Oort cloud

The Oort cloud (/ɔːrt, ʊərt/),[1] named after the Dutch astronomer Jan Oort, sometimes called the Öpik–Oort cloud,[2] is a hypothetical cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from 2,000 to 200,000 AU (0.03 to 3.2 light-years).[note 1][3] It is divided into two regions: a disc-shaped inner Oort cloud (or Hills cloud) and a spherical outer Oort cloud. Both regions lie beyond the heliosphere and in interstellar space.[3][4] The Kuiper belt and the scattered disc, the other two reservoirs of trans-Neptunian objects, are less than one thousandth as far from the Sun as the Oort cloud.

The outer limit of the Oort cloud defines the cosmographical boundary of the Solar System and the extent of the Sun's Hill sphere.[5] 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 toward the inner Solar System.[3] 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.[3][6]

Astronomers conjecture that the matter composing the Oort cloud formed closer to the Sun and was scattered far into space by the gravitational effects of the giant planets early in the Solar System's evolution.[3] Although no confirmed direct observations of the Oort cloud have been made, it may be 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.[6]

The existence of the Oort cloud was first postulated by Estonian astronomer Ernst Öpik in 1932. Oort independently proposed it in 1950.

PIA17046 - Voyager 1 Goes Interstellar
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 units. The scale is 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.
Kuiper belt - Oort cloud-en
An artist's impression of the Oort cloud and the Kuiper belt (inset). Sizes of individual objects have been exaggerated for visibility.


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.[7]

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.[8] In 1932 Estonian astronomer Ernst Öpik postulated that long-period comets originated in an orbiting cloud at the outermost edge of the Solar System.[9] Dutch astronomer Jan Oort independently revived the idea in 1950 as a means to resolve a paradox:[10]

  • Over the course of the Solar System's existence the orbits of comets are unstable, and 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.[10][11][7] He 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.[7]

Structure and composition

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

The Oort cloud is thought to occupy a vast space from somewhere between 2,000 and 5,000 AU (0.03 and 0.08 ly)[7] to as far as 50,000 AU (0.79 ly)[3] from the Sun. Some estimates place the outer edge at between 100,000 and 200,000 AU (1.58 and 3.16 ly).[7] The region can be subdivided into a spherical outer Oort cloud of 20,000–50,000 AU (0.32–0.79 ly), and a torus-shaped inner Oort cloud of 2,000–20,000 AU (0.0–0.3 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.[3] The inner Oort cloud is also known as the Hills cloud, named after Jack G. Hills, who proposed its existence in 1981.[12] Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;[12][13][14] 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.[15]

The outer Oort cloud may have trillions of objects larger than 1 km (0.62 mi),[3] and billions with absolute magnitudes[16] brighter than 11 (corresponding to approximately 20-kilometre (12 mi) diameter), with neighboring objects tens of millions of kilometres apart.[6][17] 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 3×1025 kilograms (6.6×1025 lb), or five times that of Earth.[3][18] Earlier it was thought to be more massive (up to 380 Earth masses),[19] 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 estimated.

If analyses of comets are representative of the whole, the vast majority of Oort-cloud objects consist of ices such as water, methane, ethane, carbon monoxide and hydrogen cyanide.[20] However, the discovery of the object 1996 PW, an object whose appearance was consistent with a D-type asteroid[21][22] 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.[23] 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,[24] a conclusion also supported by studies of granular size in Oort-cloud comets[25] and by the recent impact study of Jupiter-family comet Tempel 1.[26]


The Oort cloud is thought to have developed after the formation of planets from the primordial protoplanetary disc approximately 4.6 billion years ago.[3] 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 minor planets. After formation, strong gravitational interactions with young gas giants, such as Jupiter, scattered the objects into extremely wide elliptical or parabolic orbits that were subsequently modified by perturbations from passing stars and giant molecular clouds into long-lived orbits detached from the gas giant region.[3][27]

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.[28] 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.[3]

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 toward the Oort cloud, whereas 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.[29] A third of the scattered disc's population is likely to end up in the Oort cloud after 2.5 billion years.[30]

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.[31] The estimated mass of the cloud is only a small part of the 50–100 Earth masses of ejected material.[3]

