Oort_cloud - Wikiparency

# Oort cloud  **Microsim — three.js (Wikitube framework):** *The Oort cloud in the Solar System explorer* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?obj=oort&embed=1" data-title="The Oort cloud in the Solar System explorer"></div> *The Solar System explorer locked on this article's state (`?obj=oort`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.*  *Try: set the speed to 100 years/s and watch the planets turn at the centre while the cloud's points stay put; set scale to true and read the caption, which reports that true scale cannot show the cloud and keeps the logarithmic distances; then under show, choose clouds and boundaries to see the cloud alone with the heliosphere's shells and the faint outer shell of the Sun's Hill sphere near 200,000 AU.* The **Oort cloud**, sometimes called the Öpik–Oort cloud, is a theorised cloud of icy planetesimals, perhaps billions or trillions of them, surrounding the [[Sun]] at distances from about 2,000 to 200,000 [[Astronomical_unit|AU]], roughly 0.03 to 3.2 light-years.[^whipple1987][^morbidelli2006] Jan Oort proposed it in 1950 to explain where long-period [[Comet|comets]] come from: in his picture the cloud keeps topping up the supply of comets that fall into the inner [[PORTAL_Solar_System|Solar System]] and are worn away by repeated passages near the Sun.[^oort1950] No object has been seen in the outer cloud itself; its existence is inferred from the orbits of the comets it sends inward.[^nasa-oort] The cloud is usually described as two parts: a spherical outer cloud and a flatter, denser inner cloud, the [[Hills_cloud|Hills cloud]], closer to the plane of the planets.[^levison2007][^hills1981] Even its inner edge lies far beyond the [[Heliosphere|heliosphere]], some 40 times as far from the Sun as the outer edge of the [[Kuiper_belt|Kuiper belt]] at about 50 AU (derived), and well outside the [[Scattered_disc|scattered disc]] and the [[Detached_object|detached objects]].[^levison2007] Its outer edge is set by the Sun's gravitational reach against the [[Milky_Way]]: out there the Sun's pull is so weak that passing stars and the galactic tide reshape orbits and send some bodies inward as comets.[^nasa-kbo][^morbidelli2006] The material is thought to have formed much closer to the Sun, among the giant planets, and to have been scattered outward by them.[^morbidelli2006] The explorer at the top of this page draws the cloud as a static sample of ILLUSTRATIVE points, spread in all directions from 2,000 to 100,000 AU, with the flattened Hills cloud inside it; the outer half of the theorised range, out to 200,000 AU, is not filled with points. ## Development of theory By the early 20th century astronomers recognised two kinds of comets. Short-period, or ecliptic, comets travel on fairly small orbits near the plane of the planets and do not go much beyond about 50 AU. Long-period comets come in from thousands of AU and arrive from every direction on the sky, above and below the [[Ecliptic|ecliptic]] alike.[^duncan2004][^levison2007] In 1907 Armin Otto Leuschner pointed out that orbits computed from short observing runs tended to come out parabolic, and argued that better data would show long-period comets on closed ellipses, as permanent members of the Solar System returning after long absences.[^ley1967] In 1932 the Estonian astronomer Ernst Öpik proposed a reservoir of such comets in a cloud at the edge of the Solar System.[^opik1932] Oort took up the idea in 1950 to resolve a contradiction. A comet whose orbit brings it near the Sun cannot survive for the age of the Solar System: over millions of orbits it will be thrown out by the planets, hit a planet or the Sun, or lose its volatiles to sunlight until it breaks up or seals itself under a dead crust.[^oort1950] The comets seen today therefore cannot have been on their present orbits since the planets formed, more than 4.5 billion years ago; they must have spent almost all that time somewhere colder and farther away.[^oort1950][^jewitt2002] Oort also noticed that the aphelia of the long-period comets he studied clustered around 20,000 AU, which suggested a spherical reservoir at about that distance. Comets with aphelia near 10,000 AU he interpreted as bodies that had already passed through the planetary region at least once and had their orbits shrunk by the planets' gravity.