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.

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Fig. 1: Ca ii K residual spectra, and summed residual power as a function of time, stellar rotational phase and planetary orbital phase.
Fig. 2: Observed powers in the Ca ii K line residuals as a function of relevant magnetic SPI parameters.
Fig. 3: Magnetic field strengths.

Data availability

This work made use of archived data from the PolarBase archive ( and the Canada–France–Hawaii Telescope data archive ( The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request. The reduced spectra used here are also publicly available via the PolarBase archive and the Canada–France–Hawaii Telescope data archive.


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We thank T. Barman for discussions concerning details of the PHOENIX models. P.W.C. and E.L.S. acknowledge support from NASA Origins of the Solar System grant no. NNX13AH79G (PI: E.L.S.). This work has made use of NASA’s Astrophysics Data System and used the facilities of the Canadian Astronomy Data Centre operated by the National Research Council of Canada with the support of the Canadian Space Agency.

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E.L.S. was responsible for most of the original observing proposals and data collection. P.W.C. was responsible for the flux-calibration and SPI signal analysis, as well as the manuscript preparation. J.L. was responsible for some original SPI signal analysis and also contributed to the manuscript. A.F.L. provided interpretation of the SPI theories and oversight of the theory application. All authors contributed material to the manuscript.

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Correspondence to P. Wilson Cauley.

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Supplementary Table 1; Supplementary Figs. 1–3.

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Cauley, P.W., Shkolnik, E.L., Llama, J. et al. Magnetic field strengths of hot Jupiters from signals of star–planet interactions. Nat Astron 3, 1128–1134 (2019).

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