Constraining an exoplanet’s magnetic field using star-planet interactions
D. Revilla, P. J. Amado, R. Luque, P. Schöfer, A. F. Lanza, A. Binnenfeld, J. A. Caballero, A. P. Hatzes, G. W. Henry, S. V. Jeffers, S. Kaur, E. Pallé, L. Peña-Moñino, M. Pérez-Torres, A. Quirrenbach, A. Reiners, I. Ribas, D. Viganò, M. R. Zapatero-Osorio, S. ZuckerTheory predicts that a planet with a sufficiently strong magnetic field orbiting close to its host star could induce star-planet magnetic interactions. This is potentially observable as an optical or radio stellar activity signal synchronised with the planet’s orbital period. We analyze 18 years of high-resolution optical spectroscopy of GJ 436, a low mass star orbited by a Neptune-sized exoplanet on a polar eccentric orbit. Stellar activity indicators show enhancements at a period corresponding to the exoplanet orbit, modulated by stellar rotation, and the star’s 8-year magnetic cycle. We interpret this as a signal of star-planet magnetic interaction. Using a geometric model, we reproduce these periods if GJ 436 b has a magnetic field strength of 6 to 110 Gauss.