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 20,000 to 30,000 astronomical units (AU) from the Sun, and whose inner border, less well defined, is hypothetically located at , well beyond planetary 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. Overview The need for the Hills cloud hypothesis is intimately connected with the dynamics of the Oort cloud: Oort cloud comets are continually perturbed in their environment. A non-negligible fraction leave the Solar System, or tumble into the inner system where they evaporate, or fall into the Sun or gas giants. Hence, the Oort cloud should have been depleted long ago, but it is still well supplied with comets. The Hills cloud hypothesis addresses the persistence of the Oort cloud by postulating a densely populated, inner-Oort region—the "Hills cloud". 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 has the largest concentration of comets in the whole Solar System. The existence of the Hills cloud is plausible, since many bodies have been found there already. It should be denser than the Oort cloud. Gravitational interaction with the closest stars and tidal effects from the galaxy have given circular orbits to the comets in the Oort cloud, which may not be the case for the comets in the Hills cloud. The Hills cloud's total mass is unknown; some scientists think it would be many times more massive than the outer Oort cloud. History Original Oort cloud model Between 1932 and 1981, astronomers believed that the Oort cloud proposed by Ernst Öpik and Jan Oort, and the Kuiper belt were the only reserves of comets in the Solar System. In 1932, Estonian astronomer Ernst Öpik hypothesized that comets were rooted in a cloud orbiting the outer boundary of the Solar System. In 1950, this idea was revived independently by Dutch astronomer Jan Oort to explain an apparent contradiction: Comets are destroyed after several passes through the inner Solar System, so if any had existed for several billion years (since the beginning of the Solar System), no more could be