2021MNRAS.501.4383V

The GJ 436 planetary system is an extraordinary system. The Neptune-sized planet that orbits the M3 dwarf revealed in the Ly α line an extended neutral hydrogen atmosphere. This material fills a comet-like tail that obscures the stellar disc for more than 10 h after the planetary transit. Here, we carry out a series of 3D radiation hydrodynamic simulations to model the interaction of the stellar wind with the escaping planetary atmosphere. With these models, we seek to reproduce the ∼56 per cent absorption found in Ly α transits, simultaneously with the lack of absorption in H α transit. Varying the stellar wind strength and the EUV stellar luminosity, we search for a set of parameters that best fit the observational data. Based on Ly α observations, we found a stellar wind velocity at the position of the planet to be around [250-460] km s^–1 with a temperature of [3-4] x 10⁵ K. The stellar and planetary mass-loss rates are found to be 2 x 10^–15 M_☉ yr^–1 and ∼[6-10] x 10⁹ g s^–1, respectively, for a stellar EUV luminosity of [0.8-1.6] x 10²⁷ erg s^–1. For the parameters explored in our simulations, none of our models present any significant absorption in the H α line in agreement with the observations.