Nube de Oort: Diferenzas entre revisións
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{{Outroshomónimos|este=a nube de Oort exterior|para=a nube de Oort interior|nube de Hills}}
{{Imaxe múltiple
|dirección = vertical
|align = right
|ancho = 300
|foto1=PIA17046 - Voyager 1 Goes Interstellar.jpg
|foto2=Kuiper oort-en.svg
|caption1=This graphic shows the distance from the Oort cloud to the rest of the Solar System and two of the nearest stars measured in [[astronomical unit]]s. The scale is [[logarithmic scale|logarithmic]], with each specified distance ten times further out than the previous one. Red arrow indicates location of [[Voyager 1]], a [[space probe]] that will reach the Oort cloud in about 300 years.
|caption2=An [[artist's impression]] of the Oort cloud and the [[Kuiper belt]] (inset). Sizes of individual objects have been exaggerated for visibility.
}}
A '''Nube de Oort''', nomeada polo astrónomo neerlandés [[Jan Oort]] e en ocasións chamada a '''Nube de Öpik–Oort''',<ref name="Whipple" /> é unha nube [[teoría científica|teórica]] que estaría predominantemente composta de [[planetesimal|planetesimáis]] [[volátil (astronomía|volátiles]] que arrodearía ó [[Sol]] a unha distancia de entre as 2000 [[Unidade astronómica|UA]]s e as 200000 UAs.<ref group="note">The Oort cloud's outer limit is difficult to define as it varies over the [[millennia]] as [[List of nearest stars and brown dwarfs#Future and past|different stars pass the Sun]] and thus is subject to variation. Estimates of its distance range from 50,000 to 200,000 AU.</ref><ref name="Morbidelli2006" /> A súa existencia postulouna por primeira vez o astrónomo estonio [[Ernst Öpik]] no ano [[1932]].<ref name="Öpik-1932" /> Divídese en dúas rexións: unha Nube interior de Ooort en [[disco circunestelar|forma de disco]] coñecida como [[Nube de Hills]], e unha Nube de Oort exterior [[cuberta circunestelar|esférica]]. Ámbalas dúas rexións atoparíanse máis aló da [[heliosfera]], no [[espazo exterior|espazo interestelar]].<ref name="Morbidelli2006" /><ref name="jpl.PIA17046" /> En comparación, o [[Cinto de Kuiper]] e o [[disco disperso]], os outros dous depósitos de [[obxectos transneptunianos]], atópanse a menos dunha milésima parte de distancia do Sol que a Nube de Oort.
O límite exterior da Nube de oort define o límite [[cosmografía|cosmográfico]] do [[Sistema Solar]] e a extención da [[esfera de Hill]] do Sol.<ref name="NASA_SSE_oort" /> A Nube exterior de Oort tería unha débil ligazón co Sistema solar, e veríase doadamente afectada polo pulo gravitatorio de estrelas próximas e da propia [[Vía Láctea]]. Estas forzas serían as que ocasionalmente extraerían [[cometa]]s das súas orbitas dentro da nube, enviándoos cara o [[Sistema Solar interior]].<ref name=Morbidelli2006 /> Baseándose nas súas [[órbita]]s, a maioríados [[cometa de período curto|cometas de período curto]] procederían do disco disperso, máis algúns destes poderían ter a súa orixe na Nube de Oort.<ref name=Morbidelli2006 /><ref name=emel2007 />
Os astrónomos conxecturaron que a materia que compoñería a Nube de Oort estaría formada preto do Sol e posteriormente quedaría espallada lonxe no espacio por mor dos efectos gravitatorios dos [[planetas xigantes]] nos comezos da [[Formación e evolución do Sistema Solar|evolución do Sistema Solar]].<ref name=Morbidelli2006 /> Malia que non se realizaron observacións directas confirmadas da Nube de Oort, considérase que esta podería ser a fonte dos cometas de [[cometa de período longo|período longo]] e de [[Cometa Halley|tipo Halley]] que se adentran no Sistema Solar, así como de moitos dos cometas [[Centauro (astronomía)|centauros]] e da familia de [[Xúpiter]].<ref name="emel2007" />
== Hipótese ==
<!-- There are two main classes of comet: short-period comets (also called [[ecliptic]] comets) and long-period comets (also called nearly [[isotropy|isotropic]] comets). Ecliptic comets have relatively small orbits, below 10 AU, and follow the [[plane of the ecliptic|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.<ref name=book />
[[A. O. Leuschner]] in 1907 suggested that many comets believed to have parabolic orbits, and thus making single visits to the solar system, actually had elliptical orbits and would return after very long periods.<ref name="ley1967204">{{Cite magazine
|last=Ley
|first=Willy
|author=
|last2=
|first2=
|last3=
|first3=
|date=April 1967
|title=The Orbits of the Comets
|department=For Your Information
