The Oort Cloud: Observation, Measurement & Evidence

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.

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.

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 nearly isotropic directions and can have very large semimajor axes, unlike a simple flat disk source. Numerical simulations reproduce a distant bound reservoir when planetary scattering is combined with the Sun's stellar and Galactic environment.