Hills cloud

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Artist's view of the Oort cloud, Hills cloud and Kuiper belt (inset).

In astronomy, the Hills cloud (also called the inner Oort cloud[1] and inner cloud[2]) is a vast theoretical circumstellar disc, 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.


Original Oort cloud model[edit]

Ernst Öpik

Between 1932 and 1981, astronomers believed there was a cloud: the Oort Cloud theorized by Ernst Öpik and Jan Oort and with the Kuiper Belt, it was the only comet reserve.[7]

In 1932, Estonian astronomer Ernst Öpik hypothesized that comets were rooted in a cloud orbiting the outer boundary of the Solar System. In 1950, this idea was revived independently by Dutch astronomer Jan Oort to explain this apparent contradiction: comets are destroyed after several passes through the inner solar system. So if any had existed for several billion years (since the beginning of the solar system), no more could be observed nowadays.

Oort selected for his study of 46 comets best observed between 1850 and 1952. The distribution of the inverse 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 (between 0.6 and 2.5 light years). This one, located at the limits of the sphere of influence gravity of the Sun, would be subject to stellar disturbances, likely to expel cloud comets or outwards or inwards resulting in the emergence of a new comet.

New model[edit]

In the 1980s, astronomers realized that the main cloud could have an internal cloud that would start at about 3000 AU from the Sun and continue until classic cloud to 20,000 AU. Most estimates place the population of the Hills Cloud of about five to ten times that of the outer cloud, about 20 trillion, although the number could be ten times greater than that.

The main model of an "inner cloud" was proposed in 1981 by the astronomer J.G. Hills, from the Los Alamos Laboratory, who gave the region its name. He highlighted that cloud when calculated that the passage of a star near the Solar System could have caused extinctions on Earth, triggering a "comet rain". Indeed, his research suggested that most of the total mass of the cloud comets have an orbit semi-major axis 10 4 u so much closer to the Sun than the minimum distance of the Oort Cloud. Moreover, the influence of the surrounding stars and that of the "galactic tide" should have been empty the Oort Cloud comets expelling outside or inside the solar system. He then Axa his studies on the possibility of the presence of a smaller cloud, more massive and also closer to the Sun that réapprovisionnerait external cloud comets.

In the following years other astronomers accréditèrent search Hills and studied. This is the case of Sidney van den Bergh who suggested the structure in addition to the Oort Cloud in 1982 and Mark E. Bailey in 1983. In 1986, Bailey states that the majority of comets in the solar system are not located not in the Oort cloud area, but closer, with an orbit with a semi-axis 5000 AU, and it would come as an internal cloud. The research was amplified by studies of Victor Clube and Bill Napier in 1987 and those of 1988 RB Stothers.

However the cloud Hills represents a major interest since 1991, when scientists resumed theory Hills (excluding documents written by Martin Duncan, Thomas Quinn and Scott Tremaine in 1987, which include the theory Hills and do additional research).


Like the Kuiper Belt, also called belt Edgeworth-Kuiper named after the scientists who studied the phenomenon, cometary clouds have the name astronomers have revealed their existence. The cloud Hills then named after JG Hills astronomer who first, speculated that it was a body independent of the main cloud. It is alternatively called internal Oort cloud, named after Dutch astronomer Jan Oort and internal cloud Öpik-Oort, the name of the Estonian astronomer Ernst Öpik.


Structure and composition[edit]

Comets Oort cloud are constantly disturbed by their surroundings. A significant part leaves the solar system or going into the internal system. This cloud should therefore have run out a long time, or it is not. The Hills cloud theory could provide an explanation. JG Hills and other scientists have suggested that it would be a source book of comets in outer halo of the Oort cloud replenishing when the outer halo is exhausted 13. It is therefore likely that the cloud Hills is the largest concentration of comets across the Solar System.

The cloud Hills occupy a large area of space between the outer edge of the Kuiper Belt, to 50 AU and 20 000 AU, see 30 000 AU.

The mass of the Hills cloud is not known. Some scientists believe it could be five times more massive than the Oort cloud. According to Bailey estimates the mass of the cloud would Hills 13.8 Earth masses if the majority of the bodies are located at 10000 AU.

If the analyzes of comets are representative of the whole, the vast majority of Hills cloud objects consists of various windows, 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 protoplanetary cloud originating, a conclusion also supported by studies size comet cloud 18 and the recent impact study of the comet 9P / Tempel.


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 influenced neither by Jupiter nor by Neptune nor by tidal effects.[8] It is then possible that the Hills cloud would be "younger" than the Oort cloud, which probably formed earlier.[9] 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.

Possible Hills cloud objects[edit]

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
Sedna 1,200 to 1,600 76.1 975 2003
2000 OO67 28 to 87 20.8 1,068.2 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.[10] The same goes with damocloids whose origins are doubtful, such as 5335 Damocles.


  1. ^ see Oort cloud
  2. ^ astronomie, astéroïdes et comètes
  3. ^ "The Formation and Extent of the Solar System Comet Cloud". SAO/NASA. 1987. Retrieved December 19, 2008. 
  4. ^ J. A. Fernandez (1997). "The Formation of the Oort cloud and the Primitive Galactic Environment". Icarus. Vol. 129 no. 1. pp. 106–119. doi:10.1006/icar.1997.5754. 
  5. ^ J. G. Hills (1981). "Comet showers and the steady-state infall of comets from the Oort cloud". The Astronomical Journal. Vol. 86. pp. 1730–1740. doi:10.1086/113058. 
  6. ^ "Planetary Sciences: American and Soviet Research, Proceedings from the U.S.-U.S.S.R. Workshop on Planetary Sciences, p. 251". 1991. Retrieved November 7, 2007. 
  7. ^ Ernst Öpik (1932). "Note on Stellar Perturbations of Nearby Parabolic Orbits". Proceedings of the American Academy of Arts and Sciences 67. pp. 169–182. .
  8. ^ Ciel et espace, January 2006
  9. ^ Oort cloud#Origin
  10. ^ Actualité > 2008 KV42, l'astéroïde qui tourne à l'envers

Further reading[edit]