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.[3] 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 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.[32]

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."[33][34]


Comet Hale-Bopp
Comet Hale–Bopp, an archetypical Oort-cloud comet

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.[7] Comets pass from the scattered disc into the realm of the outer planets, becoming what are known as centaurs.[35] These centaurs are then sent farther inward to become the short-period comets.[36]

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 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.[11] 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.[6]

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.[37] Dynamical studies of hypothetical Oort cloud comets have estimated that their occurrence in the 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.[38]

Tidal effects

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.[39] 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.[7]

Some scholars theorise that the galactic tide may have contributed to the formation of the Oort cloud by increasing the perihelia (smallest distances to the Sun) of planetesimals with large aphelia (largest distances to the Sun).[40] 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.[41] 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.[42]

Stellar 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[3] or giant molecular clouds.[38] 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, 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).[43] During the next 10 million years the known star with the greatest possibility of perturbing the Oort cloud is Gliese 710.[44] This process could also scatter Oort cloud objects out of the ecliptic plane, potentially also explaining its spherical distribution.[44][45]

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 or the Oort cloud have been found, and many lines of evidence (such as crater counts), have thrown their existence into doubt.[46][47] Recent scientific analysis no longer supports the idea that extinctions on Earth happen at regular, repeating intervals.[48] Thus, the Nemesis hypothesis is no longer needed to explain current assumptions.[48]

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.[49] This hypothetical gas giant was nicknamed 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.[48] In 2014, NASA announced that the WISE survey had ruled out any object as they had defined it.[50]

Future exploration

Space probes have yet to reach the area of the Oort cloud. Voyager 1, the fastest[51] and farthest[52][53] of the interplanetary space probes currently leaving the Solar System, will reach the Oort cloud in about 300 years[4][54] and would take about 30,000 years to pass through it.[55][56] However, around 2025, the radioisotope thermoelectric generators on Voyager 1 will no longer supply enough power to operate any of its scientific instruments, preventing any further exploration by Voyager 1. The 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; among its missions would be to look for the Oort cloud.[57]

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 Mission" was proposed.[58] It would monitor distant stars with a photometer, looking for transits up to 10,000 AU away.[58] The observatory was proposed for halo orbiting around L2 with a suggested 5-year mission.[58] It has been suggested that the Kepler observatory may also be able to detect objects in the Oort cloud.[59]

See also


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  57. ^ TAU (Thousand Astronomical Unit) mission
  58. ^ a b c Charles Alcock; et al. "The Whipple Mission: Exploring the Oort Cloud and the Kuiper Belt" (PDF). Archived from the original (PDF) on 2015-11-17. Retrieved 2015-11-12.
  59. ^ Scientific American – Kepler Spacecraft May Be Able to Spot Elusive Oort Cloud Objects – 2010


  1. ^ The Oort cloud's outer limit is difficult to define as it varies over the millennia as different stars pass the Sun and thus is subject to variation. Estimates of its distance range from 50,000 to 200,000 AU.

External links

(308933) 2006 SQ372

(308933) 2006 SQ372 is a trans-Neptunian object and highly eccentric centaur on a cometary-like orbit in the outer region of the Solar System, approximately 123 kilometers (76 miles) in diameter. It was discovered through the Sloan Digital Sky Survey by astronomers Andrew Becker, Andrew Puckett and Jeremy Kubica on images first taken on 27 September 2006 (with precovery images dated to 13 September 2005).

