PLANETARY PERTURBATIONS AND THE ORIGIN OF SHORT-PERIOD COMETS

To investigate the dynamical plausibility of possible sources for the short-period comets, we have integrated a representative sample of comet orbits in the field of the Sun and the giant planets. Our aim is to determine whether the distribution of orbits from a proposed source that reach observable perihelia (q < 2.5 AU) matches the observed distribution of short-period orbits. We find that the majority of the short-period comets, those with orbital period P < 20 yr (the "Jupiter family"), cannot arise from isotropic orbits with perihelia near Jupiter's orbit, because the resulting observable comet orbits have the wrong distribution in period, inclination, and argument of perihelion. This result does not depend strongly on the assumed lifetime of the comets against solar evaporation. Our simulations also show that Jupiter-family comets cannot arise from isotropic orbits with perihelia in the Uranus-Neptune region. On the other hand, a source of low-inclination Neptune-crossing orbits yields a distribution of observable Jupiter-family comets that is consistent with the data in all respects. These results imply that the Jupiter-family comets arise from a disk source in the outer solar system rather than from the Oort comet cloud.