Local Condensations in Collisional Discs

The equations for a collisional disc permit solutions which represent small local condensations of matter in a uniform medium. With parameter values appropriate to the pre-planetary disc the clouds are 50 to 1500 times as dense as the surrounding region and have a mass of 10¹⁴ to 10¹⁵ g. The formation of clouds follows from collisional instability. They maintain their equilibrium by means of a mass exchange with the adjacent matter. The clouds are proposed to replace the solid planetesimals in the theory of planetary formation.