[^levison2007] The modern picture keeps the reservoir and its role, but places its inner part much closer and its outer edge farther away. ## Structure and composition Estimates put the cloud's inner edge at 2,000–5,000 AU and its outer edge at about 50,000 AU, or as far as 100,000–200,000 AU.[^levison2007][^morbidelli2006] It divides into a spherical outer cloud, from about 20,000 to 50,000 AU, and a torus-shaped inner cloud from about 2,000 to 20,000 AU.[^levison2007] The inner part is named the Hills cloud after Jack G. Hills, who proposed it in 1981; models make it tens or hundreds of times as populous as the outer cloud, and it is thought to resupply the outer cloud as that is gradually emptied.[^hills1981][^levison2001][^kaib2024] The scale is easier to grasp in light-years and orbital periods (derived). One light-year is about 63,200 AU, so 2,000 AU is 0.03 light-years and 200,000 AU is 3.2 light-years. [[Kepler's_laws_of_planetary_motion|Kepler's third law]] gives an orbital period of about 89,000 years at 2,000 AU, 2.8 million years at 20,000 AU and 32 million years at 100,000 AU. A circular orbit at 50,000 AU would move at only about 0.13 km/s.[^nasa-fs] The outer cloud may contain trillions of bodies larger than 1 km, with neighbours typically tens of millions of kilometres apart.[^morbidelli2006][^emelyanenko2007][^weissman1998] If [[Halley's_Comet]] is a fair model for a typical nucleus, their combined mass is about 3 × 10²⁵ kg, roughly five times the mass of [[Earth]]; older estimates of up to 380 Earth masses have been lowered as the size distribution of comets has become better known.[^morbidelli2006][^weissman1983] If comets are representative, most of the material is ice of [[Water|water]], methane, ethane, carbon monoxide and hydrogen cyanide.[^gibb2003] The object 1996 PW, which looks like a D-type asteroid but travels on a long-period comet's orbit, led to estimates that 1–2% of the cloud's bodies may be rocky asteroids.[^davies1998][^weissman1997] Carbon and nitrogen isotope ratios differ little between long-period and Jupiter-family comets, which suggests that both formed from the same protosolar material.[^hutsemekers2005] ## Origin The favoured account has the cloud's bodies forming among the giant planets in the [[Protoplanetary_disk|protoplanetary disc]] about 4.6 billion years ago. Close encounters with the young [[Jupiter]], [[Saturn]], [[Uranus]] and [[Neptune]] then flung them onto very long orbits, and passing stars, giant molecular clouds and the galactic tide lifted their perihelia away from the planets, detaching them onto long-lived orbits.[^morbidelli2006] The same forces randomised their inclinations, which explains why the outer cloud is spherical; the more tightly bound Hills cloud has kept a flatter shape.[^morbidelli2006] The cloud appears to hold more material than this basic model delivers. Levison and colleagues showed that the Sun could have captured comets from the discs of other stars in its birth cluster before the cluster dispersed.[^levison2010] Brasser, Duncan and Levison found that a birth cluster of about 200–400 stars, with its much more frequent close stellar passages, is broadly compatible with how the cloud formed.[^brasser2006] Simulations suggest that the cloud's mass peaked about 800 million years after the Solar System formed, after which losses exceeded supply.[^morbidelli2006] The [[Formation_and_evolution_of_the_Solar_System|early Solar System]] also destroyed much of what it scattered. Stern and Weissman found that collisions among comets during the ejection era were frequent enough to grind down most of them before they reached the cloud, so that today's cloud holds only a small fraction of the 50–100 Earth masses thought to have been ejected.[^stern2001][^morbidelli2006] The scattered disc still feeds the cloud: in models roughly half of the bodies it scatters go outward towards the cloud, a quarter are passed inward to Jupiter's region and a quarter are ejected from the Solar System.[^fernandez1997][^fernandez2004] A 2020 proposal adds a temporary binary companion of the young Sun as a further way to capture material.[^siraj2020] ## Comets Comets are left-over building blocks from the Solar System's formation, stored in two main reservoirs, the Kuiper belt and scattered disc beyond Neptune, and the Oort cloud.