# Exoplanets as probes of the winds of host stars: the case of the M dwarf GJ 436

## TL;DR

This study uses observations of the exoplanet GJ 436b to infer the stellar wind properties of its M dwarf host, demonstrating a method to probe stellar winds via planetary atmospheric evaporation and transmission spectroscopy.

## Contribution

It introduces a novel approach combining planetary atmospheric data and stellar wind models to constrain the large-scale wind structure of M dwarf stars.

## Key findings

- Stellar wind temperature constrained to 0.36-0.43 MK.
- Mass-loss rate estimated at (0.5-2.5) x 10^{-15} Msyn/yr.
- Astrosphere size around 25 au, smaller than the heliosphere.

## Abstract

Winds of cool dwarfs are difficult to observe, with only a few M dwarfs presenting observationally-derived mass-loss rates (Mdot), which span several orders of magnitude. Close-in exoplanets are conveniently positioned in the inner regions of stellar winds and can, thus, be used to probe the otherwise-unobservable local properties of their host-stars' winds. Here, we use local stellar wind characteristics observationally-derived in the studies of atmospheric evaporation of the warm-neptune GJ436 b to derive the global characteristics of the wind of its M-dwarf host. Using an isothermal wind model, we constrain the stellar wind temperature to be in the range [0.36,0.43] MK, with Mdot=[0.5,2.5] x 10^{-15} Msyn/yr. By computing the pressure balance between the stellar wind and the interstellar medium, we derive the size of the astrophere of GJ436 to be around 25 au, significantly more compact than the heliosphere. We demonstrate in this paper that transmission spectroscopy, coupled to planetary atmospheric evaporation and stellar wind models, can be a useful tool for constraining the large-scale wind structure of planet-hosting stars. Extending our approach to future planetary systems discoveries will open new perspectives for the combined characterisation of planetary exospheres and winds of cool dwarf stars.