オールトの雲
The Oort cloud is thought to occupy a vast space from somewhere between to as far as from the Sun. Some estimates place the outer boundary at between . The region can be subdivided into a spherical outer Oort cloud of , and a torus-shaped inner Oort cloud of . The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets to inside the orbit of Neptune. The inner Oort cloud is also known as the Hills cloud, named after Jack G. Hills, who proposed its existence in 1981. Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo; it is seen as a possible source of new comets to resupply the tenuous outer cloud as the latter's numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years.