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Planets orbiting young stars are thought to experience atmosphericevaporation as a result of the host stars' high magnetic activity. We study theevaporation history and expected future of the three known transitingexoplanets in the young multiplanet system K2-198. Based on spectroscopic andphotometric measurements, we estimate an age of the K-dwarf host star between200 and 500 Myr, and calculate the high-energy environment of these planetsusing eROSITA X-ray measurements. We find that the innermost planet K2-198c haslikely lost its primordial envelope within the first few tens of Myr regardlessof the age at which the star drops out of the saturated X-ray regime. For thetwo outer planets, a range of initial envelope mass fractions is possible,depending on the not-yet-measured planetary mass and the stars' spin-downhistory. Regarding the future of the system, we find that the outermost planetK2-198b is stable against photoevaporation for a wide range of planetarymasses, while the middle planet K2-198d is only able to retain an atmospherefor a mass range between ~7 and 18 Earth-masses. Lower-mass planets are toosusceptible to mass loss, and a very thin present-day envelope for higher-massplanets is easily lost with the estimated mass-loss rates. Our results supportthe idea that all three planets started out above the radius valley in the(sub-)Neptune regime and were then transformed into their current states byatmospheric evaporation, but also stress the importance of measuring planetarymasses for (young) multiplanet systems before conducting more detailedphotoevaporation simulations.

Three young planets around the K-dwarf K2-198: High-energy environment, evaporation history and expected future