  The dynamics of solid bodies in protoplanetary disks are subject to the
properties of any underlying gas turbulence. Turbulence driven by disk
self-gravity shows features distinct from those driven by the magnetorotational
instability (MRI). We study the dynamics of solids in gravito-turbulent disks
with two-dimensional (in the disk plane), hybrid (particle and gas)
simulations. Gravito-turbulent disks can exhibit stronger gravitational
stirring than MRI-active disks, resulting in greater radial diffusion and
larger eccentricities and relative speeds for large particles (those with
dimensionless stopping times $t_{stop} \Omega > 1$, where $\Omega$ is the
orbital frequency). The agglomeration of large particles into planetesimals by
pairwise collisions is therefore disfavored in gravito-turbulent disks.
However, the relative speeds of intermediate-size particles $t_{stop} \Omega
\sim 1$ are significantly reduced as such particles are collected by gas drag
and gas gravity into coherent filament-like structures with densities high
enough to trigger gravitational collapse. First-generation planetesimals may
form via gravitational instability of dust in marginally gravitationally
unstable gas disks.
