On the Formation of Planetesimals

The past and present existence of smaller bodies in our solar system manifests itself in various ways through the appearance of asteroids, minor moons, planetary rings, surface cratering, etc. The occurrence of these planetesimals appears to be an integral intermediary step in the formation of the larger bodies from the initial grain state. In order to account for the discreteness and order of the planetary orbits and regular satellite systems, Prentice (1977, 1978) has suggested that the accumulation of this planetesimal material took place in a series of well defined orbits corresponding to the positions of a system of gaseous Laplacian rings that were shed by the primitive contracting parent cloud. Alfvén and Arrhenius (1975) have also drawn attention to the fact that planetesimal formation occurs far more effectively if the accumulation takes place along well defined ‘jet streams’. Hourigan (1977) has previously demonstrated how such concentrated rock/ice streams or ‘jet-streams’ naturally form on the mean Keplerian orbit of gaseous rings that are present in the Prentice-Laplace model.