|‡ Trans-Neptunian dwarf planets are
The Oort cloud // (named after the Dutch astronomer Jan Oort), or Öpik–Oort cloud, is a spherical cloud of predominantly icy planetesimals that is believed to surround the Sun at a distance up to 50,000 AU, nearly a light-year. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun. The Kuiper belt and the scattered disc, the other two reservoirs of trans-Neptunian objects, are less than one thousandth of the Oort cloud's distance. The outer limit of the Oort cloud defines the cosmographical boundary of the Solar System and the region of the Sun's gravitational dominance.
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 argue that the matter composing the Oort cloud 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. Although no confirmed direct observations of the Oort cloud have been made, astronomers argue that it is the source of all long-period and Halley-type comets entering the inner Solar System and many of the centaurs and Jupiter-family comets as well. The outer Oort cloud is only loosely bound to the Solar System, and thus is easily affected by the gravitational pull both of passing stars and of the Milky Way itself. These forces occasionally dislodge comets from their orbits within the cloud and send them towards the inner Solar System. Based on their orbits, most of the short-period comets may come from the scattered disc, but some may still have originated from the Oort cloud.
In 1932, the Estonian astronomer Ernst Öpik postulated that long-period comets originated in an orbiting cloud at the outermost edge of the Solar System. In 1950, the idea was independently revived by the Dutch astronomer Jan Hendrik Oort as a means to resolve a paradox: 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 the volatiles off until the comet splits or develops an insulating crust that prevents further outgassing. Thus, Oort reasoned, a comet could not have formed while in its current orbit, and must have been held in an outer reservoir for almost all of its existence.
There are two main classes of comet, short-period comets (also called ecliptic comets) and long-period comets (also called nearly isotropic comets). Ecliptic comets have relatively small orbits, below 10 AU, and follow the ecliptic plane, the same plane in which the planets lie. All long-period comets have very large orbits, on the order of thousands of AU, and appear from every direction in the sky. Oort noted that there was a peak in numbers of long-period 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. Those relatively rare comets with orbits of about 10,000 AU have probably gone through one or more orbits through the Solar System and have had their orbits drawn inward by the gravity of the planets.
Structure and composition
The Oort cloud is thought to occupy a vast space from somewhere between 2,000 and 5,000 AU (0.03 and 0.08 ly) to as far as 50,000 AU (0.79 ly) from the Sun. Some estimates place the outer edge at between 100,000 and 200,000 AU (1.58 and 3.16 ly). The region can be subdivided into a spherical outer Oort cloud of 20,000–50,000 AU (0.32–0.79 ly), and a doughnut-shaped inner Oort cloud of 2,000–20,000 AU (0.03–0.32 ly). The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets to inside the orbit of Neptune. The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981. Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo; it is seen as a possible source of new comets to resupply the relatively tenuous outer cloud as the latter's numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years.
The outer Oort cloud is thought to contain several trillion individual objects larger than approximately 1 km (0.62 mi), and many billions with absolute magnitudes brighter than 11 (corresponding to approximately 20-kilometre (12 mi) diameter), with neighboring objects typically tens of millions of kilometres apart. 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 3×1025 kilograms (6.6×1025 pounds), or roughly five times the mass of the Earth. Earlier it was thought to be more massive (up to 380 Earth masses), but improved knowledge of the size distribution of long-period comets has led to much lower estimates. 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. However, the discovery of the object 1996 PW, an asteroid in an orbit more typical of a long-period comet, suggests that the cloud may also contain rocky objects. Analysis of the carbon and nitrogen isotope ratios in both the long-period and Jupiter-family comets shows little difference between the two, despite their presumably vastly separate regions of origin. This suggests that both originated from the original protosolar cloud, a conclusion also supported by studies of granular size in Oort-cloud comets and by the recent impact study of Jupiter-family comet Tempel 1.
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. 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. Recent research has been cited by NASA hypothesizing that a large number of Oort cloud objects are the product of an exchange of materials between the Sun and its sibling stars as they formed and drifted apart, and it is suggested that many—possibly the majority—of Oort cloud objects were not formed in close proximity to the Sun. 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.
Models by Julio Ángel Fernández suggest that the scattered disc, which is the main source for periodic comets in the Solar System, might also be the primary source for Oort cloud objects. According to the models, about half of the objects 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. A third of the scattered disc's population is likely to end up in the Oort cloud after 2.5 billion years.
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. The estimated mass of the cloud is only a small part of the 50–100 Earth masses of ejected material.
Gravitational interaction with nearby stars and galactic tides modified cometary orbits to make them more circular. This explains the nearly spherical shape of the outer Oort cloud. On the other hand, the Hills cloud, which is bound more strongly to the Sun, has yet to acquire a spherical shape. 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, since the number of close stellar passages within the cluster was much higher than today, leading to far more frequent perturbations.
