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The scattered disc (or scattered disk) is a distant region of the Solar System, thinly populated by icy minor planets known as scattered disc objects (SDOs), a subset of the broader family of trans-Neptunian objects (TNOs). The innermost portion of the scattered disc overlaps with the Kuiper belt, but its outer limits extend much farther away from the Sun and farther above and below the ecliptic than the belt proper. Eris, the largest dwarf planet in the Solar System, lies within the scattered disc. The scattered disc consists of those objects that are capable of being gravitationally affected by the planet Neptune. The closest distance from the Sun approached by scattered objects is less than 40 AU, within range of Neptune's influence, but their farthest distances from the Sun extend up to 100 AU. The Kuiper belt, conversely, consists of objects in relatively stable orbits beyond the reach of Neptune. The difference between the Kuiper belt and the scattered disc is not clearcut, and many astronomers see the scattered disc not as a separate population but as an outward region of the Kuiper belt. Although the numbers of objects in the Kuiper belt and the scattered disc are believed to be more or less equal, observational bias due to their farther distance means that far fewer scattered disc objects have been observed to date.[1] Due to its unstable nature, the scattered disc is believed to be the point of origin for most periodic comets observed in the Solar System. The centaurs, a population of icy bodies between Jupiter and Neptune, are believed to be an intermediate stage between the scattered disc and the periodic comets. Many Oort cloud objects are also believed to have originated in the scattered disc.[1]
[edit] Formation
The scattered disc is still poorly understood, although prevailing astronomical opinion suggests it was formed when Kuiper belt objects (KBOs) were "scattered" by gravitational interactions with the outer planets, principally Neptune, into highly eccentric and inclined orbits. While the Kuiper belt is a relatively "round" and "flat" "doughnut" of space, extending from about 30 to 50 AU[2] with its member-objects locked in autonomously circular orbits (cubewanos) or mildly-elliptical resonant orbits (plutinos and twotinos), the scattered disc is by comparison a much more erratic milieu. Scattered disc objects can often, as in the case of Eris, travel almost as great a "vertical" distance as they do relative to what has come to be defined as "horizontal". Orbital simulations show scattered disc object orbits may well be erratic and unstable and that the ultimate fate of these objects is to be either permanently ejected from the core of the solar system into the Oort cloud or beyond, or to be sent into the inner Solar System to become a short-period comet. There is an emerging sense that centaurs may simply be objects just like SDOs that were knocked inwards from the Kuiper belt rather than outwards, making them simply "cis-Neptunian" scattered disc objects. Indeed, some objects like 29981) 1999 TD blur the distinction, and the Minor Planet Center (MPC) now lists centaurs and SDOs together.[3] In recognition of this blurring of categorisation, some scientists use "scattered Kuiper belt object" (or SKBO) as an umbrella term for both centaurs and member bodies of the scattered disc. Although the trans-neptunian object 90377 Sedna is officially considered an scattered disc object by the MPC, its discoverer Michael E. Brown has suggested that because its perihelion distance of 76 AU is too distant to be affected by the gravitational attraction of the outer planets it should be considered an inner Oort cloud object rather than a member of the scattered disc.[4] This line of thinking suggests that a lack of gravitational interaction with the outer planets disqualifies a trans-neptunian object from scattered disc membership, which would create an outer edge somewhere between Sedna and more conventional SDOs like Eris. If Sedna is beyond the scattered disc, it may not be unique; 2000 CR105, which was discovered before Sedna, may also be an inner Oort cloud object or (more likely) a transitional object between the scattered disc and the inner Oort cloud. Such objects, referred to as detached, have orbits which cannot be created by Neptune scattering. Instead, a number of explanations have been put forward including a passing star[5] or a distant, planet-sized object[6] (see Sedna). [edit] Orbits
The first scattered disc object to be recognized was (15874) 1996 TL66[7], first identified in 1996 by astronomers based at Mauna Kea. The first object presently classified as an scattered disc object to be discovered was (48639) 1995 TL8, found by Spacewatch.[citation needed] The diagram on the right illustrates the orbits of all known scattered disc objects up to 100AU together with Kuiper belt objects (in grey) and resonant objects (in green). The eccentricity of the orbits is represented by segments (extending from the perihelion to the aphelion) with the inclination represented on Y axis. Typically, the scattered disc objects are characterized by orbits with medium and high eccentricities with a semi-major axis greater than 50 AU, but their perihelia bring them no closer than 34 AU, clear from direct influence of Neptune (red segments).[8] Plutinos (grey segments for Pluto and Orcus) as well as resonant objects at 2:5 (in green) can approach Neptune closer as their orbits are protected by resonances. This perihelion > 35 AU condition is actually one of the defining characteristics of scattered objects.[9] The scattered disc is the place where extreme eccentricity and high inclination appears to be the norm and circular orbits are exceptional. Some exceptional orbits are plotted in yellow
Resonant objects (shown in green), are not considered to be members of the scattered disc. Minor resonances are also populated and some computer simulations show that many objects could be actually on weak, higher order resonances (6:11,4:9,3:7,5:12,3:8,2:7,1:4). Quoting one of the researchers:[10] the scattered disc might not be so scattered after all. 
[edit] Scattered objects versus classical objects
The inserts in the diagram on the right compare the eccentricity and inclination of the scattered disc population to the cubewanos. Each small coloured square represents a given range for both the eccentricity e and the inclination i. [11] The relative number of objects within the square is represented with cartographic colours[12] (from small numbers plotted as green valleys to brown peaks). 
It is the eccentricity, more than the orbit's inclination, that is the distinctive attribute of the family of scattered disc objects. The two populations are very different: more than 30% of all cubewanos are on low inclination, near circular orbits (the low bottom corner 'peak') and their eccentricity peaks at 0.25. Scattered disc objects on the other hand are scattered. The majority of the known population have medium eccentricity in 0.25-0.55. Two local peaks correspond to e in the 0.25--0.35 range, inclination 15-20° and e=0.5--0.55, low i<10° respectively. The extreme orbits show up as outliers in grey. Characteristically, there are no known scattered disc objects with eccentricity lower than 0.3 (with the exception of 2004 XR190). [edit] Orbit plots
More traditional, the graph on the right represents polar and ecliptic views of the (aligned) orbits of the scattered disc objects[13] (in black) on the background of cubewanos (in blue) and resonant (2:5) objects (in green). As of yet unclassified objects in 50-100AU region are plotted in grey.[14] 
The solid blue ring is not an artist's representation but a real plot of hundreds of overlapping orbits of the classical objects, fully deserving the name of the main (classical or cubewanos) belt. The minimum perihelion mentioned above is illustrated by the red circle. Unlike scattered disc objects, the resonant objects approach Neptune’s orbit (in gold) . On the ecliptic view, the arcs represent the same minimum perihelion[15] of 35AU (red) and Neptune’s orbit (at ~30AU, in yellow). As this view illustrates, the inclinations alone do not really distinguish scattered disc objects from the classical objects. Instead, the eccentricity is the distinctive attribute (long aphelion segments). In the image at right, the upper graph measures the semi-major axes of these objects in astronomical units against their orbital eccentricities. The lower graph measures their semi-major axes against their orbital inclinations in degrees. The two dotted lines represent the perihelion distances of 30 AU and 38 AU, and approximately bound the orbital distribution of the scattered disc. [edit] "Detached objects"The recently discovered objects 2000 CR105 and 2004 VN112 with a perihelion too far away from Neptune to be influenced by it, led to a discussion among astronomers about a new minor planet set, called the Extended scattered disc (E-SDO[16]). More recently, these objects are referred to as detached objects.[17] or Distant Detached Objects (DDO[6]). The classification suggested by Deep Ecliptic Survey team, introduces a formal distinction between Scattered-Near objects (which could be scattered by Neptune) from Scattered-Extended objects (e.g. 90377 Sedna) using Tisserand's parameter value of 3.[18] The diagram illustrates all known scattered and detached objects together with the largest Kuiper belt objects for reference. The very large eccentricities of Sedna and (87269) 2000 OO67 are partly shown with the red segments, extending from the perihelion to the aphelion, well outside the diagram (>900AU and >1020AU respectively). [edit] Noteworthy scattered disc objects
[edit] See also[edit] Notes and references
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