|url=https://archive.org/stream/Galaxy_v25n04_1967-04#page/n55/mode/2up
|magazine=Galaxy Science Fiction
|pages=55–63
|type=
}}</ref> In 1932 [[Estonia]]n astronomer [[Ernst Öpik]] postulated that long-period comets originated in an orbiting cloud at the outermost edge of the [[Solar System]].<ref name="Öpik-1932">{{cite journal
|author= Ernst Julius Öpik
|date= 1932
|title= Note on Stellar Perturbations of Nearby Parabolic Orbits
|journal= [[Proceedings of the American Academy of Arts and Sciences]]
|volume= 67|issue=6|pages=169–182
|doi= 10.2307/20022899
|jstor= 20022899
}}</ref> [[Dutch people|Dutch]] astronomer [[Jan Oort]] independently revived the idea in 1950 as a means to resolve a paradox:<ref name=Oort />
* Over the course of the Solar System's existence the orbits of comets are unstable and eventually [[Dynamics (physics)|dynamics]] dictate that a comet must either collide with the Sun or a planet or else be ejected from the Solar System by planetary [[Perturbation (astronomy)|perturbations]].
* Moreover, their volatile composition means that as they repeatedly approach the Sun, [[Electromagnetic radiation|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.<ref name=Oort /><ref name=dave /><ref name=book /> He noted that there was a peak in numbers of long-period comets with [[aphelion|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.<ref name=book /> -->
== Estrutura e composición ==
<!-- [[File:Oort cloud Sedna orbit.svg|thumb|upright=1.25|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 {{convert|2000|and|5000|AU|ly|2|abbr=on}}<ref name=book /> to as far as {{convert|50000|AU|ly|2|abbr=on}}<ref name=Morbidelli2006 /> from the Sun. Some estimates place the outer edge at between {{convert|100000|and|200000|AU|ly|2|abbr=on}}.<ref name= book /> The region can be subdivided into a spherical outer Oort cloud of {{convert|20000|-|50000|AU|ly|2|abbr=on}}, and a [[torus]]-shaped inner Oort cloud of {{convert|2000|-|20000|AU|ly|1|abbr=on}}. 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]].<ref name=Morbidelli2006 /> The inner Oort cloud is also known as the Hills cloud, named after [[Jack G. Hills]], who proposed its existence in 1981.<ref name="hills1981" /> Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;<ref name="hills1981" /><ref name="levison2001" /><ref name="Donahue1991" /> it is seen as a possible source of new comets to resupply the 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.<ref name="Julio1997" />
The outer Oort cloud may have trillions of objects larger than {{convert|1|km|2|abbr=on}},<ref name=Morbidelli2006 /> and billions with [[Absolute magnitude#Solar System bodies (H)|absolute magnitudes]]<ref>Absolute magnitude is a measure of how bright an object would be if it were 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 brightness. The lower an object's absolute magnitude, the brighter it is.</ref> brighter than 11 (corresponding to approximately {{convert|20|km|0|adj=on}} diameter), with neighboring objects tens of millions of kilometres apart.<ref name=emel2007 /><ref>{{cite web |title=The Oort Cloud |date=1998 |author=Paul R. Weissman |work=[[Scientific American]] |url=http://www.sciamdigital.com/index.cfm?fa=Products.ViewIssuePreview&ISSUEID_CHAR=8DB2FB44-6B4B-47AF-B46B-791A911764D&ARTICLEID_CHAR=B294C211-98B8-4374-92AB-158C4866AB1 |accessdate=2007-05-26 }}</ref> Its total mass is not known, but, assuming that [[Halley's Comet]] is a suitable prototype for comets within the outer Oort cloud, roughly the combined mass is {{convert|3E25|kg|lb}}, or five times that of Earth.<ref name=Morbidelli2006 /><ref>{{cite journal |author=Paul R. Weissman |date=1983 |title=The mass of the Oort Cloud |journal=[[Astronomy and Astrophysics]] |volume=118 |issue=1 |pages=90–94 |bibcode=1983A&A...118...90W }}</ref>
Earlier it was thought to be more massive (up to 380 Earth masses),<ref>{{cite web |author=Sebastian Buhai |title=On the Origin of the Long Period Comets: Competing theories |url=http://www.tinbergen.nl/~buhai/pictures/UCU/Physics_AppliedMathematics/Astrophysics/long_period_comets.pdf |archiveurl=https://web.archive.org/web/20060930193158/http://www.tinbergen.nl/~buhai/pictures/UCU/Physics_AppliedMathematics/Astrophysics/long_period_comets.pdf |archivedate=2006-09-30 |publisher=Utrecht University College |accessdate=2008-03-29 }}</ref>
but improved knowledge of the size distribution of long-period comets led to lower estimates. The mass of the inner Oort cloud has not been characterized.
