The Formation and Dynamical Survival of the Comet Cloud

Abstract

The theory of a huge reservoir of comets (the “comet cloud”) extending to almost interstellar distances is analyzed, paying special attention to its dynamical stability, formation process and orbital properties of the incoming cloud comets. The perturbing influence of passing stars and giant molecular clouds is considered. Giant molecular clouds may be an important perturbing element of the comet cloud, although they do not seem to change drastically former studies including only stellar perturbations. The more tightly bound inner portions of the comet cloud, say within 10⁴ AU, would have withstood the disrupting forces over the age of the solar system. The theory of a primordial comet origin in the outer planetary region close to Neptune’s orbit is specially analyzed. A primordial comet origin is consistent with the cosmogonic view that a large amount of residual material was ejected during the last stage in the formation of the Jovian planets. The smooth diffusion in the energy space of bodies scattered by Neptune guarantees that most of them will fall in the narrow range of energies close to zero (near-parabolic orbits) where passing stars and GMCs can act effectively on them. The long time scales of ~10⁹ yr required for bodies scattered by Neptune to reach near-parabolic orbits would indicate that the buildup of the comet cloud was an event that took place long after the planets formed. Depending on the field of perturbing galactic objects, it is possible to conceive that most scattered comets were stored in rather tightly bound orbits (a ~10⁴ AU), favoring the concept of their dynamical survival over several billion yr. Alternative theories of comet cloud formation, e.g. in-situ origin or interstellar capture, are also discussed. The main difficulty of the in-situ theory is to explain how comets could accumulate at large heliocentric distances where the density of the nebular material was presumably very low. The interstellar capture theory also meets severe dynamical objections as, for instance, the lack of observed comets with original strongly hyperbolic orbits and the extremely low probability of capture under most plausible conditions. Since our knowledge of the structure of giant molecular clouds and their frequency of encounters with the solar system is still very uncertain, the concept of capture of transient comet clouds during such encounters can be advanced very little beyond the speculative stage. Some other dynamical properties of relevance to theories of origin and structure of the comet cloud are also reviewed. We mention, for instance, the distribution of perihelion points on the celestial sphere. There seems to be here a well established deviation from randomness, although the debate on whether or not there is a preference of the perihelion clustering for the vicinity of the apex of the solar motion is still unsettled. The alleged correlation with the solar apex may be biased by the preference of comet discoveries in the northern hemisphere. Deviations from randomness might be caused by very close stellar passages in the recent past. The excess of retrograde orbits among the observed “new” and young comets — mainly those with q ≳ 2 AU — is another well known dynamical feature. Such as excess may probably be accounted for the by the combined action of planetary and stellar perturbation. Because of the decreasing action of planetary perturbations with increasing heliocentric distances, a significant increase in the rate of passages of long-period comets is predicted for the outer planetary region.