  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. The accretion of roughly km-sized planetesimals and the accretion of
mm-cm sized pebbles are typically discussed separately as the main solid
accretion mechanisms. 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. We combine a simple model of
pebble formation and accretion with a global model of planet formation which
considers the accretion of planetesimals. We compare synthetic populations of
planets formed in disks composed of different amounts of pebbles and 600 meter
sized planetesimals. On a system-level, we study the formation pathway of giant
planets in these disks. 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 km
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. 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.