In June 2010 Harold F. Levison and others suggested on the basis of enhanced computer simulations that the Sun "captured comets from other stars while it was in its birth cluster". Their results imply that "a substantial fraction of the Oort cloud comets, perhaps exceeding 90%, are from the protoplanetary discs of other stars".
Comets are thought to have two separate points of origin in the Solar System. Short-period comets (those with orbits of up to 200 years) are generally accepted to have emerged from either the Kuiper belt or the scattered disc, which are two linked flat discs of icy debris beyond Neptune's orbit at 30 AU and jointly extending out beyond 100 AU from the Sun. Long-period comets, such as comet Hale–Bopp, whose orbits last for thousands of years, are thought to originate in the Oort cloud. The orbits within the Kuiper belt are relatively stable, and so very few comets are thought to originate there. The scattered disc, however, is dynamically active, and is far more likely to be the place of origin for comets. Comets pass from the scattered disc into the realm of the outer planets, becoming what are known as centaurs. These centaurs are then sent farther inward to become the short-period comets.
There are two main varieties of short-period comet: Jupiter-family comets (those with semi-major axes of less than 5 AU) and Halley-family comets. Halley-family comets, named for their prototype, Halley's Comet, are unusual in that while they are short-period comets, it is hypothesized that their ultimate origin lies in the Oort cloud, not in the scattered disc. Based on their orbits, it is suggested they were long-period comets that were captured by the gravity of the giant planets and sent into the inner Solar System. This process may have also created the present orbits of a significant fraction of the Jupiter-family comets, although the majority of such comets are thought to have originated in the scattered disc.
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. 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 with Comet Shoemaker–Levy 9 in 1994.
Most of the comets seen close to the Sun seem to have reached their current positions through gravitational distortion of the Oort cloud by the tidal force exerted by the Milky Way. Just as the Moon's tidal force bends and deforms the Earth's oceans, causing the tides to rise and fall, the galactic tide also bends and distorts the orbits of bodies in the outer Solar System, pulling them towards the galactic centre. In the charted 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. 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.
Some scholars theorise that the galactic tide may have contributed to the formation of the Oort cloud by increasing the perihelia—closest distances to the Sun—of planetesimals with large aphelia. 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. 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.
Star perturbations and stellar companion hypotheses
Besides the galactic tide, the main trigger for sending comets into the inner Solar System is thought to be interaction between the Sun's Oort cloud and the gravitational fields of nearby stars or giant molecular clouds. 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. This process also serves to scatter the objects out of the ecliptic plane, potentially also explaining the cloud's spherical distribution.
In 1984, Physicist Richard A. Muller postulated that the Sun has a heretofore undetected companion, either a brown dwarf or a red dwarf, in an elliptical orbit within the Oort cloud. This object, known as Nemesis, was hypothesized to pass through a portion of the Oort cloud approximately every 26 million years, bombarding the inner Solar System with comets. However, to date no evidence of Nemesis has been found, and many lines of evidence (such as crater counts), have thrown its existence into doubt. Recent scientific analysis no longer supports the idea that extinctions on Earth happen at regular, repeating intervals. Thus, the Nemesis hypothesis is no longer needed.
A somewhat similar hypothesis was advanced by astronomer John J. Matese of the University of Louisiana at Lafayette in 2002. 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 cause is a Jupiter-mass object in a distant orbit. This hypothetical gas giant planet has been nicknamed Tyche. One all-sky survey using parallax measurements in order to clarify local star distances, the WISE mission, is currently underway with part of its mission being to help with either proving or disproving the Tyche hypothesis.
Modified Newtonian dynamics within the Oort cloud
Modified Newtonian dynamics (MOND) suggests that at their distances from the Sun, the objects comprising the Oort cloud should experience accelerations of the order of 10−10 m/s2, and thus should be within the realms at which deviations from Newtonian predictions come into effect. According to this hypothesis, which was proposed to account for the discrepancies in the galaxy rotation curve, which are more commonly attributed to dark matter, acceleration ceases to be linearly proportional to force at very low accelerations. If correct, this would have significant implications regarding the formation and structure of the Oort cloud. However, the majority of cosmologists do not consider MOND a valid hypothesis.
- Interstellar comet
- Kuiper belt
- List of possible dwarf planets
- List of trans-Neptunian objects
- Scattered disc
- Tyche (hypothetical planet)
- "Oort". Oxford English Dictionary (3rd ed.). Oxford University Press. September 2005.
- Whipple, F. L.; Turner, G.; McDonnell, J. A. M.; Wallis, M. K. (1987-09-30). "A Review of Cometary Sciences". Philosophical Transactions of the Royal Society A (Royal Society Publishing) 323 (1572): 339–347 . Bibcode:1987RSPTA.323..339W. doi:10.1098/rsta.1987.0090.
