Image:Eris and dysnomia2.jpg the largest known scattered disc object (center), and its moon Dysnomia (moon) (left of object)]] The scattered disc (or scattered disk is a distant region of the Solar System that is sparsely populated by icy minor planets 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 These extreme orbits are believed to be the result of gravitational "scattering" by the gas giant , and the objects continue to be subject to perturbation by the planet Neptune While the nearest distance to the Sun approached by scattered objects is about 30–35 Astronomical unit their orbits can extend well beyond 100 AU. This makes scattered objects "among the most distant and cold objects in the Solar System".Maggie Masetti. (2007). [http://heasarc.gsfc.nasa.gov/docs/cosmic/solar_system_info.html Cosmic Distance Scales – The Solar System] Website of NASAs High Energy Astrophysics Science Archive Research Center. Retrieved 2008 07-12. The innermost portion of the scattered disc overlaps with a torus shaped region of orbiting objects known as the Kuiper belt but its outer limits reach much farther away from the Sun and farther above and below the ecliptic than the belt proper. Because of its unstable nature, astronomers now consider the scattered disc to be the place of origin for most periodic comet observed in the Solar System, with the Centaur (minor planet) a population of icy bodies between Jupiter and Neptune, being the intermediate stage in an objects migration from the disc to the inner Solar System.lt;/ref> Eventually, perturbations from the giant planets send such objects towards the Sun, transforming them into periodic comets. Many Oort cloud objects are also believed to have originated in the scattered disc.

Discovery

During the 1980s, the introduction of the charge-coupled device in telescope in combination with higher capacity computers for image analysis allowed for more efficient deep sky surveys than was practical using photography. This led to a flood of new discoveries: between 1992 and 2006, over a thousand trans-Neptunian Objects were detected.lt;/ref> The first scattered disc object to be recognised as such was a lt;/ref>lt;/ref> originally identified in 1996 by astronomy based at Mauna Kea 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 Lutz D. Schmadel, (2003). Dictionary of Minor Planet Names(5th rev. and enlarged ed. edition). Berlin: Springer. Page 925 (Appendix 10). Also see McFadden, Lucy-Ann, Weissman, Paul & Johnson, Torrence (1999). Encyclopedia of the Solar System San Diego: Academic Press. Page 218. As of 2009, over 100 SDOs have been identified, including (discovered by Schwamb, Brown, and Rabinowitz),lt;/ref> (Near Earth Asteroid Tracking , Eris (dwarf planet) (Brown, Trujillo, and Rabinowitz)lt;/ref> 90377 Sedna (Brown, Trujillo, and Rabinowitz)lt;/ref> and (Deep Ecliptic Survey .lt;/ref> 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.lt;/ref>

Subdivisions of trans-Neptunian space

Image:TheKuiperBelt Projections 100AU Classical SDO.svg Kuiper belt objects]] 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 is believed to exist, although no confirmed direct observations of the Oort cloud have been made.lt;/ref> Some researchers further suggest a transitional space between the scattered disc and the inner Oort cloud, populated with "Detached object .

