Evaluating Star–Planet Interactions with Zeeman Doppler Imaging: Case Study in YZ Ceti

Abstract The recent detections of radio emission from the nearby exoplanet host, YZ Ceti, suggest that the star is possibly interacting with its rocky innermost planet. These radio emissions are characterized by strong circular polarization, and appear to repeat within consistent orbital phase windows dictated by the orbital position of YZ Ceti b. If confirmed, this interaction would provide a first means to concretely assess the magnetic field of a close-in rocky exoplanet. This kind of magnetic star–planet interaction (SPI) should depend on both the exoplanetary orbit and the geometry of the stellar magnetic field. In this article, we report measurements of the large-scale magnetic field topology of the star YZ Ceti for the first time, and interpret the cumulative radio data sets in that context to evaluate the plausibility of magnetic SPIs. We find evidence both against and in support of the SPI hypothesis, but crucially that the measured magnetic field does not rule out SPI scenarios. However, clear evaluation of these possibilities requires more accurate assessments of the magnetic field evolution across time. We additionally suggest that YZ Ceti may be exhibiting planet-induced flaring, potentially triggered by exoplanet crossings of the Alfvén surface as the planet’s orbit approaches the stellar magnetic equator, and YZ Ceti b experiences dramatic shifts in the ambient field, its polarity, and connectivity to the host star.