  Protoplanetary disks are routinely described as finite mass reservoirs left
over by the gravitational collapse of the protostar, an assumption that
strongly constrains both disk evolution and planet formation models. We propose
a different scenario where protoplanetary disks of pre-main sequence stars are
assembled primarily by Bondi-Hoyle accretion from the parent gas cloud. We
demonstrate that Bondi-Hoyle accretion can supply not only the mass, but also
the angular momentum necessary to explain the observed size of protoplanetary
disks, and we predict the dependence of the disk specific angular momentum on
the stellar mass. Our results are based on an analytical derivation of the
scaling of the angular momentum in a turbulent flow, which we also confirm with
a numerical simulation of supersonic turbulence. This new scenario for disk
formation and evolution may alleviate a number of observational problems as
well as compel major revisions of disk and planet formation models.
