mass stars

“…The water mass fraction is in general larger than ∼10%, and water evaporation has been claimed to be inefficient for planets in the habitable zone of lowmass stars (Ribas et al 2016). …”