# Impact of Stellar Superflares on Planetary Habitability

## TL;DR

This study quantitatively evaluates how stellar superflares and associated radiation impact the habitability of exoplanets, considering atmospheric effects and flare frequency, with implications for planets like Proxima Centauri b and TRAPPIST-1 e.

## Contribution

First to develop a quantitative impact evaluation system for stellar flares on exoplanet habitability, focusing on ionizing radiation effects and atmospheric escape.

## Key findings

- Ground-level radiation doses are below critical for complex life at 1 bar atmosphere.
- Maximum stellar flares can cause fatal radiation doses on some exoplanets.
- High XUV flux induces significant atmospheric escape, reducing habitability potential.

## Abstract

High-energy radiation caused by exoplanetary space weather events from planet-hosting stars can play a crucial role in conditions promoting or destroying habitability in addition to the conventional factors. In this paper, we present the first quantitative impact evaluation system of stellar flares on the habitability factors with an emphasis on the impact of Stellar Proton Events. We derive the maximum flare energy from stellar starspot sizes and examine the impacts of flare associated ionizing radiation on CO$_2$, H$_2$, N$_2$+O$_2$ --rich atmospheres of a number of well-characterized terrestrial type exoplanets. Our simulations based on the Particle and Heavy Ion Transport code System [PHITS] suggest that the estimated ground level dose for each planet in the case of terrestrial-level atmospheric pressure (1 bar) for each exoplanet does not exceed the critical dose for complex (multi-cellular) life to persist, even for the planetary surface of Proxima Centauri b, Ross-128 b and TRAPPIST-1 e. However, when we take into account the effects of the possible maximum flares from those host stars, the estimated dose reaches fatal levels at the terrestrial lowest atmospheric depth on TRAPPIST-1 e and Ross-128 b. Large fluxes of coronal XUV radiation from active stars induces high atmospheric escape rates from close-in exoplanets suggesting that the atmospheric depth can be substantially smaller than that on the Earth. In a scenario with the atmospheric thickness of 1/10 of Earth's, the radiation dose from close-in planets including Proxima Centauri b and TRAPPIST-1 e reach near fatal dose levels with annual frequency of flare occurrence from their hoststars.