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The recent detection of a giant exosphere surrounding the warm Neptune GJ436b has shed new light on the evaporation of close-in planets, revealing thatmoderately irradiated, low-mass exoplanets could make exceptional targets forstudying this mechanism and its impact on the exoplanet population. ThreeHST/STIS observations were performed in the Lyman-$\alpha$ line of GJ436 atdifferent epochs, showing repeatable transits with large depths and extendeddurations. Here, we study the role played by stellar radiation pressure on thestructure of the exosphere and its transmission spectrum. We found that theneutral hydrogen atoms in the exosphere of GJ436 b are not swept away byradiation pressure as shown to be the case for evaporating hot Jupiters.Instead, the low radiation pressure from the M-dwarf host star only brakes thegravitational fall of the escaping hydrogen toward the star and allows itsdispersion within a large volume around the planet, yielding radial velocitiesup to about -120 km s$^{-1}$ that match the observations. We performednumerical simulations with the EVaporating Exoplanets code (EVE) to study theinfluence of the escape rate, the planetary wind velocity, and the stellarphotoionization. While these parameters are instrumental in shaping theexosphere and yield simulation results in general agreement with theobservations, the spectra observed at the different epochs show specific,time-variable features that require additional physics.

Radiative braking in the extended exosphere of GJ436b