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Oort cloud

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Artist's rendering of the Oort cloud and the Kuiper Belt.
Presumed distance of the Oort cloud compared to the rest of the solar system.

The Oort cloud, (IPA: BrE /ɔːt klaʊd/, AmE /ɔɹt klaʊd/), alternatively termed the Öpik-Oort Cloud (/'øpik/-, like /'epik/ with a rounded /e/), is a postulated spherical cloud of comets situated about 50,000 to 100,000 AU[1] from the Sun. This is approximately 2000 times the distance from the Sun to Pluto or nearly two light years. The outer extent of the Oort cloud places the boundary of our Solar System at nearly halfway the distance to Proxima Centauri, the nearest star to the Sun.

Although no confirmed direct observations of the Oort cloud have been made, astronomers believe it to be the source of most or all comets entering the inner solar system (some short-period comets, based on their orbits, may come from the Kuiper belt).

Hypothesis

In 1932 Ernst Öpik , an Estonian astronomer, proposed[2] that comets originate in an orbiting cloud situated at the outermost edge of the solar system. In 1950 the idea was revived independently[3] by Dutch astronomer Jan Hendrick Oort to explain an apparent contradiction: comets are destroyed by several passes through the inner solar system, yet if the comets we observe had really existed for billions of years (since the generally accepted origin of the solar system), all would have been destroyed by now. According to the hypothesis, the Oort cloud contains millions of comet nuclei, which are stable because the sun's radiation is very weak at their distance. The cloud provides a continual supply of new comets, replacing those that are destroyed. In order for it to supply the necessary volume of comets, the total mass of comets in the Oort cloud must be many times that of Earth.

Structure and composition

The Oort cloud is commonly thought to consist of up to six trillion individual objects,[4] each tens of millions of kilometers apart.[5] Its mass is not known with any certianty; estimates have ranged from 2 Earth masses [6] to roughly 380 Earth masses. (about that of Jupiter).[7] Recent models have tended to favour the lower figures, however, as a mass greater than the combined masses of Uranus and Neptune (about 31 Earth masses) is inconsistent with those planets having scattered the comets to the Oort cloud in the first place.[7] Although the vast majority of Oort cloud objects are believed to be primarily ice, the discovery of the object 1996 PW suggests that it may also be home to rocky objects.[8]

The Oort cloud is thought to consist of two separate regions; a spherical outer cloud from which the more distant comets originate (since their points of origin are effectively random) and a disc-like inner cloud whose comets have inclinatins of between 10 and 50 degrees.[9] There is also a theory of a denser inner part of the Oort cloud known as the Hills cloud;[10] it would have a well-defined outer boundary at 20-30 000 AU, a less well defined inner boundary at 50 to 3000 AU, and would be about 10 to 100 times denser than the Oort cloud.[11] This hypothesis is employed to explain the continued existence of the Oort cloud over the course of billions of years.[12]

Origin

The Oort cloud is thought to be a remnant of the original protoplanetary disc that formed around the Sun approximately 4.6 billion years ago, and is loosely bound to the solar system, and thus easily affected by the motions of passing stars or other forces.[13]

The most widely-accepted hypothesis of its formation is that the Oort cloud's objects initially formed much closer to the Sun as part of the same process that formed the planets and asteroids, but that gravitational interaction with young gas giants such as Jupiter ejected them into extremely long elliptical or parabolic orbits.[14] While on the distant outer regions of these orbits, gravitational interaction with nearby stars further modified their orbits to make them more circular. Recent studies have shown that the formation of the Oort cloud is broadly compatible with the hypothesis that the Solar System formed as part of an embedded cluster among between 200 and 400 stars. These early stars likely played a role in the cloud's formation.[15]

It is thought that other stars are likely to possess Oort clouds of their own, and that the outer edges of two nearby stars' Oort clouds may sometimes overlap, causing perturbations in the comets' orbits and thereby increasing the number of comets that enter the inner solar system.

Star perturbations and Nemesis theory

The interactions of the Oort cloud with those of neighboring stars, and its deformation by the galactic tide are thought to be the main triggers which send the long-period comets into the inner Solar System.[16] This process also served to scatter the objects out of the ecliptic plane, explaining the cloud's spherical distribution.[17][18]

The known star with the greatest possibility of perturbing the Oort cloud in the next 10 million years is Gliese 710.[18] However, physicist Richard A. Muller and others have postulated that the Sun has a heretofore undetected companion star in an elliptical orbit beyond the Oort cloud. This star, known as Nemesis, is theorized to pass through a portion of the Oort cloud approximately every 26 million years, bombarding the inner solar system with comets. Although the theory has many proponents, no direct proof of the existence of Nemesis has been found.[19]

Oort cloud objects (OCO)

Types of distant minor planets

So far, only three objects with orbits which suggest that they may belong to the Oort Cloud have been discovered. 90377 Sedna, 2000 OO67 and 2000 CR105 have orbits which, unlike those of the scattered disk objects, cannot be explained by perturbations of the main known planets and may thus constitute an 'inner' Oort cloud. Their orbits can then be explained by one of two theories. Either these objects were Oort cloud bodies disrupted by the passage of a neaby star close to the solar system [20] (Morbidelli and Levison (2004)), or else their orbits were disrupted by an as yet unknown planet-sized body within the Oort Cloud[21] (Gomes et al. 2006) .


