Habitable Zones & Global Climate

The Ultraviolet Habitable Zone Of Exoplanets

By Keith Cowing
Press Release
March 29, 2023
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The Ultraviolet Habitable Zone Of Exoplanets
NUV (Near Ultra-Violet) luminosity versus star–planet separation. The UHZ (Ultraviolet Habitable zone, see §2) is defined by the inner (outer) boundary corresponding to the maximum (minimum) UV flux tolerable required) for life resilience (abiogenenesis, considering a specific prebiotic pathways). Three UHZ are shown corresponding to 100, 50 and 10% (as labelled) atmospheric transmission ( 𝑓 ), to the planet’s surface, of the UV flux at the top of the atmosphere. The planets of our sample (§3) selected for being in the CHZ (Circumstellar Habitable Zone) and having UV observations (thus providing 𝐿NUV) are shown by the circle symbols. The color coding corresponds to the 𝑇eff of their host stars (vertical color bar). The horizontal bar of each exoplanet shows the CHZ as derived by Kopparapu et al. (2013). The green shaded regions show the CHZ as derived through the 𝐿NUV ´ 𝑇eff correlation (§5). — astro-ph.EP

The dozens of rocky exoplanets discovered in the Circumstellar Habitable Zone (CHZ) currently represent the most suitable places to host life as we know it outside the Solar System.

However, the presumed presence of liquid water on the CHZ planets does not guarantee suitable environments for the emergence of life. According to experimental studies, the building blocks of life are most likely produced photochemically in presence of a minimum ultraviolet (UV) flux. On the other hand, high UV flux can be life-threatening, leading to atmospheric erosion and damaging biomolecules essential to life.

These arguments raise questions about the actual habitability of CHZ planets around stars other than Solar-type ones, with different UV to bolometric luminosity ratios. By combining the “principle of mediocricy” and recent experimental studies, we define UV boundary conditions (UV-habitable Zone, UHZ) within which life can possibly emerge and evolve. We investigate whether exoplanets discovered in CHZs do indeed experience such conditions. By analysing Swift-UV/Optical Telescope data, we measure the near ultraviolet (NUV) luminosities of 17 stars harbouring 23 planets in their CHZ.

We derive an empirical relation between NUV luminosity and stellar effective temperature. We find that eighteen of the CHZ exoplanets actually orbit outside the UHZ, i.e., the NUV luminosity of their M-dwarf hosts is decisively too low to trigger abiogenesis – through cyanosulfidic chemistry – on them. Only stars with effective temperature >3900 K illuminate their CHZ planets with enough NUV radiation to trigger abiogenesis. Alternatively, colder stars would require a high-energy flaring activity.

Riccardo Spinelli, Francesco Borsa, Giancarlo Ghirlanda, Gabriele Ghisellini, Francesco Haardt

Comments: 8 pages, 3 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2303.16229 [astro-ph.EP] (or arXiv:2303.16229v1 [astro-ph.EP] for this version)
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Submission history
From: Riccardo Spinelli
[v1] Tue, 28 Mar 2023 18:00:04 UTC (963 KB)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