Hills cloud
In astronomy, the Hills cloud (also called the inner Oort cloud and inner cloud) is a vast theoretical circumstellar disc, interior to the Oort cloud, whose outer border would be located at around 2 to 3×104Astronomical Units (AU) from the Sun, and whose inner border, less well defined, is hypothetically located at 100-3,000 AU, well beyond planet and Kuiper-belt object orbits, but distances might be much greater. If it exists, the Hills cloud contains roughly 5 times as many comets as the Oort cloud.
Oort cloud comets are continually perturbed by their environment. A non-negligible fraction leaves the Solar System or goes in the inner system. Hence it should have depleted itself long ago, but it has not. The Hills cloud theory addresses the longevity of the Oort cloud by postulating a densely populated inner Oort region. Objects ejected from the Hills cloud are likely to end up in the classical Oort cloud region, maintaining the Oort cloud. It is likely that the Hills cloud is the largest concentration of comets in the whole Solar System.