Can the giant planets of the Solar System form via pebble accretion in a smooth protoplanetary disc?

Abstract

<p><em>Context</em>. Prevailing <em>N</em>-body planet formation models typically start with lunar-mass embryos and show a general trend of rapid migration of massive planetary cores to the inner Solar System in the absence of a migration trap. This setup cannot capture the evolution from a planetesimal to embryo, which is crucial to the final architecture of the system.</p><p><em>Aims</em>. We aim to model planet formation with planet migration starting with planetesimals of ~10⁻⁶−10⁻⁴ <em>M</em>_⊕ and reproduce the giant planets of the Solar System.</p><p><em>Methods</em>. We simulated a population of 1000-5000 planetesimals in a smooth protoplanetary disc, which was evolved under the effects of their mutual gravity, pebble accretion, gas accretion, and planet migration, employing the parallelized <em>N</em>-body code SyMBAp.</p><p><em>Results</em>. We find that the dynamical interactions among growing planetesimals are vigorous and can halt pebble accretion for excited bodies. While a set of results without planet migration produces one to two gas giants and one to two ice giants beyond 6 au, massive planetary cores readily move to the inner Solar System once planet migration is in effect.</p><p><em>Conclusions</em>. Dynamical heating is important in a planetesimal disc and the reduced pebble encounter time should be considered in similar models. Planet migration remains a challenge to form cold giant planets in a smooth protoplanetary disc, which suggests an alternative mechanism is required to stop them at wide orbits.</p>