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.

A revised lower estimate of ozone columns during Earth’s oxygenated history

“…The discussion of habitability here will be limited to UV radiation, which has varying effects depending on the organism considered (note that many organisms have developed strategies to avoid excessive UV damage, as well as repair mechanisms to mitigate its effects [98][99][100][101]). Although microbial life is known to survive stronger than ambient UV irradiation [97,102,103], many animals and plant species are impacted by high doses of UV radiation, resulting in infertility [59], cell death [104] and increased mortality rates [105][106][107], with UV radiation considered an environmental stressor [108].…”

No Such Thing as a Simple Flare: Substructure and QPPs Observed in a Statistical Sample of 20 Second Cadence TESS Flares

“…Fully 1/3 of our 10 34 erg flares reach this limit during the 20 second peak epoch. None of our flare peaks reach the much larger 39,000 J m −2 needed to inactivate ∼100% of the population (Abrevaya et al 2020). Survival curves for the common bacterium P. Aeruginosa also obtained from laboratory experiments by Abrevaya et al (2020) enable us to estimate the survival fractions of this microorganism during our 20 second duration peak fluences.…”

“…None of our flare peaks reach the much larger 39,000 J m −2 needed to inactivate ∼100% of the population (Abrevaya et al 2020). Survival curves for the common bacterium P. Aeruginosa also obtained from laboratory experiments by Abrevaya et al (2020) enable us to estimate the survival fractions of this microorganism during our 20 second duration peak fluences. Our estimated fluences would result in survival fractions of ∼10 −2.1 with the largest 20 second peaks resulting in survival fractions of 10 −4.9 .…”

“…The impulse of a flare describes the morphology of the flare light curve and is defined as the amplitude of a flare's peak in fractional-flux units over its FWHM duration in minutes (Kowalski et al 2013). Because highly impulsive superflares deliver the same amount of UV flux much more rapidly than less impulsive ones, it is likely the temporal structure of the flare peak could influence the rates of key reactions in atmospheric chemistry (Loyd et al 2018) or biology (Abrevaya et al 2020). Ultra-highcadence NUV/optical observations by Kowalski et al (2013Kowalski et al ( , 2016 find that flares can occur with a range of morphologies from highly impulsive and strongly peaked flares to less impulsive and more gradual events.…”

The Flare-Activity of 2MASSJ16111534-1757214 in the Upper Scorpius association

“…Since flares significantly increase the amount of this type of radiation, they are highly important for the photochemistry of planetary atmospheres (Lammer et al 2018). Biological studies demonstrate the vulnerability of microorganisms if they are exposed to the UV-C radiation (1000-2800 Å) from super-flares (Abrevaya et al 2020). Thus, flares can make planets uninhabitable even if they are formally in the habitable zone.…”