The Role of Magnetospheric Rebound in Breaking Resonant Chains of Super-Earths and Mini-Neptunes

Abstract

Stellar magnetic fields are thought to truncate the inner regions of protoplanetary disks around T Tauri stars, creating a magnetospheric cavity near the star. As the disk evolves and disperses, the truncation radius is expected to move outward as the balance between magnetic and viscous forces shifts. Planets migrating inward can become trapped near the inner edge, but as the edge itself moves outward, the evolving disk torques can drive planets to migrate outward as well. We employ N-body simulations to assess the influence of magnetospheric cavity expansion on the dynamical evolution and orbital architectures of compact resonant chains of super-Earths and mini-Neptunes. Our results show that rebound-driven expansion of the disk's inner edge plays a pivotal role in destabilizing resonant chains by spreading planetary systems outward, thereby triggering early dynamical instabilities and giant impacts. Despite this dynamical evolution, key observable properties of close-in planetary systems -- such as the distribution of orbital period ratio, the intra-system similarity in planet sizes (``radius uniformity''), and the bimodal distribution of planet radii known as the ``radius valley'' -- remain largely consistent with those of systems formed without the rebound effect, in which the inner edge of the disk remains fixed. Thus, the primary consequence of the rebound appears to be the early disruption of resonant chains, rather than any significant alteration to the statistical properties of the resulting super-Earth and mini-Neptune populations.