If analyses of comets are representative of the whole, the vast majority of Oort-cloud objects consist of [[volatiles|ices]] such as [[ice|water]], [[methane]], [[ethane]], [[carbon monoxide]] and [[hydrogen cyanide]].<ref>{{cite journal |author=E. L. Gibb |author2=M. J. Mumma |author3=N. Dello Russo |author4=M. A. DiSanti |author5=K. Magee-Sauer |last-author-amp=yes |date=2003 |title=Methane in Oort Cloud comets |journal=[[Icarus (journal)|Icarus]] |volume=165 |issue=2 |pages=391–406 |bibcode=2003Icar..165..391G |doi=10.1016/S0019-1035(03)00201-X }}</ref>
However, the discovery of the object {{mpl|1996 PW}}, an object whose appearance was consistent with a [[D-type asteroid]]<ref>{{cite journal |last= Rabinowitz|first= D. L. |date= August 1996|title= 1996 PW|bibcode= 1996IAUC.6466....2R|journal= [[IAU Circular]]|volume= 6466|issue= |pages= 2|doi= }}</ref><ref>{{cite journal |last= Davies|first= John K. |display-authors= 4 |last2= McBride|first2= Neil |last3= Green|first3=Simon F. |last4=Mottola|first4= Stefano |last5= Carsenty|first5= Uri |last6=Basran|first6= Devinder |last7=Hudson |first7=Kathryn A. |last8= Foster|first8 = Michael J. |date= April 1998 |title= The Lightcurve and Colors of Unusual Minor Planet 1996 PW|url= |journal= [[Icarus (journal)|Icarus]] |volume= 132 |issue= 2 |pages= 418–430 |doi= 10.1006/icar.1998.5888 |bibcode=1998Icar..132..418D |subscription=yes }}</ref> in an orbit typical of a long-period comet, prompted theoretical research that suggests that the Oort cloud population consists of roughly one to two percent asteroids.<ref>{{cite journal |author=Paul R. Weissman |author2=Harold F. Levison |date=1997 |title=Origin and Evolution of the Unusual Object 1996 PW: Asteroids from the Oort Cloud? |journal=[[Astrophysical Journal]] |volume= 488|issue= 2|pages=L133–L136 |doi=10.1086/310940 |bibcode = 1997ApJ...488L.133W }}</ref> 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,<ref>{{cite journal |author=D. Hutsemekers |author2=J. Manfroid |author3=E. Jehin |author4=C. Arpigny |author5=A. Cochran |author6=R. Schulz |author7=J.A. Stüwe |author8=J.M. Zucconi |last-author-amp=yes |date=2005 |title=Isotopic abundances of carbon and nitrogen in Jupiter-family and Oort Cloud comets |journal=[[Astronomy and Astrophysics]] |volume=440 |issue=2 |pages=L21–L24 |arxiv=astro-ph/0508033 |bibcode=2005A&A...440L..21H |doi=10.1051/0004-6361:200500160 }}</ref> a conclusion also supported by studies of granular size in Oort-cloud comets<ref>{{cite journal |author=Takafumi Ootsubo |author2=Jun-ichi Watanabe |author3=Hideyo Kawakita |author4=Mitsuhiko Honda |author5=Reiko Furusho |last-author-amp=yes |date=2007 |title=Grain properties of Oort Cloud comets: Modeling the mineralogical composition of cometary dust from mid-infrared emission features |volume=55 |issue=9 | pages=1044–1049 |journal=Highlights in Planetary Science, 2nd General Assembly of Asia Oceania Geophysical Society |bibcode=2007P&SS...55.1044O |doi=10.1016/j.pss.2006.11.012 }}</ref> and by the recent impact study of Jupiter-family comet [[Tempel 1]].<ref>{{cite journal |author=Michael J. Mumma |author2=Michael A. DiSanti |author3=Karen Magee-Sauer |display-authors=etal |date=2005 |title=Parent Volatiles in Comet 9P/Tempel 1: Before and After Impact |journal=[[Science (journal)|Science Express]] |volume=310 |issue=5746 |pages=270–274 |bibcode = 2005Sci...310..270M |doi=10.1126/science.1119337 |pmid=16166477 |url=https://authors.library.caltech.edu/52069/7/Mumma.SOM.pdf |format=Submitted manuscript }}</ref> -->
== Orixe ==
<!-- The Oort cloud is thought to be a remnant of the original [[protoplanetary disc]] that [[Formation and evolution of the Solar System|formed around the Sun]] approximately 4.6 billion years ago.<ref name=Morbidelli2006 /> 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 [[planet]]s and [[minor planet]]s, but that gravitational interaction with young gas giants such as Jupiter ejected the objects into extremely long [[Elliptic orbit|elliptic]] or [[parabolic orbit]]s.<ref name=Morbidelli2006 /><ref>{{cite web
