This article is about the outer Oort cloud. For the inner Oort cloud, seeHills cloud.
The distance from the Oort cloud to the interior of the Solar System, and two of the nearest stars, is measured inastronomical units. The scale islogarithmic: each indicated distance is ten times farther out than the previous distance. The red arrow indicates the projected location, in 2025–2027, of thespace probeVoyager 1, which may reach the Oort cloud about 300 years later.
Anartist's impression of the Oort cloud and theKuiper belt (inset); the sizes of objects are over-scaled for visibility.
TheOort cloud (/ɔːrt,ʊərt/),[1] sometimes called theÖpik–Oort cloud,[2] istheorized to be a vast cloud oficyplanetesimals surrounding theSun at distances ranging from 2,000 to 200,000AU (0.03 to 3.2light-years).[3][note 1][4] The concept of such a cloud was proposed in 1950 by the DutchastronomerJan Oort, in whose honor the idea was named.[5][6] Oort proposed that the bodies in this cloud replenish and keep constant the number oflong-period comets entering theinner Solar System—where they are eventually consumed and destroyed during close approaches to the Sun.[7]
The outer limit of the Oort cloud defines thecosmographic boundary of theSolar System. This area is defined by the Sun'sHill sphere, and hence lies at the interface between solar and galactic gravitational dominion.[10] The outer Oort cloud is only loosely bound to the Solar System and its constituents are easily affected by the gravitational pulls ofpassing stars, theMilky Way itself and the cloud's own microgravity.[11][12] These forces served to moderate and render more circular the highly eccentric orbits of material ejected from the inner Solar System during itsearly phases of development. The circular orbits of material in the Oort disc are largely thanks to this galactic gravitational torquing.[13][14] By the same token, galactic interference in the motion of Oort bodies occasionally dislodgescomets from their orbits within the cloud, sending them into theinner Solar System.[4] Based on their orbits, most but not all of theshort-period comets appear to have come from the Oort disc. Other short-period comets may have originated from the far larger spherical cloud.[4][15]
Astronomers hypothesize that the material presently in the Oort cloud formed much closer to the Sun, in theprotoplanetary disc, and was then scattered far into space through the gravitational influence of thegiant planets.[4] No direct observation of the Oort cloud is possible with present imaging technology.[16] Nevertheless, the cloud is thought to be the source that replenishes mostlong-period andHalley-type comets, which are eventually consumed by their close approaches to the Sun after entering the inner Solar System. The cloud may also serve the same function for many of thecentaurs andJupiter-family comets.[15]
By the early 20th century, astronomers had identified two main types of comets: short-period comets (also calledecliptic comets) and long-period comets (also callednearlyisotropic comets).[17] Ecliptic comets have relatively small orbits aligned near theecliptic plane and are not found much farther than theKuiper cliff around 50 AU from the Sun (the orbit ofNeptune averages about 30 AU and177P/Barnard has aphelion around 48 AU). Long-period comets, on the other hand, travel in very large orbits thousands of AU from the Sun and are isotropically distributed. This means long-period comets appear from every direction in the sky, both above and below the ecliptic plane.[18] The origin of these comets was not well understood, and many long-period comets were initially assumed to be on parabolic trajectories, making them one-time visitors to the Sun from interstellar space.
In 1907,Armin Otto Leuschner suggested that many of the comets then thought to have parabolic orbits in fact moved along extremely large elliptical orbits that would return them to the inner Solar System after long intervals during which they were invisible to Earth-based astronomy.[19] In 1932, theEstonian astronomerErnst Öpik proposed a reservoir of long-period comets in the form of an orbiting cloud at the outermost edge of theSolar System.[20]Dutch astronomerJan Oort revived this basic idea in 1950 to resolve a paradox about the origin of comets. The following facts are not easily reconcilable with the highly elliptical orbits in which long-period comets are always found:
Over millions and billions of years the orbits of Oort cloud comets are unstable.Celestial dynamics will eventually dictate that a comet must be pulled away by a passing star, collide with the Sun or a planet, or be ejected from the Solar System through planetaryperturbations.
