  Recently it is proposed that porous icy dust aggregates are formed by
pairwise accretion of dust aggregates beyond the snowline. We calculate the
equilibrium random velocity of porous dust aggregates taking into account
mutual gravitational scattering, collisions, gas drag, and turbulent stirring
and scattering. We find that the disk of porous dust aggregates becomes
gravitationally unstable as they evolve through gravitational compression in
the minimum-mass solar nebula model for a reasonable range of turbulence
strength, which leads to rapid formation of planetesimals.
