Jump to content

Oort cloud

From Wikipedia, the free encyclopedia

This is an old revision of this page, as edited by Serendipodous (talk | contribs) at 12:52, 5 April 2008 (→Structure and composition). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

An artist's rendering of the Oort cloud, the Hills cloud, and the Kuiper belt (inset)
Types of distant minor planets

The Oort cloud (Template:PronEng ort, alternatively the Öpik-Oort Cloud (IPA: ['øp?k]) is a spherical cloud of comets believed to lie roughly 50,000 AU (nearly a light year) from the Sun.[1] The outer extent of the Oort cloud defines the boundary of our Solar System; placing it at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun. The Kuiper belt and scattered disc, the other two known reservoirs of trans-Neptunian objects, are roughly one thousand times closer in.

The Oort cloud is thought to comprise two separate regions: a spherical outer Oort cloud and a disc-shaped inner Oort cloud (or Hills cloud). Objects in the Oort cloud are largely composed of ices such as water, ammonia and methane. Astronomers believe that the matter comprising the Oort cloud was formed closer to the Sun, and was scattered far out into space by the gravitational effects of the giant planets early in the Solar System's evolution.[1]

Although no confirmed direct observations of the Oort cloud have been made, astronomers believe that it is the source of all long period and Halley-type comets entering the inner Solar System, and many of the Centaurs and the Jupiter family of comets as well.[2] The outer Oort cloud is only loosely bound to the Solar System, and thus is easily affected by the gravitational pull of both the Milky Way galaxy and passing stars. These forces occasionally dislodge comets from their orbits within the cloud and send them towards the inner Solar System.[1] Based on their orbits, most of the short-period comets may come from the Kuiper belt, but some may still have originated from the Oort Cloud.[1][2] Although the Kuiper belt and the farther scattered disc have been observed and mapped, only two currently known trans-Neptunian objects, 90377 Sedna and 2000 CR105, are considered possible members of the inner Oort cloud.

Hypothesis

File:Oort10.jpg
Jan Oort at Leiden University, in 1976

In 1932 Estonian astronomer Ernst Öpik postulated that long-period comets originated in an orbiting cloud at the outermost edge of the Solar System.[3] In 1950 the idea was independently revived by Dutch astronomer Jan Hendrik Oort as a means to resolve a paradox:[4] over the course of the Solar System's existence, the orbits of comets are unstable: eventually, dynamics dictate that a comet must either collide with the Sun or a planet, or else be ejected from the Solar System by planetary perturbations. Moreover, their volatile composition means that as they repeatedly approach the Sun, radiation gradually boils off the volatiles until the comet splits or develops an insulating crust that prevents further outgassing. Thus, reasoned Oort, a comet could not have formed on its current orbit, and must have been held in an outer reservoir for almost all of its existence.[4][5][6]

There are two main classes of comets: ecliptic comets and nearly isotropic comets. Ecliptic comets have relatively short orbits below 10 AU, and follow the ecliptic plane, the same plane in which the planets lie. Nearly isotropic comets have very long orbits, on the order of thousands of AU, and appear from every corner of the sky.[6] Oort noted that there was a peak in numbers of nearly isotropic comets with aphelia—their farthest distance from the Sun—of roughly 20,000 AU, which suggested a reservoir at that distance with a spherical, isotropic distribution.[6] Those relatively rare comets with orbits of about 10,000 AU have probably gone through one or more cycles through the Solar System and have had their orbits drawn inward by the gravity of the planets.[6]

Structure and composition

The presumed distance of the Oort cloud compared to the rest of the Solar System

The Oort cloud is thought to occupy a vast space from somewhere between 2,000 and 5,000 AU[6] to as far as 50,000 AU[1] from the Sun. Some estimates even place the outer edge at between 100,000 and 200,000 AU;[6] the outermost extent of the Sun's gravitational pull. It can be subdivided into a spherical outer Oort cloud (20,000–50,000 AU) and a doughnut-shaped inner Oort cloud (2,000–20,000 AU). The outer cloud is only weakly bound to the Sun and supplies the long period (and possibly Halley-type) comets to the inner part of the Solar System.[1] The inner Oort cloud is also known as the Hills cloud after J. G. Hills, who proposed its existence in 1981.[7] Models predict that the inner cloud has tens or hundreds times as many cometary nuclei as the outer halo,[8][7][9] and thus is seen as a possible source of new comets for the relatively tenuous outer cloud as that gradually becomes depleted. This hypothesis is employed to explain the continued existence of the Oort cloud after billions of years.[10]

