Hills cloud.html |
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Hills Cloud, also called Inner Oort Cloud 1 and Inner Cloud2 is, in astronomy, a vast hypothetic spherical body set of the Oort Cloud, whose inner border would be located at around 2 to 3×104 Astronomical Units (au) from the Sun, and whose outer border, less well defined, is hypothetically located at 1,000 au, well beyond planet and Kuiper-belt object orbits, but distances might be much greater.
Oort Cloud comets are continually perturbated by their environment. A non-negligible fraction leaves the Solar System or goes in the inner system. Hence it should have deplenished itself long ago, but it is not the case. Thus the Hills Cloud theory could explain it all. J.G. Hills and other scientists have suggested that there could be a source of comets in an halo external to the Oort cloud replenishing it when it is empty3. It is thus very likely that Hills Cloud is the largest concentration of comets of the whole Solar System.
Hills Cloud is one of astronomy's most plausible theories, since many bodies have been found already. It would be thicker than Oort's Cloud4 · 5, but however much smaller. Gravitational interaction with the closest stars and tidal effects from the galaxy have given circular orbits to the comets in Oort's cloud, which must not be the case for the comets in Hills cloud. Hills cloud's total mass is unknown; some scientists think it would be more massive than Oort's Cloud.
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Formation: A Chaotic Encounter
Many scientists think that Hills Cloud was born 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 influenced either by Jupiter nor by Neptune nor by tidal effects6. It is then possible that Hills Cloud would be "younger" than the Oort Cloud, which probably formed earlier7. Only (90377) Sedna bears those irregularities, for 2000 OO67 and 2006 SQ372 this theory doesn't seem to be needed, because both orbit close to gas giants.
A Shaky Next Encounter
In about 1.4 million years, Hills Cloud can be perturbated once again by the passing by of another star, Gliese 7108. Then, most comets, be they from the Oort Cloud or the Hills Cloud, will be perturbated: some will be ejected and will change the shape and the looks of Hills Cloud. The problem is that it might send comets to the inner Solar System and cause an hypothetical impact with Earth, remindful of the one that wiped out dinosaurs 65 million years ago: mass extinction.
Discovery
Between 1932 and 1981, astronomers thought that there was only one cloud: the Oort Cloud theorized by Ernst Öpik and Jan Oort and that with the Kuiper Belt, it was the only comet reservoir.
During the 1980s, astronomers realized that the main cloud could have an inner cloud from between about 3,000 au and 20,000 au from the Sun. Most evaluations place the Hills Cloud population somewhere between 5 and 10 times the outer cloud's, around 20 billion, even though the figure could be as much as 10 times greater9.
The main "inner cloud" model has been proposed in 1981 by astronomer J.G. Hills, who gave it its name.
However the Hills Cloud attracted attention only from 199110, when scientists started thinking about Hills' theory.
Notable Bodies
Bodies in the Hills Cloud are made mostly of water ice, methane and ammonia. We know of many comets originating from the Hills Cloud, such as Comet Hyakutake. However, three are outstanding:
| Name | Diametre (km) |
Perihelion (UA) |
Aphelion (UA) |
Discovery |
|---|---|---|---|---|
| Sedna | 1,180 to 1,800 | 76.1 | 935 | 2003 |
| 2000 0067 | 28 to 87 | 20.8 | 1 014,2 | 2000 |
| 2006 SQ372 | 50 to 100 | 24.17 | 2 005,38 | 2006 |
Some very strange bodies could be part of the Hills Cloud. A lot of mystery surrounds 2008 KV42, with its retrograde orbit that could make it originate from the Hills Cloud, maybe even from the Oort Cloud11. The same goes with damocloids whose origins are doubtful, such as the 5335 Damocles.
References
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Links
See also
- Oort Cloud -Complementary comet cloud.
- Comet -Minor body originating from the cloud.
- (90377) Sedna -Possible main body from the Hills Cloud.
External links
- Heisler, Julia; Scott Tremaine (1986). "The influence of the galactic tidal field on the Oort comet cloud". Icarus 65: 13, http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1986Icar...65...13H&db_key=AST&data_type=HTML&format=&high=44b52c369006969.
- www.lpi.usra.edu/books/CometsII/7031.pdf
(This article is a translation of the one found in the French Wikipedia: http://fr.wikipedia.org/en/Nuage_de_Hills)