interest in 1991, when scientists resumed Hills' theory. Characteristics Structure and composition Oort cloud comets are constantly disturbed by their surroundings and distant objects. A significant number either leave the Solar System or go much closer to the Sun. The Oort cloud should therefore have broken apart long ago, but it still remains intact. The Hills cloud proposal could provide an explanation; J. G. Hills and other scientists suggest that it could replenish the comets in the outer Oort cloud. It is also likely that the Hills cloud is the largest concentration of comets across the Solar System. The Hills cloud should be much denser than the outer Oort cloud: If it exists, it is somewhere between 5,000 and 20,000 AU in size. In contrast, the Oort cloud is between in size. The mass of the Hills cloud is not known. Some scientists believe it could be five times more massive than the Oort cloud. Mark E. Bailey estimates the mass of the Hills cloud to be 13.8 Earth masses, if the majority of the bodies are located at 10,000 AU. If the analyses of comets are representative of the whole, the vast majority of Hills cloud objects consists of various ices, such as water, methane, ethane, carbon monoxide and hydrogen cyanide. However, the discovery of the object 1996 PW, an asteroid on a typical orbit of a long-period comet, suggests that the cloud may also contain rocky objects. The carbon analysis and isotopic ratios of nitrogen firstly in the comets of the families of the Oort cloud and the other in the body of the Jupiter area shows little difference between the two, despite their distinctly remote areas. This suggests that both come from a protoplanetary disk, a conclusion also supported by studies of comet cloud sizes and the recent impact study of comet Tempel 1. Formation Many scientists think that the Hills cloud formed from a close (800 AU) encounter between the Sun and another star within the first 800 million years of the Solar System, which could explain the eccentric orbit of 90377 Sedna, which should not be where it is, being neither influenced by Jupiter nor Neptune, nor tidal effects. It is then possible that the Hills cloud would be "younger" than the Oort cloud. However, only Sedna and two other sednoids ( and 541132 Leleākūhonua) bear those irregularities; for and this theory is not necessary,