Orbital evolution of planets embedded in a planetesimal disk

The existence of the Oort comet cloud, the Kuiper belt, and plausible ineff iciencies in planetary core formation all suggest that there was once a res idual planetesimal disk of mass similar to 10-100 M+ in the vicinity of the giant planets following their formation. Since removal of this disk requir es an exchange of orbital energy and angular momentum with the planets, sig nificant planetary migration can ensue. The planet migration phenomenon is examined numerically by evolving the orbits of the giant planets while they are embedded in a planetesimal disk having a mass of M-D = 10-200 M+. We f ind that Saturn, Uranus, and Neptune evolve radially outward as they scatte r the planetesimals, while Jupiter's orbit shrinks as it ejects mass. Highe r mass disks result in more rapid and extensive planet migration. If orbita l expansion and resonance trapping by Neptune are invoked to explain the ec centricities of Pluto and its cohort of Kuiper belt objects at Neptune's 3: 2 mean motion resonance, then our simulations suggest that a disk mass of o rder M-D similar to 50 M+ is required to expand Neptune's orbit by Delta a similar to 7 AU, in order to pump up Plutino eccentricities to e similar to 0.3. Such planet migration implies that the solar system was more compact in the past, with the initial Jupiter-Neptune separation having been smalle r by about 30%. We discuss the fate of the remnants of the primordial planetesimal disk. We point out that most of the planetesimal disk beyond Neptune's 2:1 resonanc e should reside in nearly circular, low-inclination orbits, unless there ar e (or were) additional, unseen, distant perturbers. The planetesimal disk i s also the source of the Oort cloud of comets. Using the results of our sim ulations together with a simple treatment of Oort cloud dynamics, we estima te that similar to 12 M+ of disk material was initially deposited in the Oo rt cloud, of which similar to 4 M+ will persist over the age of the solar s ystem. The majority of these comets originated from the Saturn-Neptune regi on of the solar nebula.