  The kinetic theory is used to study the evolution of the self-gravitating
disk of planetesimals. The effects of frequent collisions between planetesimals
are taken into account by using a Krook integral in the Boltzmann kinetic
equation. It is shown that as a result of an aperiodic collision-dissipative
instability of small gravity disturbances the disk is subdivided into numerous
dense fragments. These can eventually condense into the planetary sequence.
