奥尔特云
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.