  We present three-dimensional numerical simulations of particle clumping and
planetesimal formation in protoplanetary disks with varying amounts of solid
material. As centimeter-size pebbles settle to the mid-plane, turbulence
develops through vertical shearing and streaming instabilities. We find that
when the pebble-to-gas column density ratio is 0.01, corresponding roughly to
solar metallicity, clumping is weak, so the pebble density rarely exceeds the
gas density. Doubling the column density ratio leads to a dramatic increase in
clumping, with characteristic particle densities more than ten times the gas
density and maximum densities reaching several thousand times the gas density.
This is consistent with unstratified simulations of the streaming instability
that show strong clumping in particle dominated flows. The clumps readily
contract gravitationally into interacting planetesimals of order 100 km in
radius. Our results suggest that the correlation between host star metallicity
and exoplanets may reflect the early stages of planet formation. We further
speculate that initially low metallicity disks can be particle enriched during
the gas dispersal phase, leading to a late burst of planetesimal formation.
