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 $2\times10^{-5}$ 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.

• The paper demonstrates that Proxima Centauri b may host surface liquid water when water inventories exceed 0.6 Earth oceans and CO₂ levels are adequately maintained.
• Using a 3D Global Climate Model, the study compares synchronous 1:1 and asynchronous 3:2 spin states, revealing that climatic outcomes are highly sensitive to volatile and greenhouse gas pressures.
• Observational prospects with instruments like the E-ELT and JWST are promising, as the analysis shows that key molecular signatures may soon be detectable via high-resolution spectroscopy.

The Habitability of Proxima Centauri b: Atmospheric Conditions and Observability

This essay critically examines the research paper "The Habitability of Proxima Centauri b: II. Possible Climates and Observability," which presents an in-depth analysis of the potential climate regimes and habitability of Proxima Centauri b, along with the prospects for its observation using next-generation telescopes.

Proxima Centauri b, orbiting within the habitable zone of our closest stellar neighbor, has been identified as a potentially habitable terrestrial planet. Researchers used a 3D Global Climate Model (GCM) to simulate the planet's atmosphere and water cycle under varying conditions of surface water inventory and atmospheric greenhouse effects. The paper investigates two primary scenarios, corresponding to different spin-orbit resonances: a synchronous 1:1 resonance and a 3:2 resonance, similar to Mercury.

One of the key findings is that a wide range of atmospheric compositions could support surface liquid water, provided the surface water inventory exceeds 0.6 Earth oceans. For a tidally-locked configuration, liquid water is typically present in the substellar region, especially with a greenhouse gas composition involving a CO₂ partial pressure of at least 1 bar. In the absence of sufficient water, water can become trapped as ice, forming glaciers or lakes on the planet's night side, depending on the atmospheric CO₂ levels.

A significant observation is that climate regimes on Proxima Centauri b are sensitive to the amount of available volatiles and the planet's spin state. The asynchronous rotation model necessitates a minimum CO₂ pressure of approximately 10 mbar to avoid transition to a snowball state if water is bountiful. Conversely, in drier conditions, around 0.5 bar of CO₂ is sufficient to prevent water entrapment in polar ice caps. These findings highlight the pivotal role of volatile and atmospheric gases in defining habitability conditions, offering implications for the climate regimes of other exoplanets.

The study extends to the methodology for observing Proxima b, forecasting that direct imaging might soon be feasible using large telescopes like the E-ELT. With a potential angular separation of 7 λ/D at 1 μm and a contrast of ~10⁻⁷, the planet could allow high-resolution spectroscopy and the detection of atmospheric molecular signatures such as H₂O, O₂, and CO₂.

Moreover, analyzing thermal phase curves using JWST has been mentioned as viable, given the flux contrast, despite the challenges posed by stellar variability. Such observations could provide vital insights into Proxima b's atmosphere and climate.

In conclusion, this research systematically elucidates Proxima Centauri b's possible climate conditions, emphasizing how varied atmospheric compositions and surface water inventories could influence habitability. As observational techniques advance, the theoretical frameworks developed in this study will guide empirical investigations, enhancing our understanding of terrestrial planet climates in nearby stellar systems. Future research should focus on refining these climate models' assumptions, incorporating potential variations in topography and oceanic transport, and considering stellar influences on atmospheric retention.