90377 Sedna

90377 Sedna is a large minor planet in the outer reaches of the Solar System that was, as of 2015, at a distance of about 86 astronomical units (1.29×1010 km; 8.0×109 mi) from the Sun, about three times as far as Neptune. Spectroscopy has revealed that Sedna's surface composition is similar to that of some other trans-Neptunian objects, being largely a mixture of water, methane, and nitrogen ices with tholins. Its surface is one of the reddest among Solar System objects. It is most likely a dwarf planet. Among the eight largest trans-Neptunian objects, Sedna is the only one not known to have a moon.For most of its orbit, it is even farther from the Sun than at present, with its aphelion estimated at 937 AU (31 times Neptune's distance), making it one of the most distant-known objects in the Solar System other than long-period comets.Sedna has an exceptionally long and elongated orbit, taking approximately 11,400 years to complete and a distant point of closest approach to the Sun at 76 AU. These facts have led to much speculation about its origin. The Minor Planet Center currently places Sedna in the scattered disc, a group of objects sent into highly elongated orbits by the gravitational influence of Neptune. This classification has been contested because Sedna never comes close enough to Neptune to have been scattered by it, leading some astronomers to informally refer to it as the first known member of the inner Oort cloud. Others speculate that it might have been tugged into its current orbit by a passing star, perhaps one within the Sun's birth cluster (an open cluster), or even that it was captured from another star system. Another hypothesis suggests that its orbit may be evidence for a large planet beyond the orbit of Neptune.Astronomer Michael E. Brown, co-discoverer of Sedna and the dwarf planets Eris, Haumea, and Makemake, thinks that it is the most scientifically important trans-Neptunian object found to date, because understanding its unusual orbit is likely to yield valuable information about the origin and early evolution of the Solar System.

C/2007 Q3

C/2007 Q3 (Siding Spring), is an Oort cloud comet that was discovered by Donna Burton in 2007 at Siding Spring Observatory in New South Wales, Australia. Siding Spring came within 1.2 astronomical units of Earth and 2.25 AU of the Sun on October 7, 2009. The comet was visible with binoculars until January 2010.Images of the comet taken in March 2010 by N.Howes using the Faulkes telescope, showed that the nucleus had fragmented.The comet has an observation arc of 1,327 days and is still been observed as of April 2011. The orbit of a long-period comet is properly obtained when the osculating orbit is computed at an epoch after leaving the planetary region and is calculated with respect to the center of mass of the solar system. Using JPL Horizons, the barycentric orbital elements for epoch 2030-Jan-01 generate a semi-major axis of 7,500 AU, an apoapsis distance of 15,000 AU, and a period of approximately 650,000 years.Before entering the planetary region (epoch 1950), C/2007 Q3 had a calculated barycentric orbital period of ~6.4 million years with an apoapsis (aphelion) distance of about 69,000 AU (1.09 light-years). The comet was probably in the outer Oort cloud for millions or billions of years with a loosely bound chaotic orbit until it was perturbed inward.

C/2010 X1 (Elenin)

Comet C/2010 X1 (Elenin) is an Oort cloud comet discovered by Russian amateur astronomer Leonid Elenin on December 10, 2010, through remote control of the International Scientific Optical Network's robotic observatory near Mayhill in the U.S. state of New Mexico. The discovery was made using the automated asteroids discovery program CoLiTec. At the time of discovery, the comet had an apparent magnitude of 19.5, making it about 150,000 times fainter than can be seen with the naked eye. The discoverer, Leonid Elenin, originally estimated that the comet nucleus was 3–4 km in diameter, but more recent estimates place the pre-breakup size of the comet at 2 km. Comet Elenin started disintegrating in August 2011, and as of mid-October 2011 was not visible even using large ground-based telescopes.

C/2012 K1

C/2012 K1 (PANSTARRS) is a retrograde Oort cloud comet discovered at magnitude 19.7, 8.7 AU from the Sun on 17 May 2012 using the Pan-STARRS telescope located near the summit of Haleakalā, on the island of Maui in Hawaii (U.S.).The comet started 2014 as a Northern Hemisphere object. By late April 2014 it had brightened to roughly apparent magnitude ~8.8 making it a small telescope/binoculars target for experienced observers. In June and July 2014 the comet was near the Sickle of Leo. As of 3 July 2014 the comet had brightened to magnitude 7.9.From 12 July 2014 until 6 September 2014 it had an elongation less than 30 degrees from the Sun. The comet came to perihelion (closest approach to the Sun) on 27 August 2014 at a distance of 1.05 AU (157,000,000 km; 98,000,000 mi) from the Sun. It crosses the celestial equator on 15 September 2014 becoming a Southern Hemisphere object.The comet peaked around magnitude 6.9 in mid-October 2014 when it had an elongation of around 75 degrees from the Sun. It is visible in binoculars and small telescopes.

