Magnetic star-planet interaction in the young exoplanet system DS Tucanae Ab

Atmospheric escape is thought to be an important ingredient in the evolution of exoplanets, particularly close-in planets that are smaller than Neptune, the most abundant population of known worlds. Models of evaporation predict that these exoplanets experience intense mass loss in their early lives, when their host stars are the most magnetically active. These models are, however, limited by the lack of observational constraints on the effects of all physical processes involved in planetary mass loss, such as interactions with stellar magnetic fields. Here, we report on observations of the 45 Myr-old, transiting Neptune-sized exoplanet DS Tucanae Ab with the Hubble Space Telescope aiming to detect and characterize its hydrogen-rich exosphere fed by mass loss. In the epochs mid-2022 and early-2023 we observed a significant absorption in the stellar Lyman-alpha line about four hours before mid-transit and again immediately after transit egress, between Doppler velocities +100 to +400 km/s in the stellar rest frame. This repeated signal suggests the presence of a large cloud of neutral H infalling into the host star at high velocities, likely originating from atmospheric escape in DS~Tucanae~Ab. Different from previous observations of other exoplanets, this signal cannot be explained with models that include only stellar wind and radiation pressure. We propose that the extended planetary exosphere is being shaped by interactions with the magnetized wind of the host star. Critically, we show that, under sub-Alfvénic conditions and for stellar surface magnetic fields in the order of tens to hundreds of Gauss, which is typical for young solar-type stars, such interactions can happen and produce the signals we observed. Our study underscores the importance of star-planet interactions in observations of atmospheric escape and the role of magnetized stellar winds in shaping the early evolution of planetary systems.