|title=Oort Cloud & Sol b?
|url=http://www.solstation.com/stars/oort.htm
|publisher=SolStation
|accessdate=2007-05-26
}}</ref> 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 did not form in close proximity to the Sun.<ref name="nasax" /> 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.<ref name=Morbidelli2006 />
Models by [[Julio Ángel Fernández]] suggest that the [[scattered disc]], which is the main source for [[periodic comet]]s 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 toward the Oort cloud, whereas a quarter are shifted inward to Jupiter's orbit, and a quarter are ejected on [[Hyperbola|hyperbolic]] orbits. The scattered disc might still be supplying the Oort cloud with material.<ref>{{cite journal
|author=Julio A. Fernández
|author2=Tabaré Gallardo
|author3=Adrián Brunini
|last-author-amp=yes
|date=2004
|title=The scattered disc population as a source of Oort Cloud comets: evaluation of its current and past role in populating the Oort Cloud
|journal=[[Icarus (journal)|Icarus]]
|volume=172 |issue=2 |pages=372–381
|bibcode=2004Icar..172..372F
|doi=10.1016/j.icarus.2004.07.023
}}</ref> A third of the scattered disc's population is likely to end up in the Oort cloud after 2.5 billion years.<ref>{{cite book
|author=Davies, J. K.
|author2=Barrera, L. H.
|date=2004
|title=The First Decadal Review of the Edgeworth-Kuiper Belt.
|url=https://books.google.com/?id=WuDdVbJf_d8C&pg=PA43&dq=+oort+cloud
|publisher=Kluwer Academic Publishers
|isbn=978-1-4020-1781-0
}}</ref>
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.<ref>{{cite journal
|author=S. Alan Stern
|author2=Paul R. Weissman
|date=2001
|title=Rapid collisional evolution of comets during the formation of the Oort Cloud
|journal=[[Nature (journal)|Nature]]
|volume=409 |issue=6820 |pages=589–591
|bibcode=2001Natur.409..589S
|doi=10.1038/35054508
|pmid=11214311
}}</ref> The estimated mass of the cloud is only a small part of the 50–100 Earth masses of ejected material.<ref name=Morbidelli2006 />
Gravitational interaction with nearby stars and [[galactic tide]]s modified cometary orbits to make them more circular. This explains the nearly spherical shape of the outer Oort cloud.<ref name=Morbidelli2006 /> On the other hand, the Hills cloud, which is bound more strongly to the Sun, has not acquired 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 [[star cluster|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.<ref>{{cite journal
|author=R. Brasser
|author2=M. J. Duncan
|author3=H.F. Levison
|date=2006
|title=Embedded star clusters and the formation of the Oort Cloud
|journal=[[Icarus (journal)|Icarus]]
|volume=184 |issue=1 |pages=59–82
|bibcode=2006Icar..184...59B
|doi=10.1016/j.icarus.2006.04.010
}}</ref>
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 [[open cluster|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".<ref>{{cite journal | author1 =Levison, Harold | title =Capture of the Sun's Oort Cloud from Stars in Its Birth Cluster | journal =Science | volume =329 | issue =5988 | pages =187–190 | date =10 June 2010 | doi =10.1126/science.1187535|bibcode = 2010Sci...329..187L |display-authors=etal | pmid=20538912}}</ref><ref>{{cite web | title =Many famous comets originally formed in other solar systems | work =Southwest Research Institute® (SwRI®) News | date =10 June 2010 | url =http://www.swri.org/9what/releases/2010/cometorigins.htm | deadurl =yes | archiveurl =https://www.webcitation.org/6H9vGLJk6?url=http://www.swri.org/9what/releases/2010/cometorigins.htm | archivedate =5 June 2013 | df = }}</ref> -->
== Cometas ==
<!-- [[File:Comet Hale-Bopp.jpg|thumb|upright|[[Comet Hale–Bopp]], an archetypical Oort-cloud comet]]
[[Comet]]s 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.<ref name=book /> Comets pass from the scattered disc into the realm of the outer planets, becoming what are known as [[centaur (minor planet)|centaurs]].<ref>{{cite journal
