The Surface UV Environment on Planets Orbiting M Dwarfs: Implications for Prebiotic Chemistry and the Need for Experimental Follow-up

Ranjan, Sukrit; Wordsworth, R.; Sasselov, Dimitar

doi:10.3847/1538-4357/aa773e

Cited by 132 publications

(141 citation statements)

References 90 publications

(177 reference statements)

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“…Different absorption of the UV (200-380 nm) flux for the different atmospheric scenarios were obtained (see Fig. A, appendix), consistent with previous studies by (Ranjan et al 2017)(see comparison in Fig. B, appendix).…”

Section: Uv Radiation On the Planetary Surfacesupporting

confidence: 90%

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

Abrevaya

Leitzinger

Oppezzo

et al. 2020

Monthly Notices of the Royal Astronomical Society: Letters

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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.

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Section: Uv Radiation On the Planetary Surfacesupporting

confidence: 90%

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

Abrevaya

Leitzinger

Oppezzo

et al. 2020

Monthly Notices of the Royal Astronomical Society: Letters

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“…We note in passing that the prospects for abiotic

{NO}_{X}^{-}

buildup may be enhanced on planets orbiting M‐dwarfs, due to their much lower surface UV irradiation and consequently much slower

{NO}_{X}^{-}

photolysis rate (Ranjan et al, ). Consequently,

{NO}_{X}^{-}

‐dependent prebiotic chemistries may proceed especially well on such worlds relative to early Earth.…”

Section: Discussionmentioning

confidence: 99%

Nitrogen Oxide Concentrations in Natural Waters on Early Earth

Ranjan

Todd

Rimmer

et al. 2019

Geochem Geophys Geosyst

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A key challenge in origins‐of‐life studies is estimating the abundances of species relevant to the chemical pathways proposed to have contributed to the emergence of life on early Earth. Dissolved nitrogen oxide anions ( NOX−), in particular nitrate ( NO3−) and nitrite ( NO2−), have been invoked in diverse origins‐of‐life chemistry, from the oligomerization of RNA to the emergence of protometabolism. Recent work has calculated the supply of NOX− from the prebiotic atmosphere to the ocean and reported steady state [ NOX−] to be high across all plausible parameter space. These findings rest on the assumption that NOX− is stable in natural waters unless processed at a hydrothermal vent. Here, we show that NOX− is unstable in the reducing environment of early Earth. Sinks due to ultraviolet photolysis and reactions with reduced iron (Fe2+) suppress [ NOX−] by several orders of magnitude relative to past predictions. For pH = 6.5–8 and T = 0–50 °C, we find that it is most probable that [ NOX−] <1μM in the prebiotic ocean. On the other hand, prebiotic ponds with favorable drainage characteristics may have sustained [ NOX−] ≥1μM. As on modern Earth, most NOX− on prebiotic Earth should have been present as NO3−, due to its much greater stability. These findings inform the kind of prebiotic chemistries that would have been possible on early Earth. We discuss the implications for proposed prebiotic chemistries and highlight the need for further studies of NOX− kinetics to reduce the considerable uncertainties in predicting [ NOX−] on early Earth.

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“…This result is consistent with Rimmer et al (2018), who place LHS 1140b out of the abiogenesis zone. In this context it is still not clear if the UV-dependent prebiotic pathways that were important for the origin of life on Earth can still proceed in a lower UV environment (Ranjan et al 2017) such as that of LHS 1140b. In any case, the activity evolution of M-dwarfs, even though still not completely understood, seems to decrease with age (Ribas et al 2016;Claire et al 2012;Selsis et al 2007;Guinan et al 2016) implying that LHS 1140b could have suffered more high-energy irradiance in the past.…”

Section: Discussionmentioning

confidence: 99%

The high-energy radiation environment of the habitable-zone super-Earth LHS 1140b

Spinelli

Borsa

Ghirlanda

et al. 2019

A&A

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Context. In the last few years many exoplanets in the habitable zone (HZ) of M-dwarfs have been discovered, but the X-ray/UV activity of cool stars is very different from that of our Sun. The high-energy radiation environment influences the habitability, plays a crucial role for abiogenesis, and impacts the chemistry and evolution of planetary atmospheres. LHS 1140b is one of the most interesting exoplanets discovered. It is a super-Earth-size planet orbiting in the HZ of LHS 1140, an M4.5 dwarf at "15 parsecs. Aims. In this work, we present the results of the analysis of a Swift X-ray/UV observing campaign. We characterize for the first time the X-ray/UV radiation environment of LHS 1140b. Methods. We measure the variability of the near ultraviolet (NUV) flux and estimate the far ultraviolet (FUV) flux with a correlation between FUV 1344´1786Å and NUV 1771´2831Å flux obtained using the sample of low-mass stars in the GALEX archive. We highlight the presence of a dominating X-ray source close to the J2000 coordinates of LHS 1140, characterize its spectrum, and derive an X-ray flux upper limit for LHS 1140. We find that this contaminant source could have influenced the previously estimated spectral energy distribution.Results. No significant variation of the NUV 1771´2831Å flux of LHS 1140 is found over 3 months, and we do not observe any flare during the 38 ks on the target. LHS 1140 is in the 25th percentile of least variable M4-M5 dwarfs of the GALEX sample. Analyzing the UV flux experienced by the HZ planet LHS 1140b, we find that outside the atmosphere it receives a NUV 1771´2831Å flux ă 2% with respect to that of the present-day Earth, while the FUV 1344´1786Å /NUV 1771´2831Å ratio is "100-200 times higher. This represents a lower limit to the true FUV/NUV ratio since the FUV 1344´1786Å band does not include Lyman-alpha, which dominates the FUV output of low-mass stars. This is a warning for future searches for biomarkers, which must take into account this high ratio.Conclusions. The relatively low level and stability of UV flux experienced by LHS 1140b should be favorable for its present-day habitability.

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The Surface UV Environment on Planets Orbiting M Dwarfs: Implications for Prebiotic Chemistry and the Need for Experimental Follow-up

Cited by 132 publications

References 90 publications

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

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

Nitrogen Oxide Concentrations in Natural Waters on Early Earth

The high-energy radiation environment of the habitable-zone super-Earth LHS 1140b

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