Scattered disc versus Kuiper belt

The Kuiper belt is a relatively thick torus (or "doughnut") of space, extending from about 30 to 50 AUlt;/ref> comprising two main populations: the classical Kuiper belt object (or "cubewanos"), which lie in orbits untouched by Neptune, and the resonant Kuiper belt object ; those which Neptune has locked into a precise orbital ratio such as 3:2 (the KBO goes around twice for every three Neptune orbits) and 2:1 (the object goes around once for every two Neptune orbits). These ratios, called orbital resonance , allow KBOs to persist in regions which Neptunes 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 3:2 resonances are known as "plutino ", because Pluto is the largest member of their group, whereas those in 2:1 resonances are known as "resonant Kuiper belt object#1:2 resonance ("twotinos", period ~330 years) . 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.lt;/ref> Ongoing researchlt;/ref> suggests that the Centaur (minor planet) a class of icy planetoid 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.Remo notes that Cis-Neptunian bodies "include terrestrial and large gaseous planets, planetary moons, asteroids, and main-belt comets within Neptunes orbit."(Remo 2007) Some objects, like (29981) 1999 TD₁₀, blur the distinctionlt;/ref> and the Minor Planet Center (MPC), which officially catalogues all trans-Neptunian object , now lists centaurs and SDOs together. The MPC also 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).lt;/ref> However, the difference between the Kuiper belt and the scattered disc is not clearcut, and many astronomy 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.lt;/ref> Morbidelli and Brown propose that the difference between objects in the Kuiper belt and scattered 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 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 distance of 76 AU, it is too remote to be affected by the gravitational attraction of the outer planets.lt;/ref> Under this definition, an object with a perihelion greater than 40 AU could be classified as outside the scattered disc.lt;/ref> Sedna is not the only such object: (discovered before Sedna) and have a perihelion too far away from Neptune 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 .lt;/ref> 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"(http://www2.ess.ucla.edu/~jewitt/papers/2006/DJ06.pdf 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 starlt;/ref> or a distant, planet-sized object.lt;/ref> 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).lt;/ref> 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 Brett J. Gladman Brian G. Marsden and C. VanLaerhoven in 2007, uses 10-million-year orbit integration instead of the Tisserand parameter.lt;/ref> 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. 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

Image:TheKuiperBelt 100AU SDO.svg (in green). The eccentricity (orbit) of the orbits is represented by segments (extending from the perihelion to the aphelion with the inclination represented on Y axis.]] 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 objects can be inclined as much as 40° from the ecliptic lt;/ref> SDOs are typically characterized by orbits with medium and high eccentricities with a semi-major axis greater than 50 AU, but their perihelia bring them within influence of Neptune.lt;/ref> 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.lt;/ref> The classical objects (cubewanos 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.lt;/ref> 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 resonant orbits within the scattered disc include 1:3, 2:7, 3:11, 5:22 and 4:79.

Formation

Image:Lhborbits.png 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.lt;/ref> According to contemporary models, the scattered disc formed when Kuiper belt objects (KBOs) were "scattered" into eccentricity (orbit) and inclination orbits by gravitational interaction with Neptune and the other outer planets lt;/ref> 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;lt;/ref> a second posits that the scattering took place relatively quickly, during Neptunes early Neptune#Formation and migration 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 nor Neptune could have formed in situbeyond 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 with scattered objects.(subscription required) Once the orbits of Jupiter and Saturn shifted to a 2:1 Orbital 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.lt;/ref> As Neptune traveled outward, it scattered many trans-Neptunian object into higher and more eccentric orbits.lt;/ref>lt;/ref> This model states that 90% or more of the objects in the scattered disc may have been "promoted into these eccentric orbits by Neptunes resonances during the migration epoch...therefore] the scattered disc might not be so scattered."lt;/ref>

Composition

Scattered objects, like other trans-Neptunian objects, have low densities and are composed largely of frozen volatiles such as water and 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 redsurface colour, as they were believed to have originated in the same region and subjected to the same physical processes.lt;/ref> Specifically, SDOs were expected to have large amounts of surface methane, chemically altered into complex organic molecules by energy 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 regions, leaving low-albedo tholin covered regions bare of ice.lt;/ref>

Comets

Image:PIA02127.jpg a Jupiter-family comet]] The Kuiper belt was initially believed to be the source of the Solar Systems ecliptic comets. However, studies of the region since 1992 have revealed that the orbits within what is now called the Kuiper belt are relatively stable, and that these comets originate from the more dynamic scattered disc.lt;/ref> Comets can loosely be divided into two categories: short-period and long period—the latter being believed to originate in the Oort cloud There are two major categories of short-period comets: Jupiter-family comets and Halley-family comets. The latter group, which is named for its prototype, Halley's Comet are believed to have emerged from the Oort cloud but to have been drawn into the inner Solar System by the gravity of the giant planets. The former type, the Jupiter family, are believed to have originated from the scattered disc.lt;/ref> The Centaur (minor planet) are thought to be a dynamically intermediate stage between the scattered disc and the Jupiter family. There are many differences between SDOs and Jupiter-family comets, even though many of the latter 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.lt;/ref>

See also

* List of trans-Neptunian objects * List of plutoid candidates

References

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