|author=Harold E. Levison
|author2=Luke Dones
|last-author-amp=yes
|date=2007
|title=Comet Populations and Cometary dynamics
|journal=Encyclopedia of the Solar System
|pages=575–588
|doi=10.1016/B978-012088589-3/50035-9
|isbn=978-0-12-088589-3
|bibcode=2007ess..book..575L
}}</ref> These centaurs are then sent farther inward to become the short-period comets.<ref>{{cite journal
|author = J Horner
|author2 = NW Evans
|author3 = ME Bailey
|author4 = DJ Asher
|date = 2003
|title = The Populations of Comet-like Bodies in the Solar System
|journal = Monthly Notices of the Royal Astronomical Society
|volume = 343
|issue = 4
|pages = 1057–1066
|doi = 10.1046/j.1365-8711.2003.06714.x
|arxiv= astro-ph/0304319
|bibcode= 2003MNRAS.343.1057H
}}</ref>
There are two main varieties of short-period comet: Jupiter-family comets (those with [[Semi-major axis|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 although 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.<ref name=dave /> 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.<ref name="emel2007" />
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 dynamical process are known to explain the smaller number of observed comets than Oort estimated. 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.<ref>{{cite book
|author=Luke Dones
|author2=Paul R Weissman
|author3=Harold F Levison
|author4=Martin J Duncan
|chapter=Oort Cloud Formation and Dynamics
|chapterurl=http://www.lpi.usra.edu/books/CometsII/7031.pdf
|editor=Michel C. Festou
|editor2=H. Uwe Keller
|editor3=Harold A. Weaver
|date=2004
|title=Comets II
|url=http://www.uapress.arizona.edu/books/bid1580.htm
|publisher=University of Arizona Press
|pages=153–173
|accessdate=2008-03-22
}}</ref> Dynamical studies of hypothetical Oort cloud comets have estimated that their occurrence in the [[outer planets|outer-planet]] region would be 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.<ref name=julio>{{cite journal
|author=Julio A. Fernández
|date=2000
|title=Long-Period Comets and the Oort Cloud
|journal=[[Earth, Moon, and Planets]]
|volume=89 |issue=1–4 |pages=325–343
|bibcode = 2002EM&P...89..325F
|doi=10.1023/A:1021571108658
}}</ref> -->
== Efectos de marea ==
{{AP|Marea galáctica}}
<!-- Most of the comets seen close to the Sun seem to have reached their current positions through gravitational perturbation of the Oort cloud by the [[tidal force]] exerted by the [[Milky Way]]. Just as the [[Moon]]'s tidal force deforms Earth's oceans, causing the tides to rise and fall, the galactic tide also distorts the orbits of bodies in the [[outer Solar System]]. In the charted regions of the Solar System, these effects are negligible compared to the gravity of the Sun, but in the outer reaches of the system, the Sun's gravity is weaker and the gradient of the Milky Way's gravitational field has substantial effects. Galactic tidal forces stretch the cloud along an axis directed toward the galactic centre and compress it along the other two axes; these small perturbations can shift orbits in the Oort cloud to bring objects close to the Sun.<ref>{{cite journal
|author=Marc Fouchard
|author2=Christiane Froeschlé
|author3=Giovanni Valsecchi
|author4=Hans Rickman
|date=2006
|title=Long-term effects of the galactic tide on cometary dynamics
|journal=[[Celestial Mechanics and Dynamical Astronomy]]
|volume=95 |issue=1–4 |pages=299–326
|bibcode=2006CeMDA..95..299F
|doi=10.1007/s10569-006-9027-8
}}</ref> 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.<ref name=book />
Some scholars theorise that the galactic tide may have contributed to the formation of the Oort cloud by increasing the [[Perihelion and aphelion|perihelia]] (smallest distances to the Sun) of [[planetesimal]]s with large aphelia (largest distances to the Sun).<ref>{{cite journal
|author=Higuchi A.
