# Upper Atmosphere Dynamics and Drivers of Volatiles Loss from Terrestrial-Type (Exo)Planets

## TL;DR

This paper explores how exoplanets lose their atmospheres due to high X-ray and UV exposure from their host stars, especially M- and K-dwarfs, and how this affects their potential for habitability.

## Contribution

The paper provides a comparative analysis of volatile loss in terrestrial planets across different stellar environments, including our solar system and exoplanetary systems.

## Key findings

- M- and K-dwarf stars emit high X-ray and UV fluxes that may strip atmospheres from nearby terrestrial planets.
- The long-term habitability of exoplanets depends on their volatile inventories and stellar activity levels.
- Comparative studies of solar system planets help understand volatile loss in exoplanetary systems.

## Abstract

Volatile loss from exoplanetary atmospheres and its possible implications for the longevity of habitable surface conditions is a topic of vigorous debate currently. The vast majority of the habitable zone terrestrial-like exoplanets known to date orbit low-mass M- and K-dwarf stars and are subject to the conditions drastically different to those of terrestrial planets in the Solar System. In particular, they orbit far closer to their host stars than similar planets around G-dwarfs similar to the Sun. Therefore they receive higher X-ray and UV fluxes, even though luminosities of M- and K-dwarfs are lower than those of heavier stars. Furthermore, due to their slower evolution, M-dwarfs retain high activity on the gigayear timescales. The combination of these two effects has led to claims that most terrestrial planets orbiting M-dwarfs may have their atmospheres stripped from the higher X-ray and UV fluxes of their host stars. Opposing this are researchers who point out that volatile inventories for terrestrial exoplanets are ill-constrained, and hence, they may be able to “weather the storm” of these higher X-ray and UV fluxes. In this article, we focus on exploring volatile loss in the upper atmospheres of terrestrial planets in our solar system and applications to those in exoplanetary systems around stars of different types.