[^stern2003] Short-period comets, with periods of up to 200 years, are generally traced to the scattered disc, which is dynamically active; they pass inward through the region of the giant planets as [[Centaur_(small_Solar_System_body)|centaurs]] before some are scattered into the inner Solar System.[^levison2007][^jewitt2009][^horner2003] Very long-period comets such as C/1999 F1 (Catalina), whose orbits take millions of years, are thought to come directly from the outer cloud.[^horizons-catalina] Among the short-period comets, the Jupiter-family comets travel on small orbits controlled by Jupiter, while the Halley-type comets, named after [[Halley's_Comet]], are thought to come from the Oort cloud, as long-period comets captured onto shorter orbits by the giant planets.[^levison1997][^wang2014] The same process may have produced a fraction of the Jupiter-family comets, although most are thought to come from the scattered disc.[^emelyanenko2007] Oort's model predicts more returning comets than are observed. This "fading problem" remains unsolved: suggested causes include break-up of nuclei by tidal stress, impacts or heating, loss of all volatiles, and the growth of inert crusts that make old comets inactive.[^neslusan1999][^dones2004comets] Jupiter acts as a partial barrier, so comets fresh from the cloud should be several times more common among the outer planets than in the inner Solar System; the impact of Shoemaker–Levy 9 on Jupiter in 1994 shows one fate of such a comet.[^fernandez2000] C/2018 F4 is described as a typical dynamically old comet from the cloud.[^licandro2019] ## Sedna and similar objects Several known bodies have been proposed as members of the inner Oort cloud.[^dones2015] The first was [[Sedna_(dwarf_planet)|Sedna]], reported in 2004, whose perihelion of 76 AU is too distant for Neptune to have scattered it.[^brown2004] 2012 VP113, announced in 2014, has a still larger perihelion, about 80 AU, but an aphelion only about half of Sedna's.[^trujillo2014][^witze2014] Other candidates include 2010 GB174 and 474640 Alicanto (2004 VN112).[^brasser2015][^chen2013][^becker2008] These objects never come close enough to the Sun to show cometary activity, and they are seen only because they are passing through the near ends of their orbits. They are the [[Sednoid|sednoids]] and other [[Extreme_trans-Neptunian_object|extreme trans-Neptunian objects]], and they probe the inner edge of the cloud, where it meets the detached objects beyond the Kuiper belt.[^brasser2015] In the explorer, their state and this one share the same scene: the sednoid orbits lie well inside the inner edge of the cloud's points. ## Tidal effects Most comets that reach the Sun from the cloud appear to have been nudged by the tide of the [[Milky_Way]]. Just as the Moon's differential pull raises the [[Tide|tides]] in Earth's oceans, the difference in the Galaxy's pull across the Solar System stretches orbits along the direction of the galactic centre and compresses them along the other two axes.[^heisler1986][^fouchard2006] Near the planets the effect is negligible beside the Sun's gravity, but it grows with distance while the Sun's pull fades, and in the cloud it slowly changes the perihelia of orbits until some come close to the Sun.[^fouchard2006] The distance at which the Sun's gravity gives way to the galactic tide, the tidal truncation radius, lies at 100,000–200,000 AU and marks the cloud's outer boundary.[^levison2007] It corresponds to the Sun's [[Hill_sphere|Hill sphere]] with respect to the Galaxy, the region where solar and galactic gravity meet.[^nasa-kbo] The explorer marks it as a faint shell at 200,000 AU, a simplified ILLUSTRATIVE boundary, beyond the last of its sampled points. The tide may also have helped build the cloud, by raising the perihelia of planetesimals flung onto long orbits.[^higuchi2005] Its effect on individual orbits is complicated, but in sum it is large: up to 90% of the comets arriving from the cloud may be delivered by the galactic tide, and statistical studies of new comets' orbits point to the tide as the main agent.[^nurmi2001][^matese2004] ## Stellar perturbations and stellar companion hypotheses Besides the tide, the other main trigger is the passage of nearby stars and giant molecular clouds.[^morbidelli2006][^fernandez2000] As the Sun moves through the Galaxy it occasionally passes close to other stars. Scholz's Star is thought to have crossed the outer cloud about 70,000 years ago, though its low mass and high speed limited its effect.