|author2=Kokubo E.
|author3=Mukai, T.
|last-author-amp=yes
|date=2005
|title=Orbital Evolution of Planetesimals by the Galactic Tide
|journal=[[Bulletin of the American Astronomical Society]]
|volume=37 |page=521
|bibcode=2005DDA....36.0205H
}}</ref> 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.<ref>{{cite journal
|author=Nurmi P.
|author2=Valtonen M.J. |author3=Zheng J.Q.
|date=2001
|title=Periodic variation of Oort Cloud flux and cometary impacts on the Earth and Jupiter
|journal=[[Monthly Notices of the Royal Astronomical Society]]
|volume=327 |issue=4 |pages=1367–1376
|bibcode=2001MNRAS.327.1367N
|doi=10.1046/j.1365-8711.2001.04854.x
}}</ref> 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.<ref>{{cite journal
|author=John J. Matese
|author2=Jack J. Lissauer
|last-author-amp=yes
|date=2004
|title=Perihelion evolution of observed new comets implies the dominance of the galactic tide in making Oort Cloud comets discernible
|journal=[[Icarus (journal)|Icarus]]
|volume=170|issue=2|pages=508–513
|bibcode=2004Icar..170..508M
|doi=10.1016/j.icarus.2004.03.019
|citeseerx=10.1.1.535.1013
|url=http://www.ucs.louisiana.edu/~jjm9638/dps2003/I08821w.pdf
|format=Submitted manuscript
}}</ref> -->
== Perturbacións estelares e hipóteses de compañeiros estelares ==
<!-- 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<ref name=Morbidelli2006 /> or giant [[molecular cloud]]s.<ref name=julio /> The orbit of the Sun through the plane of the Milky Way sometimes brings it in relatively [[List of nearest stars#Future and past|close proximity to other stellar systems]]. For example, it is hypothesized that 70 thousand years ago, perhaps [[Scholz's star]] passed through the outer Oort cloud (although its low mass and high relative velocity limited its effect).<ref>{{cite journal
|last1=Mamajek |first1=Eric E.
|last2=Barenfeld |first2=Scott A.
|last3=Ivanov |first3=Valentin D.
|title=The Closest Known Flyby of a Star to the Solar System
|journal=[[The Astrophysical Journal]]
|volume=800 |issue=1 |date=2015
|doi=10.1088/2041-8205/800/1/L17
|arxiv=1502.04655|bibcode = 2015ApJ...800L..17M |page=L17|url=https://authors.library.caltech.edu/55650/1/2041-8205_800_1_L17.pdf
|format=Full text
}}</ref> During the next 10 million years the known star with the greatest possibility of perturbing the Oort cloud is [[Gliese 710]].<ref name=algol /> This process could also scatter Oort cloud objects out of the ecliptic plane, potentially also explaining its spherical distribution.<ref name="algol">{{cite conference|author=L. A. Molnar|author2=R. L. Mutel|date=1997|title=Close Approaches of Stars to the Oort Cloud: Algol and Gliese 710|conference=American Astronomical Society 191st meeting|publisher=[[American Astronomical Society]]|bibcode=1997AAS...191.6906M}}</ref><ref>{{cite journal
|author=A. Higuchi
|author2=E. Kokubo
|author3=T. Mukai
|last-author-amp=yes
|date=2006
|title=Scattering of Planetesimals by a Planet: Formation of Comet Cloud Candidates
|journal=[[Astronomical Journal]]
|volume=131 |pages=1119–1129
|bibcode = 2006AJ....131.1119H
|doi = 10.1086/498892
|issue=2
|url=https://zenodo.org/record/896851/files/article.pdf
}}</ref>
In 1984, [[physics|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 (hypothetical star)|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 or the Oort cloud have been found, and many lines of evidence (such as [[crater counting|crater counts]]), have thrown their existence into doubt.<ref>{{cite journal
|author= J. G. Hills
|date=1984
|title=Dynamical constraints on the mass and perihelion distance of Nemesis and the stability of its orbit
|journal=[[Nature (journal)|Nature]]
|volume=311 |issue= 5987 |pages=636–638
|bibcode = 1984Natur.311..636H
|doi=10.1038/311636a0
}}</ref><ref>{{cite web|title=Nemesis is a myth|publisher=Max Planck Institute|url= http://www.mpg.de/4372308/nemsis_myth?page=1|date=2011|accessdate=2011-08-11}}</ref> Recent scientific analysis no longer supports the idea that extinctions on Earth happen at regular, repeating intervals.<ref name="Tyche2011-060" /> Thus, the Nemesis hypothesis is no longer needed to explain current assumptions.<ref name="Tyche2011-060">{{cite web
|date=February 18, 2011
|title=Can WISE Find the Hypothetical 'Tyche'?
