Hills cloud
In 1932, Estonian astronomer Ernst Öpik hypothesized that comets were rooted in a cloud orbiting the outer boundary of the Solar System.
The distribution of the reciprocal of the semi-major axes showed a maximum frequency which suggested the existence of a reservoir of comets between 40,000 and 150,000 AU (0.6 and 2.4 ly) away.
This reservoir, located at the limits of the Sun's sphere of influence (astrodynamics), would be subject to stellar disturbances, likely to expel cloud comets outwards or impel them inwards.
[9] The main model of an "inner cloud" was proposed in 1981 by the astronomer Jack G. Hills, from the Los Alamos Laboratory, who gave the region its name.
[5] Moreover, the influence of the surrounding stars and that of the galactic tide should have sent the Oort cloud comets either closer to the Sun or outside of the Solar System.
To account for these issues, Hills proposed the presence of an inner cloud, which would have tens or hundreds of times as many comet nuclei as the outer halo.
[10] 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.
[17] 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.
However, only Sedna and two other sednoids (2012 VP113 and 541132 Leleākūhonua) bear those irregularities; for 2000 OO67 and 2006 SQ372 this theory is not necessary, because both orbit close to the Solar System's gas giants.
In their article announcing the discovery of Sedna, Mike Brown and his colleagues asserted that they observed the first Oort cloud object.
They observed that, unlike scattered disc objects like Eris, Sedna's perihelion (76 AU) was too remote for the gravitational influence of Neptune to have played a role in its evolution.
[20][21] However, Sedna is much closer to the Sun than expected for objects in the Hills cloud and its inclination is close to that of the planets and the Kuiper belt.
Spectroscopic measures show that its surface composition is similar to that of other trans-Neptunian objects: It is mainly composed of a mixture of water ices, methane, and nitrogen with tholins.
The Trans-Neptunian object 2012 VP113 was announced on 26 March 2014 and has a similar orbit to Sedna with a perihelion point significantly detached from Neptune.
