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The scattered disc (or scattered disk) is a distant
circumstellar disc A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accretion disk of matter composed of gas, dust, planetesimals, asteroids, or collision fragments in orbit around a star. Around the youngest stars, they are the reser ...
in the
Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
that is sparsely populated by icy small solar system bodies, which are a subset of the broader family of
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 ...
s. The scattered-disc objects (SDOs) have orbital eccentricities ranging as high as 0.8,
inclinations Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Earth ...
as high as 40°, and perihelia greater than . These extreme orbits are thought to be the result of gravitational "scattering" by the
gas giant A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" ...
s, and the objects continue to be subject to
perturbation Perturbation or perturb may refer to: * Perturbation theory, mathematical methods that give approximate solutions to problems that cannot be solved exactly * Perturbation (geology), changes in the nature of alluvial deposits over time * Perturbat ...
by the planet
Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 time ...
. 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 In geometry, a torus (plural tori, colloquially donut or doughnut) is a surface of revolution generated by revolving a circle in three-dimensional space about an axis that is coplanar with the circle. If the axis of revolution does n ...
-shaped region of orbiting objects traditionally called the
Kuiper belt The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 tim ...
, but its outer limits reach much farther away from the Sun and farther above and below the
ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic agains ...
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 comet Periodic comets (also known as short-period comets) are comets with orbital periods of less than 200 years or that have been observed during more than a single perihelion passage (e.g. 153P/Ikeya–Zhang). "Periodic comet" is also sometimes us ...
s in the Solar System, with the
centaurs A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse. Centaurs are thought of in many Greek myths as bein ...
, 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 The Oort cloud (), sometimes called the Öpik–Oort cloud, first described in 1950 by the Dutch astronomer Jan Oort, is a theoretical concept of a cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from ...
are also thought to have originated in the scattered disc.
Detached object Detached objects are a dynamical class of minor planets in the outer reaches of the Solar System and belong to the broader family of trans-Neptunian objects (TNOs). These objects have orbits whose points of closest approach to the Sun ( perihel ...
s are not sharply distinct from scattered disc objects, and some such as Sedna have sometimes been considered to be included in this group.


Discovery

Traditionally, devices like a
blink comparator A blink comparator is a viewing apparatus formerly used by astronomers to find differences between two photographs of the night sky. It permits rapid switching from viewing one photograph to viewing the other, "blinking" back and forth between th ...
were used in astronomy to detect objects in the Solar System, because these objects would move between two exposures—this involved time-consuming steps like exposing and developing photographic plates or films, and people then using a blink comparator to manually detect prospective objects. During the 1980s, the use of CCD-based cameras in
telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to obse ...
s made it possible to directly produce electronic images that could then be readily
digitized DigitizationTech Target. (2011, April). Definition: digitization. ''WhatIs.com''. Retrieved December 15, 2021, from https://whatis.techtarget.com/definition/digitization is the process of converting information into a Digital data, digital (i ...
and transferred to
digital image A digital image is an image composed of picture elements, also known as ''pixels'', each with '' finite'', '' discrete quantities'' of numeric representation for its intensity or gray level that is an output from its two-dimensional functions ...
s. Because the CCD captured more light than film (about 90% versus 10% of incoming light) and the blinking could now be done at an adjustable computer screen, the surveys allowed for higher throughput. A flood of new discoveries was the result: over a thousand trans-Neptunian objects were detected between 1992 and 2006. The first scattered-disc object (SDO) to be recognised as such was , originally identified in 1996 by
astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, moons, comets and galaxies – in either ...
s based at
Mauna Kea Mauna Kea ( or ; ; abbreviation for ''Mauna a Wākea''); is a dormant volcano on the island of Hawaii. Its peak is above sea level, making it the highest point in the state of Hawaii and second-highest peak of an island on Earth. The peak ...
in Hawaii. Three more were identified by the same survey in 1999: , , and . The first object presently classified as an SDO to be discovered was , found in 1995 by
Spacewatch The Spacewatch Project is an astronomical survey that specializes in the study of minor planets, including various types of asteroids and comets at University of Arizona telescopes on Kitt Peak near Tucson, Arizona, in the United States. The Spa ...
. As of 2011, over 200 SDOs have been identified, including Gǃkúnǁʼhòmdímà (discovered by Schwamb, Brown, and Rabinowitz), (
NEAT Neat may refer to: * Neat (bartending), a single, unmixed liquor served in a rocks glass * Neat, an old term for horned oxen * Neat Records, a British record label * Neuroevolution of augmenting topologies (NEAT), a genetic algorithm (GA) for th ...
), Eris (Brown, Trujillo, and Rabinowitz), Sedna (Brown, Trujillo, and Rabinowitz) and (
Deep Ecliptic Survey The Deep Ecliptic Survey (DES) is a project to find Kuiper belt objects (KBOs), using the facilities of the National Optical Astronomy Observatory (NOAO). The principal investigator is Robert L. Millis. Since 1998 through the end of 2003, the su ...
). Although the numbers of objects in the Kuiper belt and the scattered disc are hypothesized to be roughly equal, observational bias due to their greater distance means that far fewer SDOs have been observed to date.