|publisher=NASA/JPL
|url=http://www.jpl.nasa.gov/news/news.cfm?release=2011-060
|accessdate=2011-06-15}}</ref>
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 postulated Oort cloud than can be explained by the galactic tide or stellar perturbations alone, and that the most likely cause would be a [[Jupiter]]-mass object in a distant orbit.<ref>{{cite web
|author=John J. Matese
|author2=Jack J. Lissauer
|last-author-amp=yes
|title=Continuing Evidence of an Impulsive Component of Oort Cloud Cometary Flux
|url=http://www.ucs.louisiana.edu/~jjm9638/acm2002/acm2002_05_06.pdf
|publisher=[[University of Louisiana at Lafayette]], and [[NASA]] [[Ames Research Center]]
|date=2002-05-06
|accessdate=2008-03-21
}}</ref> This hypothetical [[gas giant]] was nicknamed [[Tyche (hypothetical planet)|Tyche]]. The [[WISE mission]], an [[all-sky survey]] using [[parallax]] measurements in order to clarify local star distances, was capable of proving or disproving the Tyche hypothesis.<ref name="Tyche2011-060" /> In 2014, NASA announced that the WISE survey had ruled out any object as they had defined it.<ref name="NASA-20140307">{{cite journal
|journal=[[The Astrophysical Journal]]
|last=K. L. |first=Luhman
|title=A Search For A Distant Companion To The Sun With The Wide-field Infrared Survey Explorer
|url=http://iopscience.iop.org/0004-637X/781/1/4?fromSearchPage=true
|date=7 March 2014
|volume=781
|number=1
|doi=10.1088/0004-637X/781/1/4
|accessdate=20 March 2014 |bibcode = 2014ApJ...781....4L
|page=4}}</ref> -->
== Exploración ==
<!-- [[File:Thousandau1 space probe.jpg|thumb|[[Artist's impression]] of the [[TAU (spacecraft)|''TAU'' spacecraft]]]]
Space probes have yet to reach the area of the Oort cloud. ''[[Voyager 1]]'', the fastest<ref name="New_Horizons2006">{{cite web |url=http://pluto.jhuapl.edu/news_center/news/081706.php |title=New Horizons Salutes Voyager |date=August 17, 2006 |publisher=New Horizons |accessdate=November 3, 2009 |deadurl=yes |archiveurl=https://www.webcitation.org/5x3s4O3KH?url=http://pluto.jhuapl.edu/news_center/news/081706.php |archivedate=March 9, 2011 |df= }}</ref> and farthest<ref name="g.2013sep13">{{cite news |last=Clark |first=Stuart |title=Voyager 1 leaving solar system matches feats of great human explorers |url=https://www.theguardian.com/science/across-the-universe/2013/sep/13/voyager-1-solar-system-great-explorers |newspaper=The Guardian |date=September 13, 2013 }}</ref><ref>{{cite news |url=http://www.spacetoday.org/SolSys/Voyagers20years.html |title=Voyagers are leaving the Solar System |work=Space Today |date=2011 |accessdate=May 29, 2014}}</ref> of the interplanetary space probes currently leaving the Solar System, will reach the Oort cloud in about 300 years<ref name="jpl.PIA17046">{{cite web |url=http://photojournal.jpl.nasa.gov/catalog/PIA17046 |title=Catalog Page for PIA17046 |work=Photo Journal |publisher=NASA |date= |accessdate=April 27, 2014}}</ref><ref name="ut.104717">{{cite web |url=http://www.universetoday.com/104717/its-official-voyager-1-is-now-in-interstellar-space/ |title=It's Official: Voyager 1 Is Now In Interstellar Space |work=UniverseToday |date= |accessdate=April 27, 2014}}</ref> and would take about 30,000 years to pass through it.