Chasing Star-Planet Magnetic Interactions: The Case of Kepler-78

Observational evidence of star-planet magnetic interactions (SPMIs) in compact exosystems have been looked for in the past decades. Indeed, planets in close-in orbit can be magnetically connected to their host star and can channel Alfven waves carrying large amounts of energy toward the central star. The strength and temporal modulation of SPMIs are primarily set by the magnetic topology of the host star and the orbital characteristics of the planet. As a result, SPMI signals can be modulated over the rotational period of the star, the orbital period of the planet, or a complex combination of the two. The detection of SPMIs thus has to rely on multiple-epoch and multiple-wavelength observational campaigns. We present a new method to characterize SPMIs and apply it to Kepler-78, a late G star with a super-Earth on an 8.5 hr orbit. We model the corona of Kepler-78 using the large-scale magnetic topology of the star observed with Zeeman-Doppler imaging. We show that the closeness of Kepler-78b allows the interaction with channel energy flux densities up to a few kW m(-2) toward the central star. We show that this flux is large enough to be detectable in classical activity tracers such as Ha. It is nonetheless too weak to explain the modulation observed by Moutou et al. We furthermore demonstrate how to predict the temporal modulation of SPMI signals in observed systems such as Kepler-78. The methodology presented here thus paves the way toward denser, more specific observational campaigns that would allow proper identification of SPMIs in compact star-planet systems.