  Proposed mechanisms for the formation of km-sized solid planetesimals face
long-standing difficulties. Robust sticking mechanisms that would produce
planetesimals by coagulation alone remain elusive. The gravitational collapse
of smaller solids into planetesimals is opposed by stirring from turbulent gas.
This proceeding describes recent works showing that "particle feedback," the
back-reaction of drag forces on the gas in protoplanetary disks, promotes
particle clumping as seeds for gravitational collapse. The idealized streaming
instability demonstrates the basic ability of feedback to generate particle
overdensities. More detailed numerical simulations show that the particle
overdensities produced in turbulent flows trigger gravitational collapse to
planetesimals. We discuss surprising aspects of this work, including the large
(super-Ceres) mass of the collapsing bound cluster, and the finding that MHD
turbulence aids gravitational collapse.