<ref name="Ghose2013">{{cite web |last=Ghose |first=Tia |title=Voyager 1 Really Is In Interstellar Space: How NASA Knows |work=Space.com |publisher=TechMedia Network |date=September 13, 2013 |url=http://www.space.com/22797-voyager-1-interstellar-space-nasa-proof.html |accessdate=September 14, 2013 }}</ref><ref name="How_We_Know">{{cite web |last=Cook |first=J.-R |title=How Do We Know When Voyager Reaches Interstellar Space? |publisher=NASA / Jet Propulsion Lab | date=September 12, 2013 |url=http://www.jpl.nasa.gov/news/news.php?release=2013-278 |accessdate=September 15, 2013 }}</ref> However, around 2025, the [[radioisotope thermoelectric generator]]s on ''Voyager 1'' will no longer supply enough power to operate any of its scientific instruments, preventing any exploration by ''Voyager 1.'' The [[:Category:Spacecraft escaping the Solar System|other four probes]] currently escaping the Solar System either are already or are predicted to be non-functional when they reach the Oort cloud; however, it may be possible to find an object from the cloud that has been knocked into the inner Solar System.
In the 1980s there was a concept for a probe to reach 1,000 AU in 50 years called ''[[TAU (spacecraft)|TAU]]''; among its missions would be to look for the Oort cloud.<ref>[http://www.daviddarling.info/encyclopedia/T/TAU.html TAU (Thousand Astronomical Unit) mission]</ref>
In the 2014 Announcement of Opportunity for the [[Discovery program]], an observatory to detect the objects in the Oort cloud (and Kuiper belt) called the [[Whipple (spacecraft)|"Whipple Mission"]] was proposed.<ref name="whipple.cfa.harvard.edu">{{cite web |title=The Whipple Mission: Exploring the Oort Cloud and the Kuiper Belt |author1=Charles Alcock |author2=Michael Brown |author3=Tom Gauron |author4=Cate Heneghan |author5=Matthew Holman |author6=Almus Kenter |author7=Ralph Kraft |author8=Roger Lee |author9=John Livingston |author10=James Mcguire |author11=Stephen Murray |author12=Ruth Murray-Clay |author13=Paul Nulsen |author14=Matthew Payne |author15=Hilke Schlichting |author16=Amy Trangsrud |author17=Jan Vrtilek |author18=Michael Werner |display-authors=1 |url=http://whipple.cfa.harvard.edu/inc/documents/Alcock_AGUPoster_2014dec.pdf |accessdate=2015-11-12 |deadurl=yes |archiveurl=https://web.archive.org/web/20151117031224/http://whipple.cfa.harvard.edu/inc/documents/Alcock_AGUPoster_2014dec.pdf |archivedate=2015-11-17 |df= }}</ref> It would monitor distant stars with a photometer, looking for transits up to 10,000 AU away.<ref name="whipple.cfa.harvard.edu" /> The observatory was proposed for halo orbiting around L2 with a suggested 5-year mission.<ref name="whipple.cfa.harvard.edu" /> It has been suggested that the [[Kepler (spacecraft)|Kepler observatory]] may also be able to detect objects in the Oort cloud.<ref>[http://www.scientificamerican.com/article/kepler-oort-cloud/ Scientific American – Kepler Spacecraft May Be Able to Spot Elusive Oort Cloud Objects – 2010]</ref> -->
== Notas ==
{{Listaref|group=note}}
;Referencias
{{Todas as ref|en}}
{{Listaref|30em|refs=
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}}</ref>
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<ref name="Morbidelli2006">{{cite arXiv
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}}</ref>
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| date = 1987-09-30
| bibcode = 1987RSPTA.323..339W
| doi = 10.1098/rsta.1987.0090
}}</ref>}}
{{Commonscat}}
{{Sistema Solar}}
{{Control de autoridades}}
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