The Oort Cloud: Physical Mechanisms

Models usually distinguish an inner Oort cloud and a much more distant outer cloud extending tens of thousands of astronomical units from the Sun. Objects remain weakly bound there, making their orbits susceptible to perturbations from passing stars and the gravitational tide of the Milky Way.

Icy planetesimals formed much closer to the young Sun and were scattered outward by giant planets. Some were ejected completely, while others reached distant orbits that external stellar and Galactic perturbations later modified into a broad spherical reservoir. Occasional perturbations send objects back toward the inner Solar System as long-period comets.

Long-period comets arrive from many directions and often have very large semimajor axes, unlike a flat disk population. Dynamical simulations of early planet scattering can populate distant bound orbits, and Galactic and stellar perturbations can reproduce the injection of comets toward the Sun.

Giant planets scattered many icy planetesimals outward during Solar-System formation. Some escaped completely, while others remained weakly bound and were lifted into distant orbits by external perturbations. Later perturbations occasionally send objects back inward as long-period comets.