The space weather around the exoplanet GJ 436b

II. Stellar wind–exoplanet interactions

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands
e-mail: vidotto@strw.leidenuniv.nl
² Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51b, 1290 Versoix, Switzerland
³ Department of Physics, College of Science, United Arab Emirates University, PO Box No. 15551, Al Ain, UAE
e-mail: rim.fares@uaeu.ac.ae
⁴ Science Division, Directorate of Science, European Space Research and Technology Centre (ESA/ESTEC), Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands
⁵ Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, CNRS, IRAP/UMR 5277, 14 avenue Édouard Belin, 31400 Toulouse, France
⁶ Laboratoire Univers et Particules de Montpellier, Université de Montpellier, CNRS, 34095 Montpellier, France

Received: 20 June 2023
Accepted: 23 August 2023

Abstract

The M dwarf star GJ 436 hosts a warm-Neptune that is losing a substantial amount of atmosphere, which is shaped by the interactions with the wind of the host star. The stellar wind is formed by particles and magnetic fields that shape the exo-space weather around the exoplanet GJ 436b. Here, we use the recently published magnetic map of GJ 436 to model its 3D Alfvén-wave-driven wind. We compared our results with previous transmission spectroscopic models and measurements of non-thermal velocities at the transition region of GJ 436; our models indicate that the wind of GJ 436 is powered by a smaller flux of Alfvén waves than that powering the wind of the Sun. This suggests that the canonical flux of Alfvén waves assumed in solar wind models might not be applicable to the winds of old M dwarf stars. Compared to the solar wind, GJ 436’s wind has a weaker acceleration and an extended sub-Alfvénic region. This is important because it places the orbit of GJ 436b inside the region dominated by the stellar magnetic field (i.e. inside the Alfvén surface). Due to the sub-Alfvénic motion of the planet through the stellar wind, magnetohydrodynamic waves and particles released in reconnection events can travel along the magnetic field lines towards the star, which could power the anomalous ultraviolet flare distribution recently observed in the system. For an assumed planetary magnetic field of B_p ≃ 2 G, we derive the power released by stellar wind-planet interactions as Ƥ ~ 10²²−10²³ erg s⁻¹, which is consistent with the upper limit of 10²⁶ erg s⁻¹ derived from ultraviolet lines. We further highlight that, because star-planet interactions depend on stellar wind properties, observations that probe these interactions and the magnetic map used in 3D stellar wind simulations should be contemporaneous for deriving realistic results.