On the Orbital Evolution of Low Mass Protoplanets in Turbulent, Magnetised Disks

Abstract

(Abridged).We present the results of MHD simulations of low mass protoplanets interacting with turbulent disks. We calculate the orbital evolution of `planetesimals' and protoplanets with masses in the range 0 < m_p < 30 M_Earth. Planetesimals and protoplanets undergo stochastic migration due to interaction with turbulent density fluctuations. Over run times of ~ 150 planet orbits, stochastic migration dominates over type I migration for many models. Fourier analysis of the torques experienced by planets indicates that the torque fluctuations contain components with significant power whose time scales of variation are similar to the simulation run times. These low frequency fluctuations partly explain the dominance of stochastic torques, and may provide a powerful means of counteracting the type I migration of some planets in turbulent disks. Turbulence is a source of eccentricity driving. Planetesimals attained eccentricities in the range 0.02 < e < 0.14, m_p=1 M_Earth planets attained eccentricities 0.02 < e < 0.08, and m_p=10 M_Earth protoplanets reached 0.02 < e < 0.03. This is in basic agreement with a model in which turbulence drives e-growth, and interaction with disk material at coorbital Lindblad resonances causes e-damping. These results are significant for planet formation. Stochastic migration may prevent some planet cores migrating into their star via type I before becoming gas giants. The growth of planetary cores may be enhanced by preventing isolation. Eccentricity excitation by turbulence, however, may reduce growth rates of planetary cores during the runaway and oligarchic growth stages, and cause collisions between planetesimals to become destructive.