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Scattered disc is out by the Kuiper belt.
The Scattered disc is a distant region of our solar system. The scattered disc is