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.

Demarcating Circulation Regimes of Synchronously Rotating Terrestrial Planets within the Habitable Zone

“…The TRAPPIST-1 systems may contain several planets within the traditional liquid water habitable zone, with TRAPPIST-1e being the most promising candidate (Wolf 2017;Turbet et al 2017). With an orbital period of ∼ 6 days, TRAPPIST-1e would be close to the rapid rotator regime-especially if its lower planetary mass implies a lower atmospheric scale height (and thus a smaller value of λ R ).…”

Is Proxima Centauri b Habitable? A Study of Atmospheric Loss

“…As a consequence, we assume that the atmospheric composition of PCb is similar to that of Venus and Mars. In reality, we wish to note there are many possibilities for the atmospheric composition of PCb, provided that it even exists (Ribas et al 2016;Turbet et al 2016;Goldblatt 2016). A clearer picture regarding the existence and composition of the atmosphere may emerge via the JWST (Kreidberg & Loeb 2016).…”

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

“…We also note that our estimates for the water loss once planets are in the HZ (Table 6) are probably upper limits. If the planet is able to retain its background atmosphere, the tropopause is expected to act as an efficient cold trap, preventing water from reaching the higher parts of the atmosphere (e.g., Wordsworth & Pierrehumbert 2014;Turbet et al 2016). In that case, water escape would be limited by the diffusion of water through the cold trap.…”