C/2013 US10

C/2013 US10 (Catalina) is an Oort cloud comet discovered on 31 October 2013 by the Catalina Sky Survey at an apparent magnitude of 19 using a 0.68-meter (27 in) Schmidt–Cassegrain telescope. As of September 2015 the comet is around apparent magnitude 6.

C/2013 V5

C/2013 V5 (Oukaimeden) is a retrograde Oort cloud comet discovered on 12 November 2013 by Oukaimeden Observatory at an apparent magnitude of 19.4 using a 0.5-meter (20 in) reflecting telescope.From 5 May 2014 until 18 July 2014 it had an elongation less than 30 degrees from the Sun. By late August 2014 it had brighten to apparent magnitude 8 making it a small telescope and high-end binoculars target for experienced observers. It crossed the celestial equator on 30 August 2014 becoming a southern hemisphere object. On 16 September 2014 the comet passed 0.480 AU (71,800,000 km; 44,600,000 mi) from Earth. The comet peaked around magnitude 6.2 in mid-September 2014 but only had an elongation of about 35 degrees from the Sun. On 20 September 2014 the comet was visible in STEREO HI-1B. The comet came to perihelion (closest approach to the Sun) on 28 September 2014 at a distance of 0.625 AU (93,500,000 km; 58,100,000 mi) from the Sun.C/2013 V5 is dynamically new. It came from the Oort cloud with a loosely bound chaotic orbit that was easily perturbed by galactic tides and passing stars. Before entering the planetary region (epoch 1950), C/2013 V5 had an orbital period of several million years. After leaving the planetary region (epoch 2050), it will have an orbital period of about 6000 years.


C/2015 ER61 (PANSTARRS) is a comet, inner Oort cloud object, Amor near-Earth asteroid, and possibly a damocloid. When classified as a minor planet, it had the fourth-largest aphelion of any known minor planet in the Solar System, after 2005 VX3, 2012 DR30, and 2013 BL76. It additionally had the most eccentric orbit of any known minor planet, with its distance from the Sun varying by about 99.9% during the course of its orbit, followed by 2005 VX3 with an eccentricity of 0.9973. On January 30, 2016, it was classified as a comet when it was 5.7 AU from the Sun. It comes close to Jupiter, and a close approach in the past threw it on the distant orbit it is on now.

Though the comet nucleus was probably mildly active, early asteroidal estimates gave an absolute magnitude (H) of 12.3, which would suggest a nucleus as large as 8–20 km in diameter. But it could easily be half that size due to activity brightening the nucleus.

Colonization of trans-Neptunian objects

Freeman Dyson has proposed that trans-Neptunian objects, rather than planets, are the major potential habitat of life in space. Several hundred billion to trillion comet-like ice-rich bodies exist outside the orbit of Neptune, in the Kuiper belt and Inner and Outer Oort cloud. These may contain all the ingredients for life (water ice, ammonia, and carbon-rich compounds), including significant amounts of deuterium and helium-3. Since Dyson's proposal, the number of trans-Neptunian objects known has increased greatly.

Colonists could live in the dwarf planet's icy crust or mantle, using fusion or geothermal heat and mining the soft-ice or liquid inner ocean for volatiles and minerals. Given the light gravity and resulting lower pressure in the ice mantle or inner ocean, colonizing the rocky core's outer surface might give colonists the largest number of mineral and volatile resources as well as insulating them from cold. Surface habitats or domes are another possibility, as background radiation levels are likely to be low.Colonists of such bodies could also build rotating habitats or live in dug-out spaces and light them with fusion reactors for thousands to millions of years before moving on. Dyson and Carl Sagan envisioned that humanity could migrate to neighbouring star systems, which have similar clouds, by using natural objects as slow interstellar vessels with substantial natural resources; and that such interstellar colonies could also serve as way-stations for faster, smaller interstellar ships. Alternatively Richard Terra has proposed using the materials from the Oort-cloud objects to build vast starlight collecting arrays to power habitats, thus making an Oort-cloud community essentially independent of its central star and fusion fuel supplies.