The outer Oort cloud is believed to contain several trillion individual comet nuclei larger than approximately 1.3 km[1] (about 500 billion with absolute magnitudes[11] less than 10.9), with neighboring comets typically tens of millions of kilometres apart.[2][12] Its total mass is not known with certainty, but, assuming that Halley's comet is a suitable prototype for all comets within the outer Oort cloud, the estimated combined mass is 3x1038 grams, or roughly three times the mass of the Earth.[1][13] Earlier it was thought to be more massive; up to 380 Earth masses.[14] However, improved knowledge of the size distribution of long period comets has led to much lower values. The mass of the inner Oort cloud is not currently known.

If analyses of comets are representative of the whole, the vast majority of Oort cloud objects consist of various ices, such as water, methane, ethane, carbon monoxide and hydrogen cyanide.[15] However, the discovery of the object 1996 PW, an asteroid in an unusual eccentric orbit, suggests that it may also be home to rocky objects.[16] Analysis of the carbon and nitrogen isotope ratios in both the Oort cloud and Jupiter-family comets shows little difference between the two, despite their vastly separate regions of origin. This suggests that both originated from the original protosolar cloud.[17] This conclusion is also supported by studies of granular size in Oort cloud comets,[18] and by the recent impact study of Comet Tempel 1.[19]

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.[1] The most widely-accepted hypothesis is that the Oort cloud's objects initially coalesced much closer to the Sun as part of the same process that formed the planets and asteroids, but that gravitational interaction with young gas giant planets such as Jupiter ejected the objects into extremely long elliptic or parabolic orbits.[1][20] Simulations of the evolution of the Oort cloud from the beginnings of the Solar System to the present suggest that the cloud's mass peaked around 800 million years after formation, as the pace of accretion and collision slowed and depletion began to overtake supply.[1]

Models by Julio Ángel Fernández et al. suggest that the scattered disc, which is the main source for periodic comets in the Solar System, might also be the primary source for the Oort cloud objects. According to the models, about half of objects that are scattered travel outward towards the Oort cloud, while a quarter are shifted inward to Jupiter's orbit, and a quarter are ejected on hyperbolic orbits. The scattered disc might still be supplying the Oort cloud with material.[21] A third of the scattered disc's population is likely to end up in the Oort cloud after 2.5 billion years.[22]

Computer models suggest that collisions of cometary debris during the formation period play a far greater role than was previously thought. According to these models, the number of collisions early in the Solar System's history was so great that most comets were destroyed before they reached the Oort cloud. Therefore, the current cumulative mass of the Oort cloud is far less than was once suspected.[23] The estimated mass of the cloud is about 3 Earth masses, which is only a small part of the 50–100 Earth masses of ejected material.[1]

Gravitational interaction with nearby stars and galactic tides modified cometary orbits to make them more circular. This explains the near spherical shape of the outer Oort cloud. On the other hand, the Hills cloud, being bound more strongly to the Sun, hasn't acquired a spherical shape yet. 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 of 200–400 stars. These early stars likely played a role in the cloud's formation.[24]

Comets

Comet Hale-Bopp, an archetypal Oort cloud comet

Comets are believed to have two separate points of origin in the Solar System. Short period comets (those with orbits lasting 200 years or less) are generally accepted to have emerged from the Kuiper belt, a flat disc of icy debris between Pluto's orbit at 38 AU out to 50 AU from the Sun, while long period comets, such as Hale-Bopp, whose orbits last for thousands of years, are thought to originate in the Oort cloud. Halley-family comets, named for their prototype, Comet Halley, are unusual in that while they are short period comets, their ultimate origin lies in the Oort cloud, not the Kuiper belt. Based on their orbits, it is believed they were long period comets that were captured by the gravity of the giant planets and sent into the inner Solar System.[5] This process may have created the present orbits of a significant fraction of the Jupiter family comets (those with semi-major axes of less than 5 AU), although the majority of such comets are thought to have originated in the Kuiper belt.[2]

Oort noted that the number of returning comets was far less than his model predicted, and this issue, known as "cometary fading", has yet to be resolved. No known dynamical process can explain this undercount of observed comets. Hypotheses for this discrepancy include the destruction of comets due to tidal stresses, impact or heating; the loss of all volatiles, rendering some comets invisible, or the formation of a non-volatile crust on the surface.[25] Dynamical studies of Oort cloud comets have shown that their occurrence in the outer planet region is several times higher than in the inner planet region. This discrepancy may be due to the gravitational attraction of Jupiter, which acts as a kind of barrier, trapping incoming comets and causing them to collide with it, just as it did the comet Shoemaker-Levy 9 in 1994.[26]

