Apsidal Behavior among Planetary Orbits: Testing the Planet-Planet Scattering Model

Planets in extrasolar systems tend to interact such that their orbits lienear a boundary between apsidal libration and circulation, a "separatrix", withone eccentricity periodically reaching near-zero. One explanation, applied tothe Upsilon Andromedae system, assumed three original planets on circularorbits. One is ejected, leaving the other two with near-separatrix behavior. Wetest that model by integrating hundreds of hypothetical, unstable planetarysystems that eject a planet. We find that the probability that the remainingplanets exhibit near-separatrix motion is small (< 5% compared with nearly 50%of observed systems). Moreover, while observed librating systems are evenlydivided between aligned and anti-aligned pericenter longitudes, the scatteringmodel strongly favors alignment. Alternative scattering theories are proposed,which may provide a more satisfactory fit with observed systems.Comment: 10 pages, 2 figures, accepted to ApJ Letter