Hills cloud

The Hills cloud is a hypothesized inner region of the Oort cloud, closer to the Sun and denser than its outer region. It may replenish the outer cloud with distant comets.

Oort cloud: A hypothesized spherical reservoir of icy bodies surrounding the Solar System far beyond the planets. The Hills cloud is proposed as its denser, more Sunward inner component.
Astronomical unit: The mean distance between Earth and the Sun, about 150 million kilometers. It provides a familiar scale for comparing the proposed cloud's distance with planetary orbits.
Jack G. Hills: An American astrophysicist known for work on stellar dynamics and the proposed inner Oort cloud. His 1981 study gave the hypothesized cloud its name and proposed its comet-reservoir role.
Oort cloud formation: The process by which icy planetesimals were scattered into distant Solar System orbits. Formation models determine whether a dense inner cloud could persist alongside the outer cloud.
Long-period comet: A comet whose orbital period around the Sun exceeds 200 years. Comets from distant reservoirs can enter the inner Solar System on long-period orbits.
Orbital eccentricity: A dimensionless measure of how much an orbit deviates from a circle. Highly eccentric orbits let distant comet bodies approach the planetary region.
Comet: A small Solar System body rich in ice that can develop a coma and tail near the Sun. The cloud was proposed to explain the supply of comets arriving from great distances.
Observational astronomy: The study of celestial objects and phenomena through collected electromagnetic radiation and other signals. Directly detecting small, distant Hills-cloud bodies is an observational challenge.
Galactic tide: The gravitational influence of the Milky Way's mass distribution on objects within it. It can alter distant comet orbits and help deliver bodies toward the Sun.
Orbital inclination: The angle between an object's orbital plane and a chosen reference plane. A roughly spherical comet cloud contains bodies with a wide range of orbital inclinations.

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Tidal disruptionRelated: Passing stars and Galactic tides can perturb its bodies into disruptive close encounters.