  We study the evolution of cold, gravitationally unstable protoplanetary
gaseous disks performing 3D SPH simulations with up to a million particles on
large parallel machines. We show that self-gravitating protoplanets can form in
disks with initial $Q_{min} \sim 1.4$ provided that disks are evolved with a
locally isothermal equation of state until localized regions reach a
sufficiently high density. Lower resolution simulations carried out for as long
as a thousand years show that systems of giant protoplanets with masses and
orbits comparable to those of the observed extrasolar planets arise naturally.
