paper

Magnetic field strengths of hot giant exoplanets consistent with Solar System values

arXiv:2606.22455 · doi:10.1038/s41550-026-02870-1

Abstract

Magnetic fields are ubiquitous in the universe. They play a key role in shaping the activity of stars, the habitability of rocky planets, and the long-term retention of planetary atmospheres. Theoretical scaling laws are largely constrained by the limited set of stars and Solar System planets, leading to a wide range of possible values for hot giant planets outside of the Solar System from fractions of the Jovian field to orders of magnitude larger. Ultra-hot Jupiters, with their highly ionised atmospheres, provide a new avenue to probe magnetic effects, as their atmospheric circulation could be directly sensitive to atmospheric magnetic field strength. Using high-spectral resolution observations targeting the iron lines of ultra-hot Jupiters we measure the Doppler shift and thus the wind speed of seven transiting ultra-hot Jupiters. We find a clear decrease of wind speed with increasing planetary temperature, a trend inconsistent with purely hydrodynamic mechanisms but naturally reproduced by magnetic drag. From this relation we estimate the possible strength of magnetic fields of hot giant planets to at most a few gauss - comparable to the Jovian equatorial field. Our results support the idea that magnetic fields affect the atmospheric circulation of ultra-hot Jupiters and could provide a crucial benchmark for scaling laws used to predict magnetic fields in exoplanets, from hot Jupiters to rocky Earths with additional implications for future direct observations.

Preprint version of the article accepted under the same title in Nature Astronomy. Please find the accepted manuscript at https://www.nature.com/articles/s41550-026-02870-1 , with the DOI 10.1038/s41550-026-02870-1. Submitted 30th Sept 2025, accepted 16th April, 2026. 17 pages, 9 figures

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