ORIGIN OF COMETS IN THE EXTENDED PREPLANETARY DISK

Astronomical observations indicate that preplanetary disks are radiall y much more extended than it has been assumed until now by models. It has been shown by other authors that gravitational instabilities are n ot effective in processes of the growth of planetesimals within such p replanetary disks, but, and as discussed in this paper, the conditions are more favourable for local gravitational instabilities to occur te mporarily in the extended particle subdisks of preplanetary disks. Com ets and other Kuiper belt objects should have been influenced in their formation by related gravitationally focused collisional growth proce sses. Especially, km-sized cometary building blocks will form as a con sequence of this temporary phase of local gravitational instability. T herefore, 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 bui lt up with the preferred structural scale of the building blocks. Only at larger scales, gravitation caused compaction will increasingly cau se the disappearence of these structures. Copyright (C) 1996 Elsevier Science Ltd