The Oort Cloud: Structures & Vocabulary

The Oort cloud is an inferred, extremely distant reservoir of icy bodies surrounding the Solar System in a roughly spherical distribution. It is not directly imaged as a dense cloud; its existence is inferred primarily from the nearly isotropic arrival directions and orbital properties of long-period comets.

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.

Models often distinguish an inner and outer Oort cloud extending from thousands to tens of thousands of AU. At those distances the Sun's binding is weak enough that passing stars, Galactic tides, and molecular clouds can substantially alter comet perihelia.