Subdivisions of trans-Neptunian space

Known trans-Neptunian objects are often divided into two subpopulations: the Kuiper belt and the scattered disc. A third reservoir of trans-Neptunian objects, the
Oort cloud The Oort cloud (), sometimes called the Öpik–Oort cloud, first described in 1950 by the Dutch astronomer Jan Oort, is a theoretical concept of a cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from ...
, has been hypothesized, although no confirmed direct observations of the Oort cloud have been made. Some researchers further suggest a transitional space between the scattered disc and the inner Oort cloud, populated with "
detached object Detached objects are a dynamical class of minor planets in the outer reaches of the Solar System and belong to the broader family of trans-Neptunian objects (TNOs). These objects have orbits whose points of closest approach to the Sun ( perihel ...
s".


Scattered disc versus Kuiper belt

The Kuiper belt is a relatively thick
torus In geometry, a torus (plural tori, colloquially donut or doughnut) is a surface of revolution generated by revolving a circle in three-dimensional space about an axis that is coplanar with the circle. If the axis of revolution does n ...
(or "doughnut") of space, extending from about 30 to 50 AU comprising two main populations of Kuiper belt objects (KBOs): the classical Kuiper-belt objects (or "cubewanos"), which lie in orbits untouched by Neptune, and the resonant Kuiper-belt objects; those which Neptune has locked into a precise orbital ratio such as 2:3 (the object goes around twice for every three Neptune orbits) and 1:2 (the object goes around once for every two Neptune orbits). These ratios, called
orbital resonance In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationsh ...
s, allow KBOs to persist in regions which Neptune's gravitational influence would otherwise have cleared out over the age of the Solar System, since the objects are never close enough to Neptune to be scattered by its gravity. Those in 2:3 resonances are known as " plutinos", because
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...
is the largest member of their group, whereas those in 1:2 resonances are known as "
twotinos In astronomy, a resonant trans-Neptunian object is a trans-Neptunian object (TNO) in mean-motion orbital resonance with Neptune. The orbital periods of the resonant objects are in a simple integer relations with the period of Neptune, e.g. 1:2, ...
". In contrast to the Kuiper belt, the scattered-disc population can be disturbed by Neptune. Scattered-disc objects come within gravitational range of Neptune at their closest approaches (~30 AU) but their farthest distances reach many times that. Ongoing research suggests that the
centaurs A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse. Centaurs are thought of in many Greek myths as bein ...
, a class of icy
planetoid According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term ''minor ...
s that orbit between Jupiter and Neptune, may simply be SDOs thrown into the inner reaches of the Solar System by Neptune, making them "cis-Neptunian" rather than trans-Neptunian scattered objects. Some objects, like (29981) 1999 TD10, blur the distinction and the
Minor Planet Center The Minor Planet Center (MPC) is the official body for observing and reporting on minor planets under the auspices of the International Astronomical Union (IAU). Founded in 1947, it operates at the Smithsonian Astrophysical Observatory. Function T ...
(MPC), which officially catalogues all
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 ...
s, now lists centaurs and SDOs together. The MPC, however, makes a clear distinction between the Kuiper belt and the scattered disc, separating those objects in stable orbits (the Kuiper belt) from those in scattered orbits (the scattered disc and the centaurs). However, the difference between the Kuiper belt and the scattered disc is not clear-cut, and many astronomers see the scattered disc not as a separate population but as an outward region of the Kuiper belt. Another term used is "scattered Kuiper-belt object" (or SKBO) for bodies of the scattered disc. Morbidelli and Brown propose that the difference between objects in the Kuiper belt and scattered-disc objects is that the latter bodies "are transported in semi-major axis by close and distant encounters with Neptune," but the former experienced no such close encounters. This delineation is inadequate (as they note) over the age of the Solar System, since bodies "trapped in resonances" could "pass from a scattering phase to a non-scattering phase (and vice versa) numerous times." That is, trans-Neptunian objects could travel back and forth between the Kuiper belt and the scattered disc over time. Therefore, they chose instead to define the regions, rather than the objects, defining the scattered disc as "the region of orbital space that can be visited by bodies that have encountered Neptune" within the radius of a Hill sphere, and the Kuiper belt as its "complement ... in the ''a'' > 30 AU region"; the region of the Solar System populated by objects with semi-major axes greater than 30 AU.


