Young Planets around Young Accreting Stars. I. Migration and Inner Stalling Orbits

Abstract Planet migration in protoplanetary disks significantly influences exoplanet architectures. We investigate various migration mechanisms for young planets close to young stars. To quantify the stochastic migration driven by turbulent disks, we incorporate planets into existing 3D MHD-disk simulations of magnetospheric accretion. We further model these turbulent torques analytically using a modified Gaussian process. Migration timescales for other processes, including the smooth Type I/II migration, planet–star tidal and magnetic interactions, and aerodynamical drag with the magnetosphere are also estimated analytically. Given that our simulation reveals a very low surface density for this very inner MHD-turbulent region, the resulting disk migration here is significantly slower than previously estimated. Earth-mass planets have migration timescales t mig ≳ 10 8 yr in this region. Thus, they are unlikely to migrate past the dead zone inner boundary ( R DZIB ) into the MHD-region before the disk dissipates. Only giant planets could migrate inward within the turbulent disk, and may stall at the magnetospheric truncation radius ( R T ). A simplified planet population synthesis demonstrates that, at the end of the disk phase, all planets around solar-mass stars typically stall at ≲0.1 au, since R T ∼ R DZIB . However, around 2 M ⊙ stars, higher-mass planets stall significantly closer to the star compared to low-mass planets, due to R T ≪ R DZIB . These results are consistent with recent exoplanet demographics around different types of stars. Finally, the turbulence in our simulations may not be sufficient to break resonant planet pairs according to previous analytical criteria, which, on the other hand, needs to be confirmed by future 3D simulations.