Oort

The Oort cloud is a hypothetical spherical shell of icy bodies surrounding the Sun at distances from about 2,000 to 100,000 astronomical units (roughly 0.03 to 1.6 light-years). Proposed by Dutch astronomer Jan Oort in 1950, it is believed to be the source of long-period comets observed entering the inner solar system, though no direct detection has occurred.

The cloud is often described as consisting of an inner region, sometimes called the Hills cloud, extending roughly from 2,000 to 20,000 AU, and an outer cloud from about 20,000 to 100,000 AU. The bodies are thought to be remnants of the solar system's formation, composed mainly of ices such as water, methane, and ammonia.

The Oort cloud's structure and dynamics are influenced by the gravity of the Sun, the galaxy, and passing stars. Galactic tides and close stellar encounters can perturb cometary orbits, sending some into the inner solar system as long-period comets or ejecting others into interstellar space.

In addition to the cloud concept, the surname Oort refers to Jan Oort and associated astronomical concepts such as the Oort constants, used to describe the local rotational properties of the Milky Way. However, the primary astronomical use of the term Oort refers to the distant comet cloud.