Close-in planet induces flares on its host star

Abstract

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 star's magnetic activity, 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 first 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 TESS and dedicated CHEOPS observations over a span of 5 years, we find that the 15 flares in HIP 67522 cluster near the innermost planet's transit phase, indicating persistent magnetic star-planet interaction in the system. The stability of interaction implies that the innermost planet is continuously self-inflicting a six time higher flare rate than it would experience without interaction. The subsequent flux of energetic radiation and particles bombarding HIP 67522 b may explain the planet's remarkably extended atmosphere, 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.

Figures (11)

• Figure 1: Planet-induced flaring in the HIP 67522 system. HIP 67522 b is shown as it perturbs the star's inclined magnetic field. The perturbation travels along the magnetic field line highlighted in white, toward the stellar surface, where it triggers flares at the footpoint, which is periodically visible to the observer.
• Figure 2: Observed flare rates on HIP 67522 relative to the orbit of the innermost planet HIP 67522 b. HIP 67522 showed a significant increase in flare rate in the expected range, indicating that the majority of detected flares were triggered by the planet. The orange dashed region illustrates the expected rate of planet-induced flares from a footpoint of interaction at the sub-planetary longitude. Planet-induced flares can only be seen when the planet faces the observer ($\pm 90^\circ$), and most flares can be expected near the transit of HIP 67522 b ($0^\circ$).
• Figure 3: Best-fit models with (dark blue) and without (orange) planet-induced flaring. The blue shade indicates at which orbital phases an elevated flare rate can be expected based on the observed flares (dotted lines) and the respective phase coverage (black line). The darker the shade, the more likely flaring is elevated at that orbital phase. The relative expected rate of planet-induced flares at the sub-planetary point (orange arc) at different orbital phases is consistent with the measured phase range of elevated flaring.
• Figure 4: Realizations of the geometric model under spin-orbit commensurability that peaks in the observed phase range. Some planet-induced flaring is expected outside the peak range, encompassing all but one flare in our sample.
• Figure 5: TESS light curves of HIP 67522 . Blue scatter shows the 2-min TESS PDC_SAP flux. Light blue shades show the batmankreidberg2015batman transit models of HIP 67522 b. Orange vertical lines mark the detected flares.