  For a long time, gravitational instability in the disk of planetesimals has
been suspected to be the main engine responsible for the beginning of dust
growth, its advantage being that it provides for rapid growth. Its real
importance in planetary formation is still debated, mainly because the
potential presence of turbulence can prevent the settling of particles into a
gravitationally unstable layer. However, several mechanisms could yield
strongly inhomogeneous distributions of solids in the disk: radial drift,
trapping in vortices, perturbations by other massive bodies, etc. In this paper
we present a numerical study of a gravitationally unstable layer. This allows
us to go beyond the classical analytical study of linear perturbations,
exploring a highly non-linear regime. A hierarchical growth of structure in the
presence of dissipation (gas drag) can yield large, virialized clusters of
planetesimals, the first time such clusters have been observed in the context
of planetesimal disks.

Astrophysics