Galactic tide

A galactic tide is a tidal force experienced by objects subject to the gravitational field of a galaxy such as the Milky Way. Particular areas of interest concerning galactic tides include galactic collisions, the disruption of dwarf or satellite galaxies, and the Milky Way's tidal effect on the Oort cloud of the Solar System.

Hills cloud

In astronomy, the Hills cloud (also called the inner Oort cloud and inner cloud) is a vast theoretical circumstellar disc, interior to the Oort cloud, whose outer border would be located at around 20,000 to 30,000 astronomical units (AU) from the Sun, and whose inner border, less well-defined, is hypothetically located at 250–1500 AU, well beyond planetary and Kuiper Belt object orbits - but distances might be much greater. If it exists, the Hills cloud contains roughly 5 times as many comets as the Oort cloud.Oort cloud comets are continually perturbed by their environment. A non-negligible fraction leave the Solar System or find their way into the inner system. It should therefore have been depleted long ago, but it has not. The Hills cloud theory addresses the longevity of the Oort cloud by postulating a densely populated inner Oort region. Objects ejected from the Hills cloud are likely to end up in the classical Oort cloud region, maintaining the Oort cloud. It is likely that the Hills cloud has the largest concentration of comets in the whole Solar System.

The existence of the Hills cloud is plausible, since many bodies have been found already. It would be denser than the Oort cloud.

Gravitational interaction with the closest stars and tidal effects from the galaxy have given circular orbits to the comets in the Oort cloud, which may not be the case for the comets in the Hills cloud. The Hills cloud's total mass is unknown; some scientists think it would be more massive than the Oort cloud.

List of Solar System objects

The following is a list of Solar System objects by orbit, ordered by increasing distance from the Sun. Most named objects in this list have a diameter of 500 km or more.

The Sun, a spectral class G2V main-sequence star

The inner Solar System and the terrestrial planets


Mercury-crosser asteroids


Venus-crosser asteroids

2002 VE68, Venus's quasi-satellite



Near-Earth asteroids (including 99942 Apophis)

Earth trojan (2010 TK7)

Earth-crosser asteroids

Earth's quasi-satellites




Mars trojans

Mars-crosser asteroids

Asteroids in the asteroid belt, between the orbits of Mars and Jupiter

Ceres, a dwarf planet




Asteroids number in the hundreds of thousands. For longer lists, see list of notable asteroids, list of asteroids, or list of objects by mass.

Asteroid moons

A number of smaller groups distinct from the asteroid belt

The outer Solar System with the giant planets, their satellites, trojan asteroids and some minor planets


Rings of Jupiter

Complete list of Jupiter's natural satellites





Jupiter trojans

Jupiter-crossing minor planets


Rings of Saturn

Complete list of Saturn's natural satellites



Tethys (trojans: Telesto and Calypso)

Dione (trojans: Helene and Polydeuces)


Rings of Rhea





Saturn-crossing minor planets


Rings of Uranus

Complete list of Uranus's natural satellites






Uranus trojan (2011 QF99)

Uranus-crossing minor planets


Rings of Neptune

Complete list of Neptune's natural satellites




Neptune trojans

Neptune-crossing minor planets

Non-trojan minor planets



Trans-Neptunian objects (beyond the orbit of Neptune)

Kuiper-belt objects (KBOs)


Pluto, a dwarf planet

Complete list of Pluto's natural satellites


90482 Orcus



Cubewanos (classical objects)

Haumea, a dwarf planet



50000 Quaoar


120347 Salacia

20000 Varuna

Makemake, a dwarf planet

Scattered-disc objects

Eris, a dwarf planet


(225088) 2007 OR10

(84522) 2002 TC302

(87269) 2000 OO67

Detached objects

2004 XR190

90377 Sedna (possibly inner Oort cloud)

2012 VP113 (possibly inner Oort cloud)

Oort cloud (hypothetical)

Hills cloud/inner Oort cloud

Outer Oort cloudThe Solar System also contains:


List of periodic comets

List of non-periodic comets

Small objects, including:


Interplanetary dust

Helium focusing cone, around the Sun

Human-made objects orbiting the Sun, Mercury, Venus, Earth, Mars, and Saturn, including active artificial satellites and space junk

