Atmospheric mass-loss and stellar wind effects in young and old systems - Comparative 3D study of TOI-942 and TOI-421 systems

At young ages, when radiation from the host star is high, and the planet is hot and inflated after formation, planetary atmospheric mass-loss can be extremely strong compared to older planets. In turn, stellar winds are faster and denser for young stars compared to evolved main-sequence stars. Their interaction with escaping planetary atmospheres can substantially affect atmospheric mass-loss rates, as well as the observable signatures of escaping atmospheres, with both effects expected to occur differently for young and evolved planets. We perform a comparative study of two systems around stars of similar masses but very different ages (50 Myr and 9 Gyr): TOI-942 and TOI-421. Both stars host two sub-Neptune-like planets at similar orbits and in similar mass ranges, which allows a direct comparison of the atmospheric escape and interactions with the stellar winds in the young and old systems. We perform the 3D atmospheric modelling of the four planets in TOI-942 and TOI-421 systems and make the theoretical predictions of possible observational signatures in Ly aabsorption. We find that accounting for the stellar wind interacting with planetary atmospheres is crucial for the interpretation of the observations for young planets. Additionally, we show that a particular energy distribution along the XUV spectra has a minor effect on the atmospheric mass-loss rates, but it is of crucial importance for modelling the Ly aabsorption and therefore for interpretation of observations.

Automated summary

This article investigates the differences in atmospheric escape and interaction with stellar winds between young and aging planets. By comparing two stellar systems, it is found that accounting for the influence of stellar winds is crucial for interpreting observations of young planets.