[^mamajek2015][^dlfm2018] Within the next 10 million years, Gliese 710 is the known star most likely to disturb the cloud, and such encounters could also scatter bodies away from the ecliptic, helping to make the cloud spherical.[^molnar1997][^higuchi2006] In 1984 Davis, Hut and Muller proposed that an undetected companion star, a red or brown dwarf later nicknamed Nemesis, passes through the cloud every 26 million years or so, triggering comet showers and periodic extinctions on Earth.[^davis1984] No such star has been found, dynamical arguments and crater records have cast doubt on it, and later analyses do not support strictly periodic extinctions.[^hills1984][^jpl-tyche] In 2002 John Matese and Jack Lissauer argued that too many comets arrive from one region of the cloud to be explained by the tide and stars alone, and proposed a Jupiter-mass planet in a distant orbit, nicknamed Tyche.[^matese2002] The Wide-field Infrared Survey Explorer (WISE) survey ruled out such an object in 2014.[^luhman2014] ## Future exploration No spacecraft has reached the cloud. [[Voyager_1]], the most distant, crossed the heliopause in 2012 but will not reach the inner edge of the cloud for about 300 years, and would need about 30,000 years to cross it.[^nasa-pia17046][^space2013] A 1980s concept called TAU (Thousand Astronomical Units) would have sent a probe to 1,000 AU in 50 years, partly to look for the cloud.[^darling-tau] A solar sail might reach the cloud within a human lifetime without major space infrastructure, depending on the design.[^matloff2005] Detecting the cloud's bodies directly may be easier from a distance. The Whipple Mission, proposed to NASA's Discovery programme in 2014, would have monitored distant stars with a photometer from a halo orbit near the Sun–Earth L2 point for five years, looking for brief occultations by objects up to 10,000 AU away.[^alcock2014] It was also suggested that the Kepler space telescope might have been able to detect such occultations.[^sciam2010] ## See also - [[Hills_cloud]] · [[Hill_sphere]] - [[Heliosphere]] - [[Interstellar_object]] - [[Scattered_disc]] · [[Detached_object]] · [[Sednoid]] - [[Planet_Nine]] ## References [^whipple1987]: Whipple, F. L.; Turner, G.; McDonnell, J. A. M.; Wallis, M. K. (1987). "A review of cometary sciences". *Philosophical Transactions of the Royal Society A* 323: 339–347. https://doi.org/10.1098/rsta.1987.0090 [^morbidelli2006]: Morbidelli, A. (2006). 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"Voyager 1 really is in interstellar space: how NASA knows". *Space.com*. http://www.space.com/22797-voyager-1-interstellar-space-nasa-proof.html [^darling-tau]: Darling, D. "TAU (Thousand Astronomical Unit) mission". *The Encyclopedia of Science*. http://www.daviddarling.info/encyclopedia/T/TAU.html [^matloff2005]: Matloff, G. L. (2005). *Deep Space Probes: To the Outer Solar System and Beyond*. Praxis Publishing, p. 33. ISBN 3-540-24772-6. [^alcock2014]: Alcock, C.; Brown, M.; Gauron, T.; Heneghan, C.; Holman, M.; Kenter, A.; et al. (2014). "The Whipple Mission: exploring the Oort cloud and the Kuiper belt". Harvard-Smithsonian Center for Astrophysics. http://whipple.cfa.harvard.edu/inc/documents/Alcock_AGUPoster_2014dec.pdf [^sciam2010]: *Scientific American* (2010). "Kepler spacecraft may be able to spot elusive Oort cloud objects". http://www.scientificamerican.com/article/kepler-oort-cloud/ ## Further reading - Morbidelli, A. (2006). "Origin and dynamical evolution of comets and their reservoirs of water, ammonia and methane". arXiv:astro-ph/0512256. - Dones, L.; Brasser, R.; Kaib, N.; Rickman, H. (2015). "Origin and evolution of the cometary reservoirs". *Space Science Reviews* 197: 191–269. https://doi.org/10.1007/s11214-015-0223-2 ## External links - NASA Science. "Oort cloud". https://science.nasa.gov/solar-system/oort-cloud/ - JPL Small-Body Database lookup. https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Oort_cloud) : [Wikitube](https://en.wikitube.io/wiki/Oort_cloud) · pinned revision [1374778425](https://en.wikipedia.org/w/index.php?oldid=1374778425) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Solar_System]]. --- *Solar System portal child articles, wave 1 · 2026-09-18 · drafted · row SOL-024 · explorer state `?obj=oort`.*