On Disk-Planet Precessional Motions

The secular variations of a planetary system during the dispersal of its precursor disk is a fundamental problem of early solar system history. Secular resonances, i.e., 1:1 commensurabilities between various apsidal and nodal precession rates among the planets can be temporarily generated by the decaying background gravitational potential of the gas disk. There are powerful torques associated with secular resonances that transport angular momentum and lead to significant changes in orbital eccentricities and/or inclinations. Although the importance of secular resonance sweeping has long been recognized, some models that track this event have been incomplete in that the forced motions of the disk were not taken into account. Disk motions can significantly alter the predicted orbital precession rates of the objects orbiting within it and modify the overall response to the secular sweeping mechanism. Examples of this effect will be presented and discussed.