Moreover, the volatile composition of comets means that as they repeatedly approach the Sunradiation gradually boils the volatiles off until the comet splits or develops an insulating crust that prevents furtheroutgassing.[21]
Oort reasoned that comets with orbits that closely approach the Sun cannot have been doing so since the condensation of the protoplanetary disc, more than 4.5 billion years ago. Hence long-period comets could not have formed in the current orbits in which they are always discovered and must have been held in an outer reservoir for nearly all of their existence.[21][22][18]
Oort also studied tables ofephemerides for long-period comets and discovered that there is a curious concentration of long-period comets whose farthest retreat from the Sun (theiraphelia) cluster around 20,000 AU. This suggested a reservoir at that distance with a spherical,isotropic distribution. He also proposed that the relatively rare comets with orbits of about 10,000 AU probably went through one or more orbits into the inner Solar System and there had their orbits drawn inward by thegravity of the planets.[18]
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 somewhere between 2,000 and 5,000 AU (0.03 and 0.08 ly)[18] from the Sun to as far out as 50,000 AU (0.79 ly) or even 100,000 to 200,000 AU (1.58 to 3.16 ly).[4][18] The region can be subdivided into a spherical outer Oort cloud with a radius of some 20,000–50,000 AU (0.32–0.79 ly) and atorus-shaped inner Oort cloud with a radius of 2,000–20,000 AU (0.03–0.32 ly).
The inner Oort cloud is sometimes known as the Hills cloud, named forJack G. Hills, who proposed its existence in 1981.[23] Models predict the inner cloud to be the much denser of the two, having tens or hundreds of times as many cometary nuclei as the outer cloud.[23][24][25] The Hills cloud is thought to be necessary to explain the continued existence of the Oort cloud after billions of years.[26]
Because it lies at the interface between the dominion of Solar and galactic gravitation, the objects comprising the outer Oort cloud are only weakly bound to the Sun. This in turn allows small perturbations from nearby stars or the Milky Way itself to inject long-period (and possiblyHalley-type) comets inside the orbit ofNeptune.[4] This process ought to have depleted the sparser, outer cloud and yet long-period comets with orbits well above or below the ecliptic continue to be observed. The Hills cloud is thought to be a secondary reservoir of cometary nuclei and the source of replenishment for the tenuous outer cloud as the latter's numbers are gradually depleted through losses to the inner Solar System.[27]
The outer Oort cloud may have trillions of objects larger than 1 km (0.6 mi),[4] and billions with diameters of 20-kilometre (12 mi). This corresponds to anabsolute magnitude of more than 11.[28] On this analysis, "neighboring" objects in the outer cloud are separated by a significant fraction of 1 AU, tens of millions of kilometres.[15][29] The outer cloud's total mass is not known, but assuming thatHalley's Comet is a suitable proxy for the nuclei composing the outer Oort cloud, their combined mass would be roughly 3×1025 kilograms (6.6×1025 lb), or five Earth masses.[4][30]Formerly the outer cloud was thought to be more massive by two orders of magnitude, containing up to 380 Earth masses,[31]but improved knowledge of the size distribution of long-period comets has led to lower estimates. No estimates of the mass of the inner Oort cloud have been published as of 2023.
If analyses of comets are representative of the whole, the vast majority of Oort-cloud objects consist ofices such aswater,methane,ethane,carbon monoxide andhydrogen cyanide.[32]However, the discovery of the object1996 PW, an object whose appearance was consistent with aD-type asteroid[33][34] 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.[35] Analysis of the carbon and nitrogenisotope 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,[36] a conclusion also supported by studies of granular size in Oort-cloud comets[37] and by the recent impact study of Jupiter-family cometTempel 1.[38]
In 2020 researchers discovered an unexpected spiral-shaped pattern in theinner part of the Oort Cloud. This discovery sheds light on how gravity shapes the outer edges of the Solar System.[39]
The Oort cloud is thought to have developed after theformation of planets from the primordialprotoplanetary disc approximately 4.6 billion years ago.[4] 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 theplanets andminor planets. After formation, strong gravitational interactions with young gas giants, such as Jupiter, scattered the objects into extremely wideelliptical orparabolic orbits that were subsequently modified by perturbations from passing stars and giant molecular clouds into long-lived orbits detached from the gas giant region.[4][40]
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.[41] 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.[4]
Models byJulio Ángel Fernández suggest that thescattered disc, which is the main source forperiodic 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 toward the Oort cloud, whereas a quarter are shifted inward to Jupiter's orbit, and a quarter are ejected onhyperbolic orbits. The scattered disc might still be supplying the Oort cloud with material.[42][43] A third of the scattered disc's population is likely to end up in the Oort cloud after 2.5 billion years.[44]
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.[45] The estimated mass of the cloud is only a small part of the 50–100 Earth masses of ejected material.[4]
Gravitational interaction with nearby stars andgalactic tides modified cometary orbits to make them more circular. This explains the nearly spherical shape of the outer Oort cloud.[4] On the other hand, theHills 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 theSolar System formed as part of an embeddedcluster 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.[46]