Heliosphere, a bubble in space produced by the solar wind



Hydrogen wall, a pile up of hydrogen from the interstellar medium

Nemesis (hypothetical star)

Nemesis is a hypothetical red dwarf or brown dwarf, originally postulated in 1984 to be orbiting the Sun at a distance of about 95,000 AU (1.5 light-years), somewhat beyond the Oort cloud, to explain a perceived cycle of mass extinctions in the geological record, which seem to occur more often at intervals of 26 million years. As of 2012, more than 1800 brown dwarfs have been identified. There are actually fewer brown dwarfs in our cosmic neighborhood than previously thought. Rather than one star for every brown dwarf, there may be as many as six stars for every brown dwarf. The majority of solar-type stars are single. The previous idea stated half or perhaps most stellar systems were binary, trinary, or multiple-star systems associated with clusters of stars, rather than the single-star systems that tend to be seen most often. In a 2017 paper, Sarah Sadavoy and Steven Stahler argued that the Sun was likely part of a binary system at the time of its formation, leading them to suggest "there probably was a Nemesis, a long time ago.” Such a star would have separated from this binary system over four billion years ago, meaning it could not be responsible for the more recent perceived cycle of mass extinctions, Douglas Vakoch told Business Insider, adding that "If the sun really was part of a binary star system in its early days, its early twin deserves a benign name like Companion, rather than the threatening Nemesis."More recent theories suggest that other forces, like close passage of other stars, or the angular effect of the galactic gravity plane working against the outer solar orbital plane, may be the cause of orbital perturbations of some outer Solar System objects. In 2011, Coryn Bailer-Jones analyzed craters on the surface of the Earth and reached the conclusion that the earlier findings of simple periodic patterns (implying periodic comet showers dislodged by a hypothetical Nemesis star) were statistical artifacts, and found that the crater record shows no evidence for Nemesis. However, in 2010, A.L. Melott and R.K. Bambach found evidence in the fossil record confirming the extinction event periodicity originally claimed by Raup & Sepkoski in 1984, but at a higher confidence level and over a time period nearly twice as long. The Infrared Astronomical Satellite (IRAS) failed to discover Nemesis in the 1980s. The 2MASS astronomical survey, which ran from 1997 to 2001, failed to detect an additional star or brown dwarf in the Solar System.Using newer and more powerful infrared telescope technology which is able to detect brown dwarfs as cool as 150 kelvins out to a distance of 10 light-years from the Sun, the Wide-field Infrared Survey Explorer (WISE survey) has not detected Nemesis. In 2011, David Morrison, a senior scientist at NASA known for his work in risk assessment of near Earth objects, has written that there is no confidence in the existence of an object like Nemesis, since it should have been detected in infrared sky surveys.

Scattered disc

The scattered disc (or scattered disk) is a distant circumstellar disc in the Solar System that is sparsely populated by icy small solar system bodies, which are a subset of the broader family of trans-Neptunian objects. The scattered-disc objects (SDOs) have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater than 30 astronomical units (4.5×109 km; 2.8×109 mi). These extreme orbits are thought to be the result of gravitational "scattering" by the gas giants, and the objects continue to be subject to perturbation by the planet Neptune.

Although the closest scattered-disc objects approach the Sun at about 30–35 AU, their orbits can extend well beyond 100 AU. This makes scattered objects among the coldest and most distant objects in the Solar System. The innermost portion of the scattered disc overlaps with a torus-shaped region of orbiting objects traditionally called the Kuiper belt, but its outer limits reach much farther away from the Sun and farther above and below the ecliptic than the Kuiper belt proper.Because of its unstable nature, astronomers now consider the scattered disc to be the place of origin for most periodic comets in the Solar System, with the centaurs, a population of icy bodies between Jupiter and Neptune, being the intermediate stage in an object's migration from the disc to the inner Solar System. Eventually, perturbations from the giant planets send such objects towards the Sun, transforming them into periodic comets. Many objects of the proposed Oort cloud are also thought to have originated in the scattered disc. Detached objects are not sharply distinct from scattered disc objects, and some such as Sedna have sometimes been considered to be included in this group.

