Origin of comets in the extended preplanetary disk

Astronomical observations indicate that preplanetary disks are radially much more extended than it has been assumed until now by models. It has been shown by other authors that gravitational instabilities are not effective in processes of the growth of planetesimals within such preplanetary disks, but, and as discussed in this paper, the conditions are more favourable for local gravitational instabilities to occur temporarily in the extended particle subdisks of preplanetary disks. Comets and other Kuiper belt objects should have been influenced in their formation by related gravitationally focused collisional growth processes. Especially, km-sized cometary building blocks will form as a consequence of this temporary phase of local gravitational instability. Therefore, comets up to sizes in the 10 km range are expected not to be of a fractal structure in the sense of self-similarity, but to be built up with the preferred structural scale of the building blocks. Only at larger scales, gravitation caused compaction will increasingly cause the disappearence of these structures. Copyright (C) 1996 Elsevier Science Ltd