Evidence for dust grain growth in young circumstellar disks

Hundreds of circumstellar disks in the Orion nebula are being rapidly destr oyed by the intense ultraviolet radiation produced by nearby bright stars. These young, million-year-old disks may not survive long enough to form pla netary systems. Nevertheless, the first stage of planet formation-the growt h of dust grains into larger particles-may have begun in these systems. Obs ervational evidence for these large particles in Orion's disks is presented . A model of grain evolution in externally irradiated protoplanetary disks is developed and predicts rapid particle size evolution and sharp outer dis k boundaries. We discuss implications for the formation rates of planetary systems.