Pebbles versus planetesimals

Abstract

Context. In the core accretion scenario of giant planet formation, a massive core forms first and then accretes a gaseous envelope. In the discussion of how this core forms, some divergences appear. The first scenarios of planet formation predict the accretion of kilometre-sized bodies called planetesimals, while more recent works suggest growth by the accretion of pebbles, which are centimetre-sized objects. Aims. These two accretion models are often discussed separately and our aim here is to compare the outcomes of the two models with identical initial conditions. Methods. The comparison is done using two distinct codes, one that computes the planetesimal accretion and the other the pebble accretion. All the other components of the simulated planet growth are computed identically in the two models: the disc, the accretion of gas, and the migration. Using a population synthesis approach, we compare planet simulations and study the impact of the two solid accretion models, focusing on the formation of single planets. Results. We find that the outcomes of the populations are strongly influenced by the accretion model. The planetesimal model predicts the formation of more giant planets, while the pebble accretion model forms more super-Earth-mass planets. This is due to the pebble isolation mass (Miso) concept, which prevents planets formed by pebble accretion to accrete gas efficiently before reaching Miso. This translates into a population of planets that are not heavy enough to accrete a consequent envelope, but that are in a mass range where type I migration is very efficient. We also find higher gas mass fractions for a given core mass for the pebble model compared to the planetesimal model, caused by luminosity differences. This also implies planets with lower densities, which could be confirmed observationally. Conclusions. We conclude that the two models produce different outputs. Focusing on giant planets, the sensitivity of their formation differs: for the pebble accretion model, the time at which the embryos are formed and the period over which solids are accreted strongly impact the results, while the population of giant planets formed by planetesimal accretion depends on the planetesimal size and on the splitting in the amount of solids available to form planetesimals.