The UV surface habitability of Proxima b: first experiments revealing probable life survival to stellar flares

Abstract: We use a new interdisciplinary approach to study the UV surface habitability of Proxima $b$ under quiescent and flaring stellar conditions. We assumed planetary atmospheric compositions based on CO$_2$ and N$_2$ and surface pressures from 100 to 5000 mbar. Our results show that the combination of these atmospheric compositions and pressures provide enough shielding from the most damaging UV wavelengths, expanding the "UV-protective" planetary atmospheric compositions beyond ozone. Additionally, we show that the UV radiation reaching the surface of Proxima $b$ during quiescent conditions would be negligible from the biological point of view, even without an atmosphere. Given that high UV fluxes could challenge the existence of life, then, we experimentally tested the effect that flares would have on microorganisms in a "worst-case scenario" (no UV-shielding). Our results show the impact that a typical flare and a superflare would have on life: when microorganisms receive very high fluences of UVC, such as those expected to reach the surface of Proxima $b$ after a typical flare or a superflare, a fraction of the population is able to survive. Our study suggests that life could cope with highly UV irradiated environments in exoplanets under conditions that cannot be found on Earth.

• The paper demonstrates that CO₂ and N₂ based atmospheres at 100–5000 mbar effectively attenuate harmful UV wavelengths.
• The paper shows through radiative transfer models that quiescent stellar phases produce minimal UV radiation, enhancing surface habitability.
• The paper finds that microorganisms can endure high UVC fluence during stellar flares via protective mechanisms like phenotypic persistence.

UV Surface Habitability of Proxima b: Assessing Life Survival through Stellar Flares

The study titled "The UV Surface Habitability of Proxima b: First Experiments Revealing Probable Life Survival to Stellar Flares" systematically explores the potential for life on Proxima b amidst the challenges posed by ultraviolet radiation (UVR) from its host star, particularly during stellar flare events. Utilizing an interdisciplinary approach, the authors incorporate both theoretical modeling and experimental methods to examine the atmospheric conditions and biological resilience necessary for life to persist on this exoplanet located within the habitable zone of the M-type star Proxima Centauri.

Key Findings

1. Atmospheric Shielding Capability:
   • The paper details that various CO₂ and N₂ based atmospheric compositions, even at pressures ranging from 100 to 5000 mbar, are significantly effective in attenuating harmful UV wavelengths. These conditions broaden the scope of atmospheric types that could offer UV protection, extending beyond the traditionally considered ozone-rich compositions.
2. UV Radiation Assessment in Quiescence:
   • Numerical analysis using radiative transfer models indicates that during quiescent stellar phases, the UVR reaching Proxima b's surface is minimal from a biological damage perspective. Notably, this remains true even in the absence of an atmosphere, suggesting a more favorable environment for life than previously assumed.
3. Biological Resilience During Flares:
   • The study recognizes that stellar flares pose a transient but intense increase in UVR, particularly in the UVC range, which is most detrimental to DNA. Experimental exposure of microorganisms (Haloferax volcanii and Pseudomonas aeruginosa) to simulated flare conditions reveals that substantial fractions of these populations can withstand high UVC fluence rates. This resilience is attributed to potential protective biological mechanisms like phenotypic persistence, which might enable microbial survival during and proliferation post-flare events.

Implications for Habitability

The findings suggest that conditions on Proxima b, particularly with regard to its ability to sustain life during quiescent periods, might be less hostile than previously thought, even without the presence of an Earth-like atmosphere. The survival of microorganisms under intense UVC exposure argues for a reconsideration of how life's resilience to UVR impacts the potential habitability of planets orbiting M-dwarfs.

Future Directions

The study informs the need to refine atmospheric models and understand the variability of stellar activity better, particularly the frequency and intensity of flares, which determine the long-term survivability prospects of microbial life on Proxima b. Future research should focus on understanding the repair mechanisms in microorganisms activated by concurrent visible light exposure during the flares, as well as assessing the potential for biofilms and other forms of microbial community structuring that could mitigate extreme conditions on exoplanetary surfaces.

This work underscores the importance of incorporating interdisciplinary techniques combining astrophysics, microbiology, and climate science to holistically approach planetary habitability assessment. It sets the stage for future explorations into the adaptation mechanisms that might support life on planets in extreme space environments.