Magnetic field strengths of hot Jupiters from signals of star-planet interactions

Evidence of star-planet interactions in the form of planet-modulated chromospheric emission has been noted for a number of hot Jupiters. Magnetic star-planet interactions involve the release of energy stored in the stellar and planetary magnetic fields. These signals thus offer indirect detections of exoplanetary magnetic fields. Here, we report the derivation of the magnetic field strengths of four hot Jupiter systems, using the power observed in calcium II K emission modulated by magnetic star-planet interactions. By approximating the fractional energy released in the calcium II K line, we find that the surface magnetic field values for the hot Jupiters in our sample range from 20 G to 120 G, around 10-100 times larger than the values predicted by dynamo scaling laws for planets with rotation periods of around 2-4 days. However, these values are in agreement with scaling laws relating the magnetic field strength to the internal heat flux in giant planets. Large planetary magnetic field strengths may produce observable electron cyclotron maser radio emission by preventing the maser from being quenched by the planet's ionosphere. Intensive radio monitoring of hot Jupiter systems will help to confirm these field values and inform the generation mechanism of magnetic fields in this important class of exoplanets.