Disk Dispersal and Planet Formation Time Scales
  Well before the existence of exo-solar systems was confirmed, it was accepted knowledge that most -- if not all -- stars possess circumstellar material during the first one-to-several million years of their pre-main sequence lives, and thus that they commonly have the potential to form planets. Here I summarize current understanding regarding the evolution of proto-planetary dust and gas disks, emphasizing the diversity in evolutionary paths. 
Lynne A. Hillenbrand