- Alessandro Morbidelli (2006). "Origin and dynamical evolution of comets and their reservoirs of water ammonia and methane.". arXiv:astro-ph/0512256 [astro-ph].
- "Kuiper Belt & Oort Cloud". NASA Solar System Exploration web site. NASA. Retrieved 2011-08-08.
- 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): 779–789. Bibcode:2007MNRAS.381..779E. doi:10.1111/j.1365-2966.2007.12269.x.
- Ernst Julius Öpik (1932). "Note on Stellar Perturbations of Nearby Parabolic Orbits". Proceedings of the American Academy of Arts and Sciences 67 (6): 169–182. doi:10.2307/20022899. JSTOR 20022899.
- Jan Oort (1950). "The structure of the cloud of comets surrounding the Solar System and a hypothesis concerning its origin". Bulletin of the Astronomical Institutes of the Netherlands 11: 91–110. Bibcode:1950BAN....11...91O.
- David C. Jewitt (2001). "From Kuiper Belt to Cometary Nucleus: The Missing Ultrared Matter". Astronomical Journal 123 (2): 1039–1049. Bibcode:2002AJ....123.1039J. doi:10.1086/338692.
- 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. Encyclopedia of the Solar System (2nd ed.). Amsterdam; Boston: Academic Press. pp. 575–588. ISBN 0-12-088589-1.
- Jack G. Hills (1981). "Comet showers and the steady-state infall of comets from the Oort Cloud". Astronomical Journal 86: 1730–1740. Bibcode:1981AJ.....86.1730H. doi:10.1086/113058.
- Harold F. Levison, Luke Dones, Martin J. Duncan (2001). "The Origin of Halley-Type Comets: Probing the Inner Oort Cloud". Astronomical Journal 121 (4): 2253–2267. Bibcode:2001AJ....121.2253L. doi:10.1086/319943.
- 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. Retrieved 2008-03-18.
- Julio A. Fernéndez (1997). "The Formation of the Oort Cloud and the Primitive Galactic Environment". Icarus 219: 106–119. Bibcode:1997Icar..129..106F. doi:10.1006/icar.1997.5754. Retrieved 2008-03-18.
- 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.
- Paul R. Weissman (1998). "The Oort Cloud". Scientific American. Retrieved 2007-05-26.
- Paul R. Weissman (1983). "The mass of the Oort Cloud". Astronomy and Astrophysics 118 (1): 90–94. Bibcode:1983A&A...118...90W.
- Sebastian Buhai. "On the Origin of the Long Period Comets: Competing theories". Utrecht University College. Archived from the original on 2006-09-30. Retrieved 2008-03-29.
- E. L. Gibb, M. J. Mumma, N. Dello Russo, M. A. DiSanti and K. Magee-Sauer (2003). "Methane in Oort Cloud comets". Icarus 165 (2): 391–406. Bibcode:2003Icar..165..391G. doi:10.1016/S0019-1035(03)00201-X.
- Paul R. Weissman, Harold F. Levison (1997). "Origin and Evolution of the Unusual Object 1996 PW: Asteroids from the Oort Cloud?". Astrophysical Journal 488 (2): L133–L136. Bibcode:1997ApJ...488L.133W. doi:10.1086/310940.
- 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 (2): L21–L24. arXiv:astro-ph/0508033. Bibcode:2005A&A...440L..21H. doi:10.1051/0004-6361:200500160.
- Takafumi Ootsubo, Jun-ichi Watanabe, Hideyo Kawakita, Mitsuhiko Honda and Reiko Furusho (2007). "Grain properties of Oort Cloud comets: Modeling the mineralogical composition of cometary dust from mid-infrared emission features". Highlights in Planetary Science, 2nd General Assembly of Asia Oceania Geophysical Society 55 (9): 1044–1049. Bibcode:2007P&SS...55.1044O. doi:10.1016/j.pss.2006.11.012.
- Michael J. Mumma, Michael A. DiSanti, Karen Magee-Sauer et al. (2005). "Parent Volatiles in Comet 9P/Tempel 1: Before and After Impact". Science Express 310 (5746): 270–274. Bibcode:2005Sci...310..270M. doi:10.1126/science.1119337. PMID 16166477.
- "Oort Cloud & Sol b?". SolStation. Retrieved 2007-05-26.
- "The Sun Steals Comets from Other Stars". NASA. 2010.
- Julio A. Fernández, Tabaré Gallardo and Adrián Brunini (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): 372–381. Bibcode:2004Icar..172..372F. doi:10.1016/j.icarus.2004.07.023.
- Davies, J. K.; Barrera, L. H. (2004). The First Decadal Review of the Edgeworth-Kuiper Belt.. Kluwer Academic Publishers. ISBN 978-1-4020-1781-0.
