# The ultraviolet habitable zone of exoplanets

**Authors:** Riccardo Spinelli, Francesco Borsa, Giancarlo Ghirlanda, Gabriele Ghisellini, Francesco Haardt  
**arXiv:** [2303.16229](https://arxiv.org/abs/2303.16229)  
**Date:** 2023-03-28  
**Summary page:** https://www.summarizepaper.com/en/paper/2303.16229v1/

## Key points

- Dozens of rocky exoplanets have been discovered in the Circumstellar Habitable Zone (CHZ)
- Liquid water is present on these planets but it does not guarantee their habitability
- Building blocks of life are produced photochemically in presence of a minimum ultraviolet (UV) flux
- High UV flux can be detrimental to life by leading to atmospheric erosion and damaging biomolecules essential to its survival
- Researchers from Italy's National Institute for Astrophysics and University of Milan-Bicocca have defined UV boundary conditions within which life can possibly emerge and evolve, called Ultraviolet Habitable Zone (UHZ)
- Only stars with effective temperature greater than 3900 K illuminate their CHZ planets with enough NUV radiation to trigger abiogenesis through cyanosulfidic chemistry
- Eighteen out of twenty-three CHZ exoplanets actually orbit outside the UHZ because their M-dwarf hosts do not emit enough NUV radiation to trigger abiogenesis on them.
- It is important to consider the UV radiation emitted by host stars when assessing the habitability of exoplanets.

## Summary

In recent years, the discovery of dozens of rocky exoplanets in the Circumstellar Habitable Zone (CHZ) has sparked increased interest in the search for extraterrestrial life. While these planets may have liquid water - a key ingredient for life as we know it - this alone does not guarantee their habitability. Experimental studies suggest that the building blocks of life are most likely produced photochemically in presence of a minimum ultraviolet (UV) flux. However, high UV flux can be detrimental to life by leading to atmospheric erosion and damaging biomolecules essential to its survival. To better understand the actual habitability of CHZ planets around stars other than Solar-type ones with different UV to bolometric luminosity ratios, researchers from Italy's National Institute for Astrophysics and University of Milan-Bicocca have defined UV boundary conditions within which life can possibly emerge and evolve. By combining the "principle of mediocrity" and recent experimental studies, they have established an Ultraviolet Habitable Zone (UHZ), which represents the range of UV radiation that is conducive to abiogenesis – the emergence of life from non-living matter. The team analyzed data from Swift-UV/Optical Telescope on 17 stars hosting 23 planets in their CHZs and measured their near ultraviolet (NUV) luminosities. They derived an empirical relation between NUV luminosity and stellar effective temperature and found that only stars with effective temperature greater than 3900 K illuminate their CHZ planets with enough NUV radiation to trigger abiogenesis through cyanosulfidic chemistry. Alternatively, colder stars would require a high-energy flaring activity. Their findings suggest that eighteen out of twenty-three CHZ exoplanets actually orbit outside the UHZ because their M-dwarf hosts do not emit enough NUV radiation to trigger abiogenesis on them. This study highlights how important it is to consider the UV radiation emitted by host stars when assessing the habitability of exoplanets and provides a framework for future studies in pursuit of extraterrestrial life.