Close-in planet induces flares on its host star.

In the past decade, hundreds of exoplanets have been discovered in extremely short orbits below 10 days. Unlike in the Solar System, planets in these systems orbit their host stars close enough to disturb the stellar magnetic field lines. The interaction can enhance the magnetic activity of the star, such as its chromospheric and radio emission or flaring. So far, the search for magnetic star-planet interactions has remained inconclusive. Here we report the detection of planet-induced flares on HIP 67522, a 17 million-year-old G dwarf star with two known close-in planets. Combining space-borne photometry from the Transiting Exoplanet Survey Satellite and dedicated Characterising Exoplanets Telescope observations over 5 years, we find that the 15 flares in HIP 67522 cluster near the transit phase of the innermost planet, indicating persistent magnetic star-planet interaction in the system. The stability of interaction implies that the innermost planet is continuously self-inflicting a six times higher flare rate than it would experience without interaction. The subsequent flux of energetic radiation and particles bombarding HIP 67522 b may explain the remarkably extended atmosphere of the planet, recently detected with the James Webb Space Telescope. HIP 67522 is, therefore, an archetype to understand the impact of magnetic star-planet interaction on the atmospheres of nascent exoplanets.