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The formation of planetesimals.Four stages in the accretion of planetesimals are described. The initial stage is the condensation of dust particles from the gaseous solar nebula as it cools. These dust particles settle into a thin disk which is gravitationally unstable. A first generation of planetesimals, whose radii range up to about 0.1 km, form from the dust disk by direct gravitational collapse to solid densities on a time scale of the order of 1 year. The resulting disk, composed of first-generation planetesimals, is still gravitationally unstable, and the planetesimals are grouped into clusters containing approximately 10,000 members. The contraction of these clusters is controlled by the rate at which gas drag damps their internal rotational and random kinetic energies. On a time scale of a few thousand years, the clusters contract to form a second generation of planetesimals having radii of the order of 5 km. Further coalescence of planetesimals proceeds by direct collisions which seem capable of producing growth at a rate of the order of 15 cm per year at 1 AU.
Document ID
Document Type
Reprint (Version printed in journal)
External Source(s)
Goldreich, P.
Ward, W. R. (California Institute of Technology Pasadena, Calif., United States)
Date Acquired
August 7, 2013
Publication Date
August 1, 1973
Publication Information
Publication: Astrophysical Journal
Volume: 183
Subject Category
Funding Number(s)
Distribution Limits