The time evolution of the ultraviolet habitable zone

TL;DR

This study explores how the ultraviolet habitable zone around stars evolves over time and intersects with the circumstellar habitable zone, highlighting implications for the potential emergence of life on exoplanets, especially around M-dwarfs.

Contribution

It provides the first analysis of the time evolution of the ultraviolet habitable zone and its intersection with the habitable zone using observational data and stellar models.

Findings

Intersection of CHZ and UHZ varies over time for different stars.

Hotter M-dwarfs have longer periods where UHZ and CHZ overlap.

Proxima Centauri shows a prolonged CHZ-UHZ intersection, making it a prime target.

Abstract

For stars hosting Circumstellar Habitable Zone (CHZ) exoplanets, we investigate the time-evolution of their ultraviolet habitable zone (UHZ), the annular region around a star where an exoplanet could experience a suitable ultraviolet environment for the presence and emergence of life, and the possible intersection of the UHZ with the CHZ. To estimate their UV luminosity evolution, and therefore the evolution of their UHZ, we analyse Swift-UV/Optical telescope observations and adopt the near-UV luminosity evolutionary tracks derived using GALEX observations of young moving groups. We find that an intersection between CHZ and UHZ could exist (or have existed) around all stars of our sample at different epochs, except for the coldest M-dwarfs (temperature < 2800 K, e.g. Trappist-1). For hotter M-dwarfs the formation of RNA precursors through cyanosulfidic chemistry triggered by near-UV radiation could occur during the first 1-2 Gyrs. The radial-extension and time-duration of the CHZ-UHZ intersection increase with the stellar effective temperature and the exoplanet atmospheric transmissivity at near-UV wavelengths. Within our sample, Proxima Centauri represents a golden target for the quest of life outside the Solar system because it experienced a long-lasting and more extended, compared to similar M-dwarfs, CHZ-UHZ intersection.