Upper atmosphere dynamics and drivers of volatiles loss from terrestrial-type (exo)planets

TL;DR

This paper investigates how volatile loss from the upper atmospheres of terrestrial planets affects their habitability, focusing on differences between planets around various star types, especially M- and K-dwarfs.

Contribution

It provides a comparative analysis of volatile loss mechanisms on terrestrial planets in our solar system and exoplanets orbiting different star types, highlighting the impact of stellar radiation.

Findings

Higher X-ray and UV fluxes can strip atmospheres of planets around M-dwarfs.

Volatile inventories may help planets withstand atmospheric erosion.

Stellar activity duration influences atmospheric retention.

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 chapter, 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.