Formation of planetesimals in the trans-Neptunian region of the protoplanetary disk

We consider the formation of cometlike and larger bodies in the trans-Neptunian region of the protoplanetary gas-dust disk. Once the particles have reached 1-10 cm in size through mutual collisions, they compact and concentrate toward the midplane of the disk to form a dust subdisk there. We show that after the subdisk has reached a critical density, its inner, equatorial layer that, in contrast to the two subsurface layers, contains no shear turbulence can be gravitationally unstable. The layer breaks up into similar to10(12)-cm Clumps whose small fragments (similar to109 cm) can rapidly contract to form bodies similar to10 km in size. We consider the sunward drift of dust particles at a velocity that decreases with decreasing radial distance as the mechanism of radial contraction and compaction of the layer that contributes to its gravitational instability and the formation of larger (similar to100 km) planetesimals. Given all of the above processes, it takes similar to10(6) yr for planetesimals to form, which is an order of magnitude shorter than the lifetime of the gas-dust protoplanetary disk. We discuss peculiarities of the structure of planetesimals.