Detached objects

The Minor Planet Center classifies the trans-Neptunian object 90377 Sedna as a scattered-disc object. Its discoverer
Michael E. Brown Michael E. Brown (born June 5, 1965) is an American astronomer, who has been professor of planetary astronomy at the California Institute of Technology (Caltech) since 2003. His team has discovered many trans-Neptunian objects (TNOs), includin ...
has suggested instead that it should be considered an inner Oort-cloud object rather than a member of the scattered disc, because, with a
perihelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any elli ...
distance of 76 AU, it is too remote to be affected by the gravitational attraction of the outer planets. Under this definition, an object with a perihelion greater than 40 AU could be classified as outside the scattered disc. Sedna is not the only such object: (discovered before Sedna) and have a perihelion too far away from
Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 time ...
to be influenced by it. This led to a discussion among astronomers about a new minor planet set, called the ''extended scattered disc'' (E-SDO). may also be an inner Oort-cloud object or (more likely) a transitional object between the scattered disc and the inner Oort cloud. More recently, these objects have been referred to as ''"detached"'',
Preprint version (pdf)
or ''distant detached objects'' (DDO). There are no clear boundaries between the scattered and detached regions. Gomes et al. define SDOs as having "highly eccentric orbits, perihelia beyond Neptune, and semi-major axes beyond the 1:2 resonance." By this definition, all distant detached objects are SDOs. Since detached objects' orbits cannot be produced by Neptune scattering, alternative scattering mechanisms have been put forward, including a passing star or a distant, planet-sized object. Alternative, it has been suggested that these objects have been captured from a passing star. A scheme introduced by a 2005 report from the Deep Ecliptic Survey by J. L. Elliott et al. distinguishes between two categories: ''scattered-near'' (i.e. typical SDOs) and ''scattered-extended'' (i.e. detached objects). Scattered-near objects are those whose orbits are non-resonant, non-planetary-orbit-crossing and have a Tisserand parameter (relative to Neptune) less than 3. Scattered-extended objects have a Tisserand parameter (relative to Neptune) greater than 3 and have a time-averaged eccentricity greater than 0.2. An alternative classification, introduced by B. J. Gladman, B. G. Marsden and C. Van Laerhoven in 2007, uses 10-million-year orbit integration instead of the Tisserand parameter. An object qualifies as an SDO if its orbit is not resonant, has a semi-major axis no greater than 2000 AU, and, during the integration, its semi-major axis shows an excursion of 1.5 AU or more. Gladman et al. suggest the term ''scattering disk object'' to emphasize this present mobility. If the object is not an SDO as per the above definition, but the eccentricity of its orbit is greater than 0.240, it is classified as a ''detached TNO''. (Objects with smaller eccentricity are considered classical.) In this scheme, the disc extends from the orbit of Neptune to 2000 AU, the region referred to as the inner Oort cloud.


