Formation and Evolution of Planets

Abstract

The basic stages of the formation of the planets are considered. Decay of turbulence in the solar nebula allows the precipitation of dust particles to the solar nebula’s equatorial plane, their growth occurring by sticking at collisions and transformation into kilometer-sized bodies (planetesimals) partly by direct growth and partly via gravitational instability in a dense dust layer formed near the central plane of the nebula. Analytical study of the dynamical evolution of a rotating swarm of bodies with quasi-equilibrium values of velocity and mass distributions gave a reasonable description of the final stage of accumulation. This is a robust result in that the time scale of formation of the terrestrial planets is 10⁸ yr. Numerical simulation of the earlier stage revealed an accelerated (runaway) growth of the largest bodies—planet embryos. More careful study of the intermediate stage is needed. The two-step formation of giant planets is considered: accumulation of icy-rock cores of about 10–15 Earth masses, similar to that in the terrestrial zone, and accretion of gas onto cores of Jupiter and Saturn require time scales not much longer than 10⁷ yr (mostly for the growth of their cores). Problems connected with a slow time of growth of Uranus and Neptune are discussed. Small bodies of the Solar System are byproducts of the formation of the planets. The fall of large bodies onto the planets at the last stage of accumulation brought much of stochastics into the process (planetary obliquities, etc.). It led to a much higher initial temperature of the Earth’s mantle, early beginning of partial melting and differentiation, which determined its pre-geological evolution.

Paper presented at the Conference on Planetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.