Jupiter's role in protecting Earth from collision with potentially devastating long-period comets may have played an important role in the evolution of life.[27] However, other astronomers argue that, although it may play a role in deflecting or absorbing long-period comets, Jupiter may actually negate this by disturbing the orbits of the centaur population, a class of short-period comets in transition to the inner Solar System. Models suggest that the combined effect of Jupiter's presence is little different from if it had not been there at all.[28]

Tidal effects

Main article: Galactic tide

Most of the comets seen close to the Sun are believed to have reached their current positions through gravitational distortion of the Oort cloud by the tidal force exerted by the Milky Way galaxy. Just as the Moon's tidal force bends and deforms the Earth's oceans, causing the tides to rise and fall, so the galactic tide also bends and distorts bodies in the outer Solar System, pulling them towards the galactic centre. In the inner regions of the Solar System, these effects are negligible compared to the gravity of the Sun. At the outer reaches of the system, however, the Sun's gravity is weaker and the gradient of the Milky Way's gravitational field plays a far more noticeable role. Because of this gradient, galactic tides can deform an otherwise spherical Oort cloud, stretching the cloud in the direction of the galactic centre and compressing it along the other two axes. These small galactic perturbations may be enough to dislodge members of the Oort cloud from their orbits, sending them towards the Sun.[29] The point at which the Sun's gravity concedes its influence to the galactic tide is called the tidal truncation radius. It lies at a radius of 100,000 to 200,000 AU, and marks the outer boundary of the Oort cloud.[6]

Some scholars theorize that the galactic tide may have contributed to the formation of the Oort cloud, by increasing the perihelia of planetesimals with large aphelia.[30] The effects of the galactic tide are quite complex, and depend heavily on the behaviour of individual objects within a planetary system. Cumulatively, however, the effect can be quite significant: up to 90% of all comets originating from the Oort cloud may be the result of the galactic tide.[31] Statistical models of the observed orbits of long period comets argue that the galactic tide is the principal means by which their orbits are perturbed toward the inner Solar System.[32]

Star perturbations and stellar companion hypotheses

Besides the galactic tide, the main trigger for sending comets into the inner Solar System is believed to be interaction between the Sun's Oort cloud and the gravitational fields of near-by stars[1] or giant molecular clouds.[26] The orbit of the Sun through the plane of the Milky Way sometimes brings it in relatively close proximity to other stellar systems. For example, during the next 10 million years the known star with the greatest possibility of perturbing the Oort cloud is Gliese 710.[33] This process also serves to scatter the objects out of the ecliptic plane, potentially also explaining the cloud's spherical distribution.[34][33]

Physicist Richard A. Muller and others have also postulated that the Sun has a heretofore undetected companion, either a brown dwarf or gaseous giant planet, in an elliptical orbit beyond the Oort cloud. This object, 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.[35] Furthermore, many argue that a companion star at such a great distance could not have a stable orbit, as it would probably be ejected by perturbations from other stars.

A similar hypothesis has been advanced by astronomer John J. Matese of the University of Louisiana. He contends that more comets are arriving in the inner Solar System from a particular region of the Oort cloud than can be explained by the galactic tide or stellar perturbations alone, and that the most likely scenario is a bound Jupiter-mass object in a distant orbit.[36]

Matese has also offered an alternative to the Nemesis hypothesis; that the possible periodicity in terrestrial mass extinctions may be due instead to vertical oscillations made by the Sun as it travels through the Milky Way. When the Sun's orbit takes it outside the galactic disc, the influence of the galactic tide is weaker; as it re-enters the galactic disc, as it does every 20–25 million years, it comes under the influence of the far stronger "disc tides", which, according to mathematical models, increase the flux of Oort cloud comets into the Solar System by a factor of 4.[37]

Oort cloud objects (OCOs)

