It is thought that planets form from solid particles in a flattened, r
otating, 99% gaseous nebula. These grains gradually coagulate into mil
limeter-to-meter sized aggregates which settle toward the midplane of
the nebula. It is widely believed that the resulting dense layer event
ually becomes gravitationally unstable and collapses into ''planetesim
als.'' A new numerical model is presented to simulate the predominant
processes (gravitation, vertical convection, and shear-driven turbulen
ce) during the stage while the particulate material is still dispersed
about the midplane of the nebula. In our previous work, particles wer
e assumed to be spheres of a single radius; in the present work, parti
cles are spheres of different radii. Results indicate that neither a b
road nor a narrow distribution of particle sizes is likely to become g
ravitationally unstable. (C) 1995 American Institute of Physics.