Oort cloud object candidates
Number Name Equatorial diameter
(km) 		Perihelion (AU) Aphelion (AU) Date discovered Discoverer Diameter method
90377 Sedna 1180 - 1800 km 76 (±7) 975 2003 Brown, Trujillo, Rabinowitz thermal
— 2000 CR105 265 km 44.3 397 2000 Lowell Observatory ???
87269 2000 OO67 28 - 87 km 20.8 1005.5 2000 Cerro Tololo telescope ???

See also

References

  1. ^ "Space Topics: Trans-Neptunian Objects". Teh Planetary Society. Retrieved 2007-05-27.
  2. ^ Öpik, E., Note on Stellar Perturbations of Nearby Parabolic Orbits, Proceedings of the American Academy of Arts and Sciences, Vol. 67, pp. 169-182 (1932)
  3. ^ Oort, J. H., The structure of the cloud of comets surrounding the Solar System and a hypothesis concerning its origin, Bull. Astron. Inst. Neth., 11, p. 91-110 (1950) Text at Harvard server (PDF)
  4. ^ PAUL R. WEISSMAN (1998). "The Oort Cloud". Scientific American. Retrieved 2007-05-26.
  5. ^ Rosanna L. Hamilton (1999). "The Oort Cloud". Retrieved 2007-05-26.
  6. ^ "The mass of the Oort cloud". California Institute of Technology. 1981. Retrieved 2007-05-26. {{cite web}}: Unknown parameter |auhtor= ignored (|author= suggested) (help)
  7. ^ a b "(meteorobs) Excerpts from "CCNet 19/2001". 2001. Retrieved 2007-05-26.
  8. ^ PAUL R. WEISSMAN, HAROLD F. LEVISON (1997). "Origin and Evolution of the Unusual Object 1996 PW: Asteroids from the Oort Cloud?". Earth and Space Sciences Division, Jet Propulsion Laboratory, Space Sciences Department, Southwest Research Institute. Retrieved 2007-05-26.
  9. ^ Harold F. Levison, Luke Dones, Martin J. Duncan (2001). "The Origin of Halley-Type Comets: Probing the Inner Oort Cloud". Retrieved 2007-06-27.{{cite web}}: CS1 maint: multiple names: authors list (link)
  10. ^ Hills, J. G. (1981). "Comet showers and the steady-state infall of comets from the Oort cloud". Astronomical Journal. 86: 1730–1740. doi:10.1086/113058. {{cite journal}}: Unknown parameter |month= ignored (help)
  11. ^ Planetary Sciences: American and Soviet Research, Proceedings from the U.S.-U.S.S.R. Workshop on Planetary Sciences, 1991, p. 251
  12. ^ Julio A. Ferna´ndez (1996). "The Formation of the Oort Cloud and the Primitive Galactic Environment" (PDF). Departamento de Astronomı´a, Facultad de Ciencias, Tristan Narvaja. Retrieved 2007-05-26.
  13. ^ "The Oort cloud". Retrieved 2007-05-26.
  14. ^ "Oort Cloud & Sol b?". SolStation. Retrieved 2007-05-26.
  15. ^ R. Brasser, M. J. Duncan, H.F. Levison (2006). "Embedded star clusters and the formation of the Oort Cloud". Retrieved 2007-05-26.{{cite web}}: CS1 maint: multiple names: authors list (link)
  16. ^ Rosanna L. Hamilton (1999). "The Oort Cloud". Retrieved 2007-05-26.
  17. ^ A. Higuchi, E. Kokubo (2006). "Evolution of the Oort Cloud and Distribution of New Comets Due to the Galactic Tide". National Astronomical Observatory of Japan. Retrieved 2007-05-27.
  18. ^ a b L. A. Molnar, R. L. Mutel. "Close Approaches of Stars to the Oort Cloud: Algol and Gliese 710". University of Iowwa. Retrieved 2007-05-27.
  19. ^ J. G. Hills (1984). "Dynamical constraints on the mass and perihelion distance of Nemesis and the stability of its orbit". Nature. Retrieved 2006-06-23.
  20. ^ Morbidelli, Alessandro (2004). "Scenarios for the Origin of the Orbits of the Trans-Neptunian Objects 2000 CR105 and 2003 VB12 (Sedna)". Astron. J. 128: 2564–2576. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  21. ^ Gomes, Rodney S. (2006). "A distant planetary-mass solar companion may have produced distant detached objects". Icarus. 184: 589–601. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

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