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At young ages, when radiation from the host star is high, and the planet ishot and inflated after formation, planetary atmospheric mass loss can beextremely strong compared to older planets. In turn, stellar winds are fasterand denser for young stars compared to evolved main-sequence stars. Theirinteraction with escaping planetary atmospheres can substantially affectatmospheric mass loss rates, as well as the observable signatures of escapingatmospheres, with both effects expected to occur differently for young andevolved planets. We perform a comparative study of two systems around stars ofsimilar 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 similarmass ranges, which allows a direct comparison of the atmospheric escape andinteractions with the stellar winds in the young and old systems. We performthe 3D atmospheric modeling of the four planets in TOI-942 and TOI-421 systemsand make the theoretical predictions of possible observational signatures inLy-alpha absorption. We find that accounting for the stellar wind interactingwith planetary atmospheres is crucial for the interpretation of theobservations for young planets. Additionally, we show that a particular energydistribution along the XUV spectra has a minor effect on the atmosphericmass-loss rates, but it is of crucial importance for modeling the Ly-alphaabsorption and therefore for interpretation of observations.

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