The role of magnetic coupling in exoplanet atmospheres: insights from star-planet magnetic interactions

Since the discovery in 1995 of the first exoplanet orbiting a solar-type star, more than 5,500 exoplanets have been identified, revealing a remarkable diversity of exoplanetary systems. While many of their physical parameters are now well understood, the characterisation of exoplanetary magnetic fields remains largely unexplored, despite its critical role in atmospheric retention. To better understand exoplanetary magnetic fields, star-planet magnetic interactions present a promising avenue of investigation. Such interactions have been observed in HD 189733 which features a hot Jupiter discovered in 2005 with an atmosphere composed of H2O, CO, CH4, CO2, and Na. This system is therefore ideal for studying magnetic coupling in exoplanetary atmospheres, by better understanding star-planet magnetic interactions.

The poster presents a stellar wind model that simulates the theoretical power generated by the magnetic interactions between a star and its planet in the HD 189733 system. Two versions of the model are discussed: a polytropic version, and a more sophisticated version in which the stellar wind is heated and accelerated by Alfvén waves. These same waves can also be excited by the presence of a planet when it is sufficiently close to its star (within the so-called Alfvén surface), propagating towards the star and forming what are known as “Alfvén wings”. Star-planet magnetic interactions can only occur when the planet's orbit falls within the theoretical surface delimited by these wings.

The results of the HD 189733 system models have revealed that the planetary orbit of HD 189733b primarily resides within the Alfvén surface, making this system likely to host star-planet magnetic interactions. This suggests that HD 189733b could host a magnetic field capable of driving these interactions. The two models are driven using a Zeeman Doppler-Imaging magnetic map from 2023 spectro-polarimetric observations, showing that incorporating Alfvén wave heating is crucial for accurately reproducing star-planet magnetic interactions. Modelling such physical processes is thus a promising approach for characterising exoplanetary magnetic fields, and could significantly help us understanding magnetic coupling in exoplanetary atmospheres.