  In models of planetary accretion, pebbles form by dust coagulation and
rapidly migrate toward the central star. Planetesimals may continuously form
from pebbles over the age of the protoplanetary disk by yet uncertain
mechanisms. Meanwhile, large planetary embryos grow efficiently by accumulation
of leftover pebbles that are not incorporated in planetesimals. Although this
process, called pebble accretion, is offering a new promising pathway for
formation of giant planets' cores, architectures of planetary systems formed
through the process remain elusive. In the present paper, we perform
simulations of formation of planetary systems using a particle-based hybrid
code, to which we implement most of the key physical effects as precisely as
possible. We vary the size of a protoplanetary disk, the turbulent viscosity,
the pebble size, the planetesimal formation efficiency, and the initial mass
distribution of planetesimals. Our simulations show that planetesimals first
grow by mutual collisions if their initial size is the order of $100$ km or
less. Once planetesimals reach $\sim$ 1000 km in size, they efficiently grow by
pebble accretion. If pebble supply from the outer region continues for a long
period of time in a large protoplanetary disk, planetary embryos become massive
enough to commence runaway gas accretion, resulting in gas giant planets. Our
simulations suggest that planetary systems like ours form from protoplanetary
disks with moderately high turbulent viscosities. If the disk turbulent
viscosity is low enough, a planet opens up a gap in the gas disk and halts
accretion of pebbles even before the onset of runway gas accretion. Such a disk
produces a planetary system with several Neptune-size planets.

Earth and Planetary Astrophysics