The ultraviolet habitable zone of exoplanets

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 mediocrity' 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 18 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 greater than or similar to 3900 K illuminate their CHZ planets with enough NUV radiation to trigger abiogenesis. Alternatively, colder stars would require a high-energy flaring activity.

Automated summary

The rocky exoplanets discovered in the circumstellar habitable zone (CHZ) are currently the most suitable places for hosting life outside the Solar system. However, the presence of liquid water on these CHZ planets does not guarantee habitable environments for the emergence of life. Experimental studies suggest that the building blocks of life are likely produced photochemically in the presence of a minimal ultraviolet (UV) flux. High UV flux, on the other hand, can be detrimental to life. This raises questions about the habitability of CHZ planets around stars with different UV to bolometric luminosity ratios. By combining the principle of mediocrity and recent experimental studies, the researchers define UV boundary conditions within which life can possibly emerge and evolve. They analyze data from the Swift-UV/Optical Telescope and find that most of the CHZ exoplanets actually orbit outside the UV-habitable zone, where the NUV luminosity of their M-dwarf hosts is too low to trigger abiogenesis.