TOI-700 d: Assessing Habitability Under UV Radiation and Atmospheric Haze Conditions

Planets orbiting within the habitable zone (HZ) of M stars face the risk of high-energy radiation, which can erode their atmospheres over time. Among these stars, TOI-700 is an intriguing target. This quiet star hosts four planets, including TOI-700 d, a rocky planet situated within the HZ. This study focuses on TOI-700 d, evaluating its potential atmosphere and habitability, with a particular emphasis on the role of hazes in the modeled atmospheres, which offers a novel insight into this Earth-like exoplanet. To achieve this, we employed a 1D photochemistry code coupled with the climate ATMOS model, simulating both modern Earth-like and Archean atmospheres to elucidate the atmospheric composition. Although TOI-700 is often quiescent, we simulated a scenario with enhanced flare sufficient to match the relative NUV irradiance levels at the top of the atmosphere to those of the young Sun and Earth, based on stars of the same spectral type. We also conducted simulations without UV amplification for comparison. The impact on habitability is not considered critical. Both common and extremophile bacteria could potentially survive on the planet under various atmospheric conditions in most scenarios. Interestingly, haze presence could significantly influence habitability, possibly having a greater impact than ozone under certain conditions. However, these UV radiation absorbers might also hinder the formation of RNA precursors.