Orbits

The scattered disc is a very dynamic environment. Because they are still capable of being perturbed by Neptune, SDOs' orbits are always in danger of disruption; either of being sent outward to the Oort cloud or inward into the centaur population and ultimately the Jupiter family of comets. For this reason Gladman et al. prefer to refer to the region as the scattering disc, rather than scattered. Unlike Kuiper-belt objects (KBOs), the orbits of scattered-disc objects can be inclined as much as 40° from the
ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic agains ...
. SDOs are typically characterized by orbits with medium and high eccentricities with a
semi-major axis In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the lon ...
greater than 50 AU, but their perihelia bring them within influence of Neptune. Having a perihelion of roughly 30 AU is one of the defining characteristics of scattered objects, as it allows Neptune to exert its gravitational influence. The classical objects (
cubewanos A classical Kuiper belt object, also called a cubewano ( "QB1-o"), is a low-eccentricity Kuiper belt object (KBO) that orbits beyond Neptune and is not controlled by an orbital resonance with Neptune. Cubewanos have orbits with semi-major axe ...
) are very different from the scattered objects: more than 30% of all cubewanos are on low-inclination, near-circular orbits whose eccentricities peak at 0.25. Classical objects possess eccentricities ranging from 0.2 to 0.8. Though the inclinations of scattered objects are similar to the more extreme KBOs, very few scattered objects have orbits as close to the ecliptic as much of the KBO population. Although motions in the scattered disc are random, they do tend to follow similar directions, which means that SDOs can become trapped in temporary resonances with Neptune. Examples of possible resonant orbits within the scattered disc include 1:3, 2:7, 3:11, 5:22 and 4:79.


Formation

The scattered disc is still poorly understood: no model of the formation of the Kuiper belt and the scattered disc has yet been proposed that explains all their observed properties. According to contemporary models, the scattered disc formed when
Kuiper belt The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 tim ...
objects (KBOs) were "scattered" into
eccentric Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-center, in geometry * Eccentricity (graph theory) of a v ...
and
inclined Incline, inclined, inclining, or inclination may refer to: *Grade (slope), the tilt, steepness, or angle from horizontal of a topographic feature (hillside, meadow, etc.) or constructed element (road, railway, field, etc.) *Slope, the tilt, steepn ...
orbits by gravitational interaction with Neptune and the other outer planets. The amount of time for this process to occur remains uncertain. One hypothesis estimates a period equal to the entire age of the Solar System; a second posits that the scattering took place relatively quickly, during Neptune's early
migration Migration, migratory, or migrate may refer to: Human migration * Human migration, physical movement by humans from one region to another ** International migration, when peoples cross state boundaries and stay in the host state for some minimum le ...
epoch. Models for a continuous formation throughout the age of the Solar System illustrate that at weak resonances within the Kuiper belt (such as 5:7 or 8:1), or at the boundaries of stronger resonances, objects can develop weak orbital instabilities over millions of years. The 4:7 resonance in particular has large instability. KBOs can also be shifted into unstable orbits by close passage of massive objects, or through collisions. Over time, the scattered disc would gradually form from these isolated events. Computer simulations have also suggested a more rapid and earlier formation for the scattered disc. Modern theories indicate that neither
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus ( Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars), grandfather of Zeus (Jupiter) and father of ...
nor Neptune could have formed ''in situ'' beyond Saturn, as too little primordial matter existed at that range to produce objects of such high mass. Instead, these planets, and Saturn, may have formed closer to Jupiter, but were flung outwards during the early evolution of the Solar System, perhaps through exchanges of
angular momentum In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syst ...
with scattered objects. Once the orbits of Jupiter and Saturn shifted to a 2:1 resonance (two Jupiter orbits for each orbit of Saturn), their combined gravitational pull disrupted the orbits of Uranus and Neptune, sending Neptune into the temporary "chaos" of the proto-Kuiper belt. As Neptune traveled outward, it scattered many trans-Neptunian objects into higher and more eccentric orbits. This model states that 90% or more of the objects in the scattered disc may have been "promoted into these eccentric orbits by Neptune's resonances during the migration epoch... hereforethe scattered disc might not be so scattered."


