Oort Cloud
The Oort Cloud is a region of space surrounding the Sun where comets are thought to originate. Proposed by Jan Oort in the 1950s, the Oort Cloud is believed to extend from roughly 2,000 to 100,000 Astronomical Units (AU), with its greatest concentration around 50,000 AU (1 AU is the average Earth-Sun distance, which equals 149 million kilometers, or 93 million miles). The outer edge is difficult to define, so 100,000 AU may be conservative.
The Oort Cloud is a theoretical construct, filled with planetesimals that likely formed in the Solar System while the planets were forming, but eventually got ejected to the outer realms due to gravitational interactions with the planets.
Occasionally, these small, icy planetesimals are perturbed into an elliptical orbit that brings them close to the Sun. The Sun’s energy and heat cause their ices to vaporize, they grow a tail, and a comet is born. It passes quickly around the Sun and is typically trapped in an elliptical orbit around the Sun.
The Oort Cloud replenishes the Solar System with its comets. Comets don’t last too long before they’re broken up by the increased stresses and energy as they approach the Sun. Some comets plunge directly into the Sun, absorbed completely and gone forever.