In June 2010Harold F. Levison and others suggested on the basis of enhanced computer simulations that the Sun "captured comets from other stars while it was in itsbirth cluster." Their results imply that "a substantial fraction of the Oort cloud comets, perhaps exceeding 90%, are from the protoplanetary discs of other stars."[47][48] In July 2020 Amir Siraj andAvi Loeb found that a captured origin for the Oort Cloud in the Sun'sbirth cluster could address the theoretical tension in explaining the observed ratio of outer Oort cloud toscattered disc objects, and in addition could increase the chances of a capturedPlanet Nine.[49][50][51]
Comets are thought to have two separate points of origin in the Solar System.[52] Short-period comets (those with orbits of up to 200 years) are generally accepted to have emerged from either theKuiper 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. Very long-period comets, such asC/1999 F1 (Catalina), whose orbits last for millions of years, are thought to originate directly from the outer Oort cloud.[53] Other comets modeled to have come directly from the outer Oort cloud includeC/2006 P1 (McNaught),C/2010 X1 (Elenin),Comet ISON,C/2013 A1 (Siding Spring),C/2017 K2, andC/2017 T2 (PANSTARRS). The orbits within the Kuiper belt are relatively stable, 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.[18] Comets pass from the scattered disc into the realm of the outer planets, becoming what are known ascentaurs.[54][55] These centaurs are then sent farther inward to become the short-period comets.[56]
There are two main types of short-period comets: Jupiter-family comets (with orbits smaller than 5 AU) and Halley-family comets. Halley-family comets, named afterHalley's Comet, are distinct because, even though they are short-period comets, they are thought to come from the Oort Cloud rather than the scattered disc.[57][58] 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.[22] 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.[15]
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.[59] No dynamical process is 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 allvolatiles, rendering some comets invisible, or the formation of a non-volatile crust on the surface.[60] Dynamical studies of hypothetical Oort cloud comets have estimated that their occurrence in theouter-planet region would be several times higher than in the inner-planet region. This discrepancy may be due to the gravitational attraction ofJupiter, which acts as a kind of barrier, trapping incoming comets and causing them to collide with it, just as it did withComet Shoemaker–Levy 9 in 1994.[61] An example of a typical dynamically old comet with an origin in the Oort cloud could be C/2018 F4.[62]
Most of the comets seen close to the Sun seem to have reached their current positions through gravitational perturbation of the Oort cloud by thetidal force exerted by theMilky Way. Just as theMoon's tidal force deforms Earth's oceans, causing the tides to rise and fall, the galactic tide also distorts the orbits of bodies in theouter Solar System.[63] 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 gravitationalGalactic Center compresses it along the other two axes; these small perturbations can shift orbits in the Oort cloud to bring objects close to the Sun.[64] 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.[18]
Some scholars theorize that the galactic tide may have contributed to the formation of the Oort cloud by increasing theperihelia (smallest distances to the Sun) ofplanetesimals with large aphelia (largest distances to the Sun).[65] 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.[66] 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.[67]
Stellar perturbations and stellar companion hypotheses
Besides thegalactic 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[4] or giantmolecular clouds.[61] The orbit of the Sun through the plane of the Milky Way sometimes brings it in relativelyclose proximity to other stellar systems. For example, it is hypothesized that 70,000 years agoScholz's Star passed through the outer Oort cloud (although its low mass and high relative velocity limited its effect).[68][69] During the next 10 million years the known star with the greatest possibility of perturbing the Oort cloud isGliese 710.[70] This process could also scatter Oort cloud objects out of the ecliptic plane, potentially also explaining its spherical distribution.[70][71]
In 1984,physicistRichard A. Muller postulated that the Sun has an as-yet undetected companion, either abrown dwarf or ared dwarf, in an elliptical orbit within the Oort cloud.[72] This object, known asNemesis, 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 ascrater counts), have thrown its existence into doubt.[73][74] Recent scientific analysis no longer supports the idea that extinctions on Earth happen at regular, repeating intervals.[75] Thus, the Nemesis hypothesis is no longer needed to explain current assumptions.[75]
A somewhat similar hypothesis was advanced by astronomerJohn J. Matese of theUniversity 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 aJupiter-mass object in a distant orbit.[76] This hypotheticalgas giant was nicknamedTyche. TheWISE mission, anall-sky survey usingparallax measurements in order to clarify local star distances, was capable of proving or disproving the Tyche hypothesis.[75] In 2014, NASA announced that the WISE survey had ruled out any object as they had defined it.[77]
Space probes have yet to reach the area of the Oort cloud.Voyager 1, the fastest[78] and farthest[79][80] of the interplanetary space probes currently leaving the Solar System, will reach the Oort cloud in about 300 years[8][81] and would take about 30,000 years to pass through it.[82][83] However, around 2025, theradioisotope thermoelectric generators onVoyager 1 will no longer supply enough power to operate any of its scientific instruments, preventing any further exploration byVoyager 1. Theother four probes currently escaping the Solar System have either already stopped functioning (Pioneer 10,Pioneer 11) or are predicted to also stop functioning before they reach the Oort cloud (Voyager 2,New Horizons).