Composition

Scattered objects, like other trans-Neptunian objects, have low densities and are composed largely of frozen
volatiles Volatiles are the group of chemical elements and chemical compounds that can be readily vaporized. In contrast with volatiles, elements and compounds that are not readily vaporized are known as refractory substances. On planet Earth, the term ...
such as water and
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane ...
. Spectral analysis of selected Kuiper belt and scattered objects has revealed signatures of similar compounds. Both Pluto and Eris, for instance, show signatures for methane. Astronomers originally supposed that the entire trans-Neptunian population would show a similar red surface colour, as they were thought to have originated in the same region and subjected to the same physical processes. Specifically, SDOs were expected to have large amounts of surface methane, chemically altered into tholins by sunlight from the Sun. This would absorb blue light, creating a reddish hue. Most classical objects display this colour, but scattered objects do not; instead, they present a white or greyish appearance. One explanation is the exposure of whiter subsurface layers by impacts; another is that the scattered objects' greater distance from the Sun creates a composition gradient, analogous to the composition gradient of the terrestrial and gas giant planets. Michael E. Brown, discoverer of the scattered object Eris, suggests that its paler colour could be because, at its current distance from the Sun, its atmosphere of methane is frozen over its entire surface, creating an inches-thick layer of bright white ice. Pluto, conversely, being closer to the Sun, would be warm enough that methane would freeze only onto cooler, high-
albedo Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refle ...
regions, leaving low-albedo tholin-covered regions bare of ice.


Comets

The Kuiper belt was initially thought to be the source of the Solar System's ecliptic comets. However, studies of the region since 1992 have shown that the orbits within the Kuiper belt are relatively stable, and that ecliptic comets originate from the scattered disc, where orbits are generally less stable. Comets can loosely be divided into two categories: short-period and long-period—the latter being thought to originate in the Oort cloud. The two major categories of short-period comets are Jupiter-family comets (JFCs) and Halley-type comets. Halley-type comets, which are named after their prototype,
Halley's Comet Halley's Comet or Comet Halley, officially designated 1P/Halley, is a short-period comet visible from Earth every 75–79 years. Halley is the only known short-period comet that is regularly visible to the naked eye from Earth, and thus the on ...
, are thought to have originated in the Oort cloud but to have been drawn into the inner Solar System by the gravity of the giant planets, whereas the JFCs are thought to have originated in the scattered disc. The centaurs are thought to be a dynamically intermediate stage between the scattered disc and the Jupiter family. There are many differences between SDOs and JFCs, even though many of the Jupiter-family comets may have originated in the scattered disc. Although the centaurs share a reddish or neutral coloration with many SDOs, their nuclei are bluer, indicating a fundamental chemical or physical difference. One hypothesis is that comet nuclei are resurfaced as they approach the Sun by subsurface materials which subsequently bury the older material.


See also

*
List of possible dwarf planets The number of dwarf planets in the Solar System is unknown. Estimates have run as high as 200 in the Kuiper belt and over 10,000 in the region beyond. However, consideration of the surprisingly low densities of many large trans-Neptunian objects ...
* List of trans-Neptunian objects


Notes


References

{{DEFAULTSORT:Scattered Disc Trans-Neptunian region 19961009