  We present a model in which planetesimal disks are built from the combination
of planetesimal formation and accretion of radially drifting pebbles onto
existing planetesimals. In this model, the rate of accretion of pebbles onto
planetesimals quickly outpaces the rate of direct planetesimal formation in the
inner disk. This allows for the formation of a high mass inner disk without the
need for enhanced planetesimal formation or a massive protoplanetary disk. Our
proposed mechanism for planetesimal disk growth does not require any special
conditions to operate. Consequently, we expect that high mass planetesimal
disks form naturally in nearly all systems. The extent of this growth is
controlled by the total mass in pebbles that drifts through the inner disk.
Anything that reduces the rate or duration of pebble delivery will
correspondingly reduce the final mass of the planetesimal disk. Therefore, we
expect that low mass stars (with less massive protoplanetary disks), low
metallicity stars and stars with giant planets should all grow less massive
planetesimal disks. The evolution of planetesimal disks into planetary systems
remains a mystery. However, we argue that late stage planet formation models
should begin with a massive disk. This reinforces the idea that massive and
compact planetary systems could form in situ but does not exclude the
possibility that significant migration occurs post-planet formation.
