The interplay between pebble and planetesimal accretion in population synthesis models and its role in giant planet formation

Context. In the core accretion scenario of planet formation, rocky cores grow by first accreting solids until they are massive enough to accrete gas. For giant planet formation, this means that a massive core must form within the lifetime of the gas disk. Inspired by observations of Solar System features such as the asteroid and Kuiper belts, the accretion of roughly kilometre-sized planetesimals is traditionally considered as the main accretion mechanism of solids but such models often result in longer planet formation timescales. The accretion of millimetre- to centimetre-sized pebbles, on the other hand, allows for rapid core growth within the disk lifetime. The two accretion mechanisms are typically discussed separately.Aims. We investigate the interplay between the two accretion processes in a disk containing both pebbles and planetesimals for planet formation in general and in the context of giant planet formation specifically. The goal is to disentangle and understand the fundamental interactions that arise in such hybrid pebble-planetesimal models laying the groundwork for informed analysis of future, more complex, simulations.Methods. We combined a simple model of pebble formation and accretion with a global model of planet formation which considers the accretion of planetesimals. We compared synthetic populations of planets formed in disks composed of different amounts of pebbles and 600 metre-sized planetesimals to identify the impact of the combined accretion scenario. On a system level, we studied the formation pathway of giant planets in these disks.Results. We find that, in hybrid disks containing both pebbles and planetesimals, the formation of giant planets is strongly suppressed, whereas, in a pebbles-only or planetesimals-only scenario, giant planets can form. We identify the heating associated with the accretion of up to 100 kilometre-sized planetesimals after the pebble accretion period to delay the runaway gas accretion of massive cores. Coupled with strong inward type-I migration acting on these planets, this results in close-in icy sub-Neptunes originating from the outer disk.Conclusions. We conclude that, in hybrid pebble-planetesimal scenarios, the late accretion of planetesimals is a critical factor in the giant planet formation process and that inward migration is more efficient for planets in increasingly pebble-dominated disks. We expect a reduced occurrence rate of giant planets in planet formation models that take the accretion of pebbles and planetesimals into account.