Bimodality in Rotational Modulation of Planet-hosting Stars

Stellar magnetic activity is governed by the interplay between rotation, convection, and the temporal evolution of surface magnetic structures, yet the role of planetary systems in shaping these processes remains unclear. In particular, it remains unclear whether the presence of planets can influence the global organization of stellar magnetic activity beyond localized or transient effects. Here, we analyze Kepler photometry of more than 1300 stars to investigate rotational modulation in stars with and without confirmed exoplanets. Using a time–frequency analysis, we measure a photometric proxy of rotational modulation dispersion, S phot , which traces the temporal coherence of surface magnetic features. Stars with confirmed exoplanets exhibit systematically enhanced values of S phot compared to stars without detected planets (Δ S phot = 0.17 ± 0.01 rad day −1 ; p < 10 −25 ). More importantly, the S phot distribution of planet hosts is bimodal, with peaks at 0.12 and 0.44 rad day −1 (Hartigan’s dip test p < 10 −6 ; ΔBIC = 188.7), a feature absent in the control sample. We interpret S phot as a proxy for rotational modulation dispersion, reflecting spot evolution rather than true differential rotation. The low- S phot regime corresponds to stable magnetic coherence, or long-lived active regions (∼6.8 rotations), while the high- S phot regime indicates rapidly evolving active regions (∼0.9 rotations). The existence of two distinct regimes exclusively among planet-hosting stars suggests that planetary systems are associated with differences in the temporal organization of stellar magnetic activity. These results suggest that planets may influence stellar dynamos indirectly, by modifying the stability and evolution of surface magnetic structures rather than solely altering differential rotation.