- S. Alan Stern, Paul R. Weissman (2001). "Rapid collisional evolution of comets during the formation of the Oort Cloud". Nature 409 (6820): 589–591. Bibcode:2001Natur.409..589S. doi:10.1038/35054508. PMID 11214311.
- R. Brasser, M. J. Duncan, H.F. Levison (2006). "Embedded star clusters and the formation of the Oort Cloud". Icarus 184 (1): 59–82. Bibcode:2006Icar..184...59B. doi:10.1016/j.icarus.2006.04.010.
- Harold F. Levison (2010), "Capture of the Sun's Oort Cloud from Stars in Its Birth Cluster" (Science June 10, 2010) (SwRI) News
- Harold E. Levison and Luke Dones (2007). "Comet Populations and Cometary dynamics". Encyclopedia of the Solar System: 575–588. doi:10.1016/B978-012088589-3/50035-9. ISBN 978-0-12-088589-3.
- J Horner, NW Evans, ME Bailey, DJ Asher (2003). "The Populations of Comet-like Bodies in the Solar System". Retrieved 2007-06-29.
- Luke Dones, Paul R Weissman, Harold F Levison, Martin J Duncan (2004). "Oort Cloud Formation and Dynamics". In Michel C. Festou; H. Uwe Keller; Harold A. Weaver. Comets II. University of Arizona Press. pp. 153–173. Retrieved 2008-03-22.
- Julio A. Fernández (2000). "Long-Period Comets and the Oort Cloud". Earth, Moon, and Planets 89 (1–4): 325–343. Bibcode:2002EM&P...89..325F. doi:10.1023/A:1021571108658.
- 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): 299–326. Bibcode:2006CeMDA..95..299F. doi:10.1007/s10569-006-9027-8.
- Higuchi A., Kokubo E. & Mukai, T.; Kokubo; Mukai (2005). "Orbital Evolution of Planetesimals by the Galactic Tide". Bulletin of the American Astronomical Society 37: 521. Bibcode:2005DDA....36.0205H.
- Nurmi P.; Valtonen M.J., Zheng J.Q. (2001). "Periodic variation of Oort Cloud flux and cometary impacts on the Earth and Jupiter". Monthly Notices of the Royal Astronomical Society 327 (4): 1367–1376. Bibcode:2001MNRAS.327.1367N. doi:10.1046/j.1365-8711.2001.04854.x.
- John J. Matese and Jack J. Lissauer (2004). "Perihelion evolution of observed new comets implies the dominance of the galactic tide in making Oort Cloud comets discernible". Icarus 170 (2): 508–513. Bibcode:2004Icar..170..508M. doi:10.1016/j.icarus.2004.03.019.
- L. A. Molnar, R. L. Mutel (1997). "Close Approaches of Stars to the Oort Cloud: Algol and Gliese 710". American Astronomical Society 191st meeting. American Astronomical Society. Retrieved 2014-01-19.
- A. Higuchi, E. Kokubo and T. Mukai (2006). "Scattering of Planetesimals by a Planet: Formation of Comet Cloud Candidates". Astronomical Journal 131 (2): 1119–1129. Bibcode:2006AJ....131.1119H. doi:10.1086/498892.
- J. G. Hills (1984). "Dynamical constraints on the mass and perihelion distance of Nemesis and the stability of its orbit". Nature 311 (5987): 636–638. Bibcode:1984Natur.311..636H. doi:10.1038/311636a0.
- "Nemesis is a myth". Max Planck Institute. 2011. Retrieved 2011-08-11.
- "Can WISE Find the Hypothetical 'Tyche'?". NASA/JPL. February 18, 2011. Retrieved 2011-06-15.
- John J. Matese and Jack J. Lissauer (2002-05-06). "Continuing Evidence of an Impulsive Component of Oort Cloud Cometary Flux". University of Louisiana at Lafayette, and NASA Ames Research Center. Retrieved 2008-03-21.
- Milgrom, M. (1983). "A modification of the newtonian dynamics as a possible alternative to the hidden mass hypothesis". Astrophysical Journal 270: 365–370. Bibcode:1983ApJ...270..365M. doi:10.1086/161130.
- Milgrom, M. (1986). "Solutions for the modified Newtonian dynamics field equation". Astrophysical Journal 302: 617–625. Bibcode:1986ApJ...302..617M. doi:10.1086/164021.
- Sean Carroll. "Dark Matter: Just Fine, Thanks". Discover. Retrieved 2011-03-04.
- Paul Gilster (2008-11-12). "An Inflatable Sail to the Oort Cloud". Centauri-dreams.org. Retrieved 2013-07-23.
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|Wikimedia Commons has media related to Oort cloud.|
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