  Recent research on the buildup of rocks from small dust grains has reaffirmed
that grain growth in protoplanetary disks should occur quickly. Calculation of
growth rates have been made for a variety of growth processes and generally
predict high probabilities of sticking in low-velocity collisions that may be
brought about in a number of ways in protoplanetary disks. Laboratory
experiments have measured sticking coefficients for some materials largely
confirming the calculations. Although the detailed velocity fields of disks are
not well understood, many of the important processes leading to particle
collisions and grain growth have been studied theoretically and demonstrate
likely paths by which dust is assembled into planets. Calculations of the
radiative properties of particles with various size distributions show that
large particles should produce observable changes in the spectral energy
distributions of disks. Changes of the sort predicted are, in fact, observed,
but their interpretation is ambiguous; there are other ways to produce the
observed changes that do not require grain growth, so the evidence is currently
inconclusive. The major uncertainties can be overcome with the next generation
of millimeter-wave interferometers, and it seems likely that a firm case for
grain growth could be established within a decade.