Scholz's Star

Scholz's Star (WISE designation WISE 0720−0846 or fully WISE J072003.20−084651.2) is a dim binary stellar system about 17–23 light-years (5.1–7.2 parsecs) from the Sun in the southern constellation Monoceros near the galactic plane. It was discovered in 2013 by astronomer Ralf-Dieter Scholz. In 2015, Eric Mamajek and collaborators reported the system passed through the solar system's Oort cloud roughly 70,000 years ago, and dubbed it Scholz's Star.


A sednoid is a trans-Neptunian object with a perihelion greater than 50 AU and a semi-major axis greater than 150 AU. Only three objects are known from this population, 90377 Sedna, 2012 VP113, and 2015 TG387, all of which have perihelia greater than 64 AU, but it is suspected that there are many more. These objects lie outside an apparently nearly empty gap in the Solar System starting at about 50 AU, and have no significant interaction with the planets. They are usually grouped with the detached objects. Some astronomers, such as Scott Sheppard, consider the sednoids to be inner Oort cloud objects (OCOs), though the inner Oort cloud, or Hills cloud, was originally predicted to lie beyond 2,000 AU, beyond the aphelia of the three known sednoids.

This definition also applies for 2013 SY99 which has a perihelion at 50.02 AU, far beyond the Kuiper cliff, but it is thought not to belong to the Sednoids, but to the same dynamical class as 2004 VN112, 2014 SR349 and 2010 GB174. With these high eccentricities > 0.8 they can easily be distinguished from the high-perihelion objects with moderate eccentricities which are in a stable resonance with Neptune, that is 2015 KQ174, 2015 FJ345, 2004 XR190, 2014 FC72 and 2014 FZ71.

Trans-Neptunian object

A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (AU).

Typically, TNOs are further divided into the classical and resonant objects of the Kuiper belt, the scattered disc and detached objects with the sednoids being the most distant ones. As of October 2018, the catalog of minor planets contains 528 numbered and more than 2,000 unnumbered TNOs.The first trans-Neptunian object to be discovered was Pluto in 1930. It took until 1992 to discover a second trans-Neptunian object orbiting the Sun directly, 15760 Albion. The most massive TNO known is Eris, followed by Pluto, 2007 OR10, Makemake and Haumea. More than 80 satellites have been discovered in orbit of trans-Neptunian objects. TNOs vary in color and are either grey-blue (BB) or very red (RR). They are thought to be composed of mixtures of rock, amorphous carbon and volatile ices such as water and methane, coated with tholins and other organic compounds.

Twelve minor planets with a semi-major axis greater than 150 AU and perihelion greater than 30 AU are known, which are called extreme trans-Neptunian objects (ETNOs).

Tyche (hypothetical planet)

Tyche () is a hypothetical gas giant located in the Solar System's Oort cloud, first proposed in 1999 by astrophysicists John Matese, Patrick Whitman and Daniel Whitmire of the University of Louisiana at Lafayette. They argued that evidence of Tyche's existence could be seen in a supposed bias in the points of origin for long-period comets. More recently, Matese and Whitmire re-evaluated the comet data and noted that Tyche, if it existed, would be detectable in the archive of data that was collected by NASA's Wide-field Infrared Survey Explorer (WISE) telescope. In 2014, NASA announced that the WISE survey had ruled out any object with Tyche's characteristics, indicating that Tyche as hypothesized by Matese, Whitman, and Whitmire does not exist.

Whipple (spacecraft)

Whipple was a proposed space observatory in the NASA Discovery Program. The observatory would try to search for objects in the Kuiper belt and the theorized Oort cloud by conducting blind occultation observations. Although the Oort cloud was hypothized in the 1950s, it has not yet been directly observed. The mission would attempt to detect Oort cloud objects by scanning for brief moments where the objects would block the light of background stars.In 2011, six finalists were selected for the 2016 Discovery Program, and Whipple was not among them, but it was awarded funding to continue its technological development efforts.

Minor planets
Asteroid belt
Centaurs (extended)
Other resonances/
unknown resonances:
Scattered disc
Area uncertain
Detached objects

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