  It is believed that planets are formed by aggregation of dust particles
suspended in the turbulent gas forming accretion disks around developing stars.
We describe a mechanism, termed 'Stokes trapping', by which turbulence limits
the growth of aggregates of dust particles, so that their Stokes number
(defined as the ratio of the damping time of the particles to the Kolmogorov
dissipation timescale) remains close to unity. We discuss possible mechanisms
for avoiding this barrier to further growth. None of these is found to be
satisfactory and we introduce a new theory which does not involve the growth of
small clusters of dust grains.
