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The Hills Cloud, also called the Inner Oort Cloud[1] and Inner Cloud[2] is, in astronomy, a vast hypothetical spherical body interior to the Oort Cloud, whose outer border would be located at around 2 to 3 × 104 Astronomical Units (AU) from the Sun, and whose inner border, less well defined, is hypothetically located at 100-3,000 AU,[citation needed] well beyond planet and Kuiper-belt object orbits, but distances might be much greater. If it exists, the Hills Cloud contains roughly 5 times as many comets as the Oort cloud.[3]
Oort Cloud comets are continually perturbed by their environment. A non-negligible fraction leaves the Solar System or goes in the inner system. Hence it should have depleted itself long ago, but it has not. The Hills Cloud theory addresses the longevity of the Oort cloud by postulating a densely populated inner Oort region. Objects ejected from the Hills Cloud are likely to end up in the classical Oort cloud region, maintaining the Oort cloud.[4] It is likely that the Hills Cloud is the largest concentration of comets in the whole Solar System.
The existence of the Hills Cloud is plausible, since many bodies have been found already. It would be thicker than the Oort Cloud,[5][6] but much smaller. Gravitational interaction with the closest stars and tidal effects from the galaxy have given circular orbits to the comets in the Oort cloud, which must 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 more massive than the Oort Cloud.
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Formation
Many scientists think that the 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 neither by Jupiter nor by Neptune nor by tidal effects.[7] It is then possible that the Hills Cloud would be "younger" than the Oort Cloud, which probably formed earlier.[8] 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.
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 greater.[9]
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 1991,[10] when scientists started thinking about Hills' theory.
Possible Hills Cloud objects
| TNOs and similar bodies |
|---|
*Trans-Neptunian dwarf planets are "plutoids"
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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 | Diameter (km) |
Perihelion (AU) |
Aphelion (AU) |
Discovery |
|---|---|---|---|---|
| Sedna | 1,180 to 1,800 | 76.1 | 935 | 2003 |
| 2000 OO67 | 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 Cloud.[11] The same goes with damocloids whose origins are doubtful, such as the 5335 Damocles.
References
- ^ see Oort Cloud
- ^ astronomie, astéroïdes et comètes
- ^ "The Formation and Extent of the Solar System Comet Cloud". SAO/NASA. 1987. http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1987AJ.....94.1330D&db_key=AST&page_ind=0&data_type=GIF&type=SCREEN_VIEW&classic=YES. Retrieved on December 19, 2008.
- ^ J. A. Fernandez (09/1997). "The Formation of the Oort Cloud and the Primitive Galactic Environment". Icarus (1): 106-119. doi:.
- ^ J. G. Hills (11/1981). "Comet showers and the steady-state infall of comets from the Oort cloud". The Astronomical Journal: 1730-1740. doi:.
- ^ "Planetary Sciences: American and Soviet Research, Proceedings from the U.S.-U.S.S.R. Workshop on Planetary Sciences, p. 251". 1991. http://fermat.nap.edu/books/0309043336/html/251.html.
- ^ Ciel et espace, January 2006
- ^ Oort Cloud#Origin
- ^ http://www.evolution-creationism.us/young_earth/comets.html
- ^ http://loloch.free.fr/acm2.htm
- ^ http://www.futura-sciences.com/fr/news/t/astronomie/d/2008-kv42-lasteroide-qui-tourne-a-lenvers_16593/
Links
See also
- Oort Cloud -Complementary comet cloud.
- Comet -Minor body originating from the cloud.
- (90377) Sedna -Possible main body from 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
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(This article is a translation of the one found in the French Wikipedia: http://fr.wikipedia.org/wiki/Nuage_de_Hills)
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