In the 1980s, there was a concept for a probe that could reach 1,000 AU in 50 years, calledTAU; among its missions would be to look for the Oort cloud.[84]
In the 2014 Announcement of Opportunity for theDiscovery program, an observatory to detect the objects in the Oort cloud (and Kuiper belt) called the"Whipple Mission" was proposed.[85] It would monitor distant stars with a photometer, looking for transits up to 10,000 AU away.[85] The observatory was proposed for halo orbiting around L2 with a suggested 5-year mission.[85] It was also suggested that theKepler space telescope could have been capable of detecting objects in the Oort cloud.[86]
^Williams, Matt (August 10, 2015)."What is the Oort Cloud?".Universe Today.Archived from the original on January 23, 2018. RetrievedMay 21, 2021.
^abcdefghijklmnoAlessandro Morbidelli (2006). "Origin and dynamical evolution of comets and their reservoirs of water ammonia and methane".arXiv:astro-ph/0512256.
^van der Kruit, Pieter C. (2020).Master of Galactic Astronomy: A Biography of Jan Hendrik Oort. Springer.Bibcode:2021mgab.book.....K.
^Whipple, Fred L. (1978). "The Oort Cloud". In Tom Gehrels (ed.).Protostars and Planets. University of Arizona Press.
^Fouchard, Marc (2007). "Long-term effects of the Galactic tide on cometary dynamics".Celestial Mechanics and Dynamical Astronomy.98 (1–2):35–54.doi:10.1007/s10569-007-9076-2 (inactive 22 March 2025).{{cite journal}}: CS1 maint: DOI inactive as of March 2025 (link)
^"Oort Cloud".NASA Solar System Exploration. 20 June 2023.Archived from the original on 2023-06-30. Retrieved2023-07-01.
^Duncan, Martin J.; Levison, Harold F.; Dones, Luke (2004). "Dynamical Evolution of Ecliptic Comets". In Michel Festou, H. U. Keller, and H. A. Weaver (ed.).Comets II(PDF). University of Arizona Press. pp. 193–204.{{cite book}}: CS1 maint: multiple names: editors list (link)
^Kaib, Nathan A.; Volk, Kathryn (2022). "Dynamical Population of Comet Reservoirs".The Planetary Science Journal.3 (6): 123.arXiv:2206.00010.doi:10.3847/PSJ/ac6e66 (inactive 23 March 2025).{{cite journal}}: CS1 maint: DOI inactive as of March 2025 (link)
^Absolute magnitude is a measure of how bright an object would be if it were 1 au from the Sun and Earth; as opposed toapparent 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.
^Horizons output."Barycentric Osculating Orbital Elements for Comet C/1999 F1 (Catalina)".Archived from the original on 2021-06-02. Retrieved2021-06-01. Solution using the Solar SystemBarycenter. Ephemeris Type:Elements and Center:@0 (To be outside planetary region, inbound epoch 1950 and outbound epoch 2050. For epoch 1950-Jan-01 orbit period is "PR= 1.6E+09 / 365.25 days" = ~4.3 million years)
^Luke Dones; Paul R Weissman; Harold F Levison; Martin J Duncan (2004)."Oort Cloud Formation and Dynamics"(PDF). In Michel C. Festou; H. Uwe Keller; Harold A. Weaver (eds.).Comets II. University of Arizona Press. pp. 153–173.Archived from the original on 2017-08-24. Retrieved2008-03-22.
^Licandro, Javier; de la Fuente Marcos, Carlos; de la Fuente Marcos, Raúl; de Leon, Julia; Serra-Ricart, Miquel; Cabrera-Lavers, Antonio (28 May 2019). "Spectroscopic and dynamical properties of comet C/2018 F4, likely a true average former member of the Oort cloud".Astronomy and Astrophysics.625: A133 (6 pages).arXiv:1903.10838.Bibcode:2019A&A...625A.133L.doi:10.1051/0004-6361/201834902.S2CID85517040.
^"New Horizons Salutes Voyager". New Horizons. August 17, 2006. Archived fromthe original on November 13, 2014. RetrievedNovember 3, 2009.Voyager 1 is escaping the solar system at 17 kilometers per second.
^The Oort cloud's outer limit is difficult to define as it varies over themillennia asdifferent stars pass the Sun and thus is subject to variation.[citation needed] Estimates of its distance range from 50,000 to 200,000 au.
Matthews, R. A. J. (1994). "The close approach of stars in the solar neighbourhood".Quarterly Journal of the Royal Astronomical Society.35: 1.Bibcode:1994QJRAS..35....1M.
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