Sedna, a possible inner Oort cloud object discovered in 2004

Only two objects have been discovered that have orbits which suggest that they may belong to the Oort Cloud: 90377 Sedna and 2000 CR105. Unlike scattered disc objects, their orbits cannot be explained by perturbations of the gas giant planets and they may be interlopers to the inner Oort cloud. If they formed in their current locations, their orbits must originally have been circular; otherwise accretion would not have been possible because the large relative velocities of planetesimals would have been too disruptive.[38] Their orbits can be explained by a number of hypotheses: these objects could have had their orbits and perihelion distances "lifted" by the passage of a nearby star when the Sun was still embedded in its birth star cluster,[39] their orbits could have been disrupted by an as-yet-unknown planet-sized body within the Oort cloud,[40] they could have been scattered by Neptune during a period of particularly high eccentricity or by the gravity of a far larger primordial trans-Neptunian disc, or they could have been captured from around smaller passing stars. Of these, the stellar disruption and "lift" hypothesis appear to agree more closely with observations.[39] Some astronomers prefer to refer to Sedna and 2000 CR105 as belonging to the "extended scattered disc" rather than to the inner Oort cloud.[38]

Oort cloud object candidates
Number Name Equatorial diameter
(km) 		Perihelion (AU) Aphelion (AU) Year discovered Discoverer Diameter method
90377 Sedna 1180–1800 km 76.1 892 2003 Brown, Trujillo, Rabinowitz thermal[41]
148209 2000 CR105 ~250 km 44.3 397 2000 Lowell Observatory assumed[42]

See also

References

  1. ^ a b c d e f g h i j k l m Alessandro Morbidelli (2008-02-03). "Origin and dynamical evolution of comets and their reservoirs" (PDF). arxiv. Retrieved 2007-05-26. {{cite web}}: Check date values in: |year= / |date= mismatch (help)
  2. ^ a b c d V. V. Emelyanenko, D. J. Asher, M. E. Bailey (2007). "The fundamental role of the Oort cloud in determining the flux of comets through the planetary system". Monthly Notices of the Royal Astronomical Society. 381 (2). Royal Astronomical Society: 779–789. doi:10.1111/j.1365-2966.2007.12269.x. Retrieved 2008-03-31.{{cite journal}}: CS1 maint: multiple names: authors list (link) CS1 maint: unflagged free DOI (link)
  3. ^ Ernst Julius Öpik (1932). "Note on Stellar Perturbations of Nearby Parabolic Orbits". Proceedings of the American Academy of Arts and Sciences. 67: 169–182.
  4. ^ a b Jan Oort (1950). "The structure of the cloud of comets surrounding the Solar System and a hypothesis concerning its origin" (PDF). Bull. Astron. Inst. Neth. 11: 91–110.
  5. ^ a b David C. Jewitt (2001). "From Kuiper Belt to Cometary Nucleus: The Missing Ultrared Matter". University of Chicago. Retrieved 2007-06-26.
  6. ^ a b c d e f g Harold F. Levison, Luke Donnes (2007). "Comet Populations and Cometary Dynamics". In Lucy Ann Adams McFadden, Lucy-Ann Adams, Paul Robert Weissman, Torrence V. Johnson (ed.). Encyclopedia of the Solar System (2nd ed.). Amsterdam; Boston: Academic Press. pp. 575–588. ISBN 0120885891.{{cite book}}: CS1 maint: multiple names: editors list (link)
  7. ^ a b Jack G. Hills (1981). "Comet showers and the steady-state infall of comets from the Oort cloud". The Astronomical Journal. 86: 1730–1740. doi:10.1086/113058. {{cite journal}}: Unknown parameter |month= ignored (help)
  8. ^ Harold F. Levison, Luke Dones, Martin J. Duncan (2001). "The Origin of Halley-Type Comets: Probing the Inner Oort Cloud". The Astronomical Journal. The American Astronomical Society. pp. 2253–2267. Retrieved 2007-06-27. {{cite web}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  9. ^ Thomas M. Donahue, ed. (1991). Planetary Sciences: American and Soviet Research, Proceedings from the U.S.-U.S.S.R. Workshop on Planetary Sciences. Kathleen Kearney Trivers, and David M. Abramson. National Academy Press. p. 251. ISBN 0-309-04333-6. {{cite book}}: |access-date= requires |url= (help); Unknown parameter |link= ignored (help)
  10. ^ Julio A. Fernéndez (1997-04-07). "The Formation of the Oort Cloud and the Primitive Galactic Environment" (PDF). Icarus (219). Elsevier: 106–119. Retrieved 2008-03-18.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  11. ^ Absolute magnitude is a measure of how bright an object would be if it were exactly 1 AU from the Sun and Earth; as opposed to apparent magnitude, which measures how bright an object appears from Earth. Because all measurements of absolute magnitude assume the same distance, absolute magnitude is in effect a measurement of an object's true brightness. The lower an object's absolute magnitude, the brighter it is.
  12. ^ Paul R. Weissman (1998). "The Oort Cloud". Scientific American. Scientific American, Inc. Retrieved 2007-05-26.
  13. ^ Paul R. Weissman (1983-02-01). "The mass of the Oort cloud". Astronomy and Astrophysics. 118 (1). American Astronomical Society: 90–94. Retrieved 2008-03-31.
  14. ^ Sebastian Buhai. "On the Origin of the Long Period Comets: Competing theories". Utrecht University College. Retrieved 2008-03-29.
  15. ^ E. L. Gibb, M. J. Mumma, N. Dello Russo, M. A. DiSanti and K. Magee-Sauer (October 2003). "Methane in Oort cloud comets". Icarus. 165 (2). Elselvier: 391–406. doi:10.1016/S0019-1035(03)00201-X. Retrieved 2008-03-31.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  16. ^ 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. University of Chicago Press. Retrieved 2007-05-26.
  17. ^ D. Hutsemekers, J. Manfroid, E. Jehin, C. Arpigny, A. Cochran, R. Schulz, J.A. St¨uwe, and J.M. Zucconi (2005). "Isotopic abundances of carbon and nitrogen in Jupiter-family and Oort Cloud comets". Astronomy and Astrophysics. 440. American Astronomical Society: L21–L24. doi:10.1051/0004-6361:200500160. Retrieved 2008-03-31. {{cite journal}}: line feed character in |author= at position 91 (help)CS1 maint: multiple names: authors list (link)
  18. ^ Takafumi Ootsubo, Jun-ichi Watanabe, Hideyo Kawakita, Mitsuhiko Honda and Reiko Furusho (June 2007). "Grain properties of Oort cloud comets: Modeling the mineralogical composition of cometary dust from mid-infrared emission features". 55 (9). Elselvier: 1044–1049. doi:doi:10.1016/j.pss.2006.11.012. Retrieved 2008-03-31. {{cite journal}}: Check |doi= value (help); Cite journal requires |journal= (help); Unknown parameter |Journal= ignored (|journal= suggested) (help)CS1 maint: date and year (link) CS1 maint: multiple names: authors list (link)
  19. ^ Michael J. Mumma, Michael A. DiSanti, Karen Magee-Sauer; et al. (2005-09-15). "Parent Volatiles in Comet 9P/Tempel 1: Before and After Impact". Science Express. 310 (5746). Nature Publishing Group: 270–274. doi:10.1126/science.1119337. Retrieved 2008-03-22. {{cite journal}}: Explicit use of et al. in: |author= (help)CS1 maint: date and year (link) CS1 maint: multiple names: authors list (link)
  20. ^ "Oort Cloud & Sol b?". SolStation. Retrieved 2007-05-26.
  21. ^ Julio A. Fernández, Tabaré Gallardo and Adrián Brunini (December 2004). "The scattered disc population as a source of Oort cloud comets: evaluation of its current and past role in populating the Oort cloud". Icarus. 172 (2). Elsevier: 372–381. Retrieved 2008-03-31. {{cite journal}}: Text "doi:10.1016/j.icarus.2004.07.023" ignored (help)
  22. ^ Davies, J. K.; Barrera, L. H. (2004). The First Decadal Review of the Edgeworth-Kuiper Belt. Kluwer Academic Publishers.{{cite book}}: CS1 maint: multiple names: authors list (link)
  23. ^ S. Alan Stern, Paul R. Weissman (2001-02-01). "Rapid collisional evolution of comets during the formation of the Oort cloud". Nature. 409 (6820). Nature Publishing Group: 589–591. doi:10.1038/35054508. Retrieved 2008-03-31.
  24. ^ R. Brasser, M. J. Duncan, H.F. Levison (2006). "Embedded star clusters and the formation of the Oort Cloud". 184 (1). Elsevier: 59–82. doi:10.1016/j.icarus.2006.04.010. Retrieved 2007-05-26. {{cite journal}}: Cite journal requires |journal= (help); Unknown parameter |Journal= ignored (|journal= suggested) (help)CS1 maint: multiple names: authors list (link)
  25. ^ Luke Dones, Paul R Weissman, Harold F Levison, Martin J Duncan. "Oort Cloud Formation and Dynamics". In Michel C. Festou; H. Uwe Keller; Harold A. Weaver (ed.). Comets II. pp. 153–173. Retrieved 2008-03-22. {{cite book}}: External link in |chapterurl= (help); Unknown parameter |Publisher= ignored (|publisher= suggested) (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help)CS1 maint: multiple names: authors list (link)
  26. ^ a b Julio A. Fernández (October 2000). "Long-Period Comets and the Oort Cloud". Earth, Moon, and Planets. 89 (1–4). Springer Netherlands: 325–343. doi:10.1023/A:1021571108658. Retrieved 2008-03-25.
  27. ^ Bortman, Henry (2004-09-29). "Coming Soon: "Good" Jupiters". Astrobiology Magazine. Retrieved 2007-08-05.
  28. ^ Anita Heward (2007). "Jupiter: Friend or Foe?". European Planetology Network. Retrieved 2008-04-01.
  29. ^ Marc Fouchard, Christiane Froeschlé, Giovanni Valsecchi, Hans Rickman (2006). "Long-term effects of the galactic tide on cometary dynamics". Celestial Mechanics and Dynamical Astronomy. 95 (1–4). Springer. doi:10.1007/s10569-006-9027-8. {{cite journal}}: Unknown parameter |Pages= ignored (|pages= suggested) (help)CS1 maint: multiple names: authors list (link)
  30. ^ Higuchi A., Kokubo E. & Mukai, T. (2005). "Orbital Evolution of Planetesimals by the Galactic Tide". Bulletin of the American Astronomical Society. 37. American Astronomical Society: 521.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  31. ^ Nurmi P. (2001). "Periodic variation of Oort Cloud flux and cometary impacts on the Earth and Jupiter". Monthly Notices of the Royal Astronomical Society. 327. Blackwell: 1367–1376. doi:10.1046/j.1365-8711.2001.04854.x. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: unflagged free DOI (link)
  32. ^ John J. Matese and Jack J. Lissauer (August 2004). "Perihelion evolution of observed new comets implies the dominance of the galactic tide in making Oort cloud comets discernable". Icarus. 170 (2). Elselvier: 508–513. doi:10.1016/j.icarus.2004.03.019. Retrieved 2008-03-21.
  33. ^ a b L. A. Molnar, R. L. Mutel (1998-01-09). Close Approaches of Stars to the Oort Cloud: Algol and Gliese 710. American Astronomcial Society 191st meeting. The American Astronomcial Society. Retrieved 2008-03-31.
  34. ^ A. Higuchi, E. Kokubo and T. Mukai (February 2006). "Scattering of Planetesimals bya a Planet: Formation of Comet cloud Candidates". The Astronomical Journal. 131. The American Astronomical Society: 1119–1129. Retrieved 2007-05-27.
  35. ^ J. G. Hills (1984-10-18). "Dynamical constraints on the mass and perihelion distance of Nemesis and the stability of its orbit". Nature. 311. Nature Publishing Group: 636–638. doi:10.1038/311636a0. Retrieved 2008-03-25.
  36. ^ John J. Matese and Jack J. Lissauer (2002-05-06). "Continuing Evidence of an Impulsive Component of Oort Cloud Cometary Flux" (PDF). 1University of Louisiana at Lafayette, and NASA Ames Research Center. Retrieved 2008-03-21.
  37. ^ Michael Szpir. "Perturbing the Oort Cloud". American Scientist. The Scientific Research Society. Retrieved 2008-03-25.
  38. ^ a b Scott S. Sheppard (2005). "Small Bodies in the Outer Solar System" (PDF). Frank N. Bash Symposium. The University of Texas at Austin. Retrieved 2008-03-25. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  39. ^ a b Alessandro Morbidelli (2004). "Scenarios for the Origin of the Orbits of the Trans-Neptunian Objects 2000 CR105 and 2003 VB12 (Sedna)". The Astronomical Journal. 128 (5). University of Chicago Press: 2564–2576. doi:10.1086/424617. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  40. ^ Rodney S. Gomes (2006). "A distant planetary-mass solar companion may have produced distant detached objects". Icarus. 184 (2). Elsevier: 589–601. doi:10.1016/j.icarus.2006.05.026. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  41. ^ W.M. Grundy, K.S. Noll and D.C. Stephens (July 2005). "Diverse albedos of small trans-Neptunian objects". Icarus. 176 (1). Elsevier: 184–191. doi:10.1016/j.icarus.2005.01.007. Retrieved 2008-03-22. (arxiv.org PDF)
  42. ^ E. L. Schaller and M. E. Brown (2007). "Volatile loss and retention on Kuiper belt objects". Astrophysical Journal. 659: I.61–I.64. Retrieved 2008-04-02. (PDF)


Retrieved from "https://en.wikipedia.org/w/index.php?title=Oort_cloud&oldid=203509364"