On the formation of compact planetary systems via concurrent core accretion and migration

Abstract

We present the results of planet formation N-body simulations based on a comprehensivephysical model that includes planetary mass growth through mutual embryo collisions andplanetesimal/boulder accretion, viscous disc evolution, planetary migration and gas accretionon to planetary cores. The main aim of this study is to determine which set of model parametersleads to the formation of planetary systems that are similar to the compact low-mass multiplanetsystems that have been discovered by radial velocity surveys and the Kepler mission.We varythe initial disc mass, solids-to-gas ratio and the sizes of the boulders/planetesimals, and for arestricted volume of the parameter space we find that compact systems containing terrestrialplanets, super-Earths and Neptune-like bodies arise as natural outcomes of the simulations.Disc models with low values of the solids-to-gas ratio can only form short-period super-Earthsand Neptunes when small planetesimals/boulders provide the main source of accretion, sincethe mobility of these bodies is required to overcome the local isolation masses for growingembryos. The existence of short-period super-Earths around low-metallicity stars providesstrong evidence that small, mobile bodies (planetesimals, boulders or pebbles) played a centralrole in the formation of the observed planets.