http://en.wikipedia.org/wiki/Scattered_disc wrote:
<<The Kuiper belt was initially believed to be the source of the Solar System's 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.
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. 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 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.
The eccentricity and inclination of the scattered disc population compared to the classical and 5:2 resonant Kuiper belt objects
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 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 giants, 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 AU, their orbits can extend well beyond 100 AU. This makes scattered objects "among the most distant and cold objects in the Solar System". 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 comets observed 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 Oort cloud objects are also believed to have originated in the scattered disc.
The first scattered disc object to be recognised as such was a (15874) 1996 TL66, originally identified in 1996 by astronomers based at Mauna Kea in Hawaii. Three more were identified by the same survey in 1999: 1999 CV118, 1999 CY118 and 1999 CF119. The first object presently classified as a scattered disc object to be discovered was (48639) 1995 TL8, found in 1995 by Spacewatch. As of 2009, over 100 scattered disc objects have been identified, including 2007 UK126 (discovered by Schwamb, Brown, and Rabinowitz), (84522) 2002 TC302 (NEAT), Eris (Brown, Trujillo, and Rabinowitz) Sedna (Brown, Trujillo, and Rabinowitz) and 2004 VN112 (Deep Ecliptic Survey). 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 scattered disc objects have been observed to date.
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. Some researchers further suggest a transitional space between the scattered disc and the inner Oort cloud, populated with "detached objects".
The Kuiper belt is a relatively thick torus (or "doughnut") of space, extending from about 30 to 50 AU comprising two main populations: 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 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 resonances, 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 3:2 resonances are known as "plutinos", because Pluto is the largest member of their group, whereas those in 2:1 resonances are known as "twotinos".
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 class of icy planetoids 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[21] and the Minor Planet Center (MPC), which officially catalogues all trans-Neptunian objects, now lists centaurs and SDOs together.[8]
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. Thus, an object with a perihelion greater than 40 AU could be classified as outside the scattered disc.
The scattered disc is a very dynamic environment. Because they are still capable of being perturbed by Neptune, scattered disc objects' 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.
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.
Simulation showing Outer Planets and Kuiper Belt: a) Before Jupiter/Saturn 2:1 resonance b) Scattering of Kuiper Belt objects into the solar system after the orbital shift of Neptune c) After ejection of Kuiper Belt bodies by Jupiter
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 objects (KBOs) were "scattered" into eccentric and inclined 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 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 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 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...[therefore] the scattered disc might not be so scattered.">>