Schedule Mar 16, 2004
Yoram Lithwick (University of California ~ Berkeley)

We address three questions regarding solar system planets. What determined their number? Why are their orbits nearly circular and coplanar? How long did they take to form? We pick up the story of planet formation at the end of oligarchy, when the oligarchs' surface mass density matched that of the small bodies they were accreting. Dynamical friction by the small bodies was no longer able to balance viscous stirring, so the oligarchs' velocity dispersion increased to the extent that their orbits crossed. An essential assumption of our investigation is that as the big bodies got bigger, the small ones got smaller as the result of undergoing a collisional fragmentation cascade. It follows that oligarchy was a brief stage in solar system evolution. What happened next differed in the inner and outer parts of the planetary system. In the inner part, where the ratio of the escape velocity from the surfaces of the planets to the escape velocity from their orbits is smaller than unity, big bodies collided and coalesced once their random velocities became comparable to their escape velocities. In the outer part, where this ratio is larger than unity, the random velocities of some of the big bodies continued to rise until they were ejected. In both parts, the number density of the big bodies eventually decreased to the extent that gravitational interactions among them no longer produced large scale chaos. After that their orbital eccentricities and inclinations were damped by dynamical friction from the remaining small bodies. A few implications of the above scenario are worth noting. Impacts among protoplanets of comparable size were common in the inner planet system but not in the outer. Ejections from the outer planet system included several bodies with masses in excess of Earth after oligarchy.

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