Oort cloud

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This image is an artist's rendering of the Oort cloud and the Kuiper belt
This diagram shows the presumed distance of the Oort cloud compared to the rest of the solar system

The Oort cloud, alternatively termed the Öpik-Oort cloud, is a hypothetical spherical cloud of comets situated about 50,000 to 100,000 astronomical unit (AU) from the Sun. This is approximately two thousand times the distance from the Sun to Pluto or roughly one light year, almost a quarter of the distance from the Sun to Proxima Centauri, the star nearest the Sun.

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

There is a theory that periodic (every about 30 million years) disturbances of the Oort cloud by nearby star have been inducing massive falls of comets onto the Earth that caused mass extinction of life on Earth. If that is true, the Oort cloud is very significant in influencing the evolution of life on Earth (by supplying water and destroying life) as well as preserving primitive materials, from which planetary scientists can study the origin and evolution of the Solar System.

Contents

History

In 1932, Ernst Öpik, an Estonian astronomer, proposed[1] that comets originate in an orbiting cloud situated at the outermost edge of the solar system. In 1950 the idea was revived and proposed[2] 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. It is believed that if the Oort cloud exists and supplies comets, 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. Estimates range between five and 100 Earth masses.[3]

There is also a theory of a denser, inner part of the Oort cloud coined the Hills cloud;[4] it would have a well-defined outer boundary at 20,000 to 30,000 AU, a less well defined inner boundary at 50 to 3,000 AU, and would be about 10 to 100 times denser than the remainder.[5]

Origin

The Oort cloud is thought to be a remnant of the original solar nebula that collapsed to form the Sun and planets approximately 4.6 billion years ago, and is loosely bound to the solar system.

The most widely-accepted hypothesis of its formation is that objects in the Oort cloud 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. This process also served to scatter the objects out of the ecliptic plane, explaining the cloud's spherical distribution. While on the distant outer regions of these orbits, gravitational interaction with nearby stars further modified their orbits to make them more circular.

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 known star with the greatest possibility of perturbing the Oort cloud in the next 10 million years is Gliese 710. 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 based on the records of heavy bombardments on the Earth that caused mass extinctions.[6] 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.

Oort Cloud Objects (OCOs)

TNOs and similar bodies
  • Cis-Neptunian objects
    • Centaurs
    • Neptune Trojan
  • Trans-Neptunian objects (TNOs)
      • Classical KBOs (Cubewanos)
      • Resonant KBOs
        • Plutinos (2:3 Resonance)
    • Scattered disc objects (SDOs)
    • Oort cloud objects (OCOs)

So far, only three potential Oort cloud objects have been discovered: 90377 Sedna,[7] 2000 OO67,[8] and 2000 CR105.[9]

90377 Sedna's orbit that ranges from roughly 76 to 925 AU, does not carry it completely out to the assumed position of the Oort cloud and is too far out for it to be truly considered as a Kuiper belt object. If Sedna indeed belongs to the Oort cloud, this may mean that the Oort cloud is both denser and closer to the Sun than previously thought.

Some astronomers include the objects 2000 CR105 and 2000 OO67 as part of the Oort cloud. The object 2000 CR105 has a perihelion of 45 AU, an aphelion of 415 AU and an orbital period of 3,241 years while the object 2000 OO67 has a perihelion of 21 AU, an aphelion of 1,000 AU and an orbital period of 12,705 years.[10]


Oort cloud objects (potential)
Number Name Equatorial diameter
(km)
Perihelion (AU) Aphelion (AU) Date discovered Discoverer Diameter method
90377 Sedna 1180 - 1800 km 76 (±7) 975 2003 Michael E. Brown, Chad Trujillo, David L. Rabinowitz thermal
2000 CR105 265 km 44.3 397 2000 Lowell Observatory assumed albedo
87269 2000 OO67 28 - 87 km 20.8 1005.5 2000 Cerro Tololo telescope assumed albedo

See also

Notes

  1. Ernst Öpik, “Note on Stellar Perturbations of Nearby Parabolic Orbits,” Proceedings of the American Academy of Arts and Sciences 67 (1932): 169-182.
  2. J. H. Oort, “The Structure of the Cloud of Comets Surrounding the Solar System and a Hypothesis Concerning its Origin,” Bull. Astron. Inst. Neth 11 (1950): 91-110. Retrieved September 20, 2007.
  3. L. S. Marochnik, L. M. Mukhin and R. Z. Sagdeev, “Estimates of Mass and Angular Momentum in the Oort Cloud,” Science 242: 547-550. Retrieved September 20, 2007.
  4. Jack G. Hills, “Comet Showers and the Steady-State Infall of Comets from the Oort Cloud,” Astronomical Journal 86 (November 1981): 1730-1740. Retrieved September 20, 2007.
  5. “Planetary Sciences: American and Soviet Research,” Proceedings from the U.S.-U.S.S.R. Workshop on Planetary Sciences (1991): 251. Retrieved September 20, 2007.
  6. Lynn Yarris, “Does A Companion Star to Sun Cause Earth's Periodic Mass Extinctions?” LBL Research Review (Spring 1987). Retrieved September 20, 2007.
  7. Robert Roy Britt, “Scientists Find Another Huge Mini-World in Outer Solar System,” Space.com (March 15, 2004). Retrieved September 20, 2007.
  8. “List Of Centaurs and Scattered-Disk Objects,” MPC. Retrieved September 20, 2007.
  9. Brett Gladman, “Evidence for an Extended Scattered Disk?” Retrieved September 20, 2007.
  10. List of Known Trans-Neptunian Objects. Retrieved September 20, 2007.

References

  • Öpik, E. 1932. Note on Stellar Perturbations of Nearby Parabolic Orbits. Proceedings of the American Academy of Arts and Sciences 67 (1932): 169-182.
  • Raup, David M. 1999. The Nemesis Affair - A Story of the Death of Dinosaurs and the Ways of Science. Revised ed. New York: W. W. Norton & Company. ISBN 0393319180

External links

All links retrieved September 20, 2007.


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For a complete listing, see: List of asteroids. See also Pronunciation of asteroid names and Meanings of asteroid names.
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