Stellar-planet magnetospheric interaction in the young AU Microscopii exoplanetary system

When planets orbit close to their stars, their magnetic fields can interact releasing energy with a periodicity linked to both the planet’s orbital motion and the star’s rotation. In the Solar System, the clearest example of such interactions is the Io-Jupiter system, where the moon’s motion through Jupiter’s intense magnetic field powers strong and periodic auroral emission. For exoplanets, similar interactions are expected to occur when a planet possesses a substantial magnetic field and orbits within the sub-Alfvénic region of the stellar magnetosphere, possibly leading to periodic enhancements in stellar emission. However, despite their diagnostic power, only a few tentative detections of star-planet magnetic interactions have been reported so far. Here we show that the X-ray luminosity of the young star AU Microscopii, monitored over two years with more than eight hundred observations collected by the Einstein Probe satellite, varies by 50% of the quiescent luminosity value and shows a 4.26 sigma significance periodicity of 5.64 days. This period corresponds to half of the synodic period of the innermost planet AU Mic-b, namely the interval after which the planet returns to the longitude of one of the two magnetic poles on the stellar surface. Through the measured period and amplitude of the luminosity variation we estimate that the magnetic field of the innermost planet AU Mic b is at least B≈40-350 G, depending on whether the interconnecting magnetic loops are active intermittently or continuously throughout the synodic cycle. This result represents the first measurement of an exoplanetary magnetic field obtained through X-ray monitoring of star-planet magnetic interactions (SPMIs) and also highlights the critical role of high cadence X-ray observations in probing the magnetospheric environments of young planetary systems. The measurement of planet’s magnetic field is fundamental for assessing the long-term stability of planetary atmospheres against the stellar X--ray and Ultra Violet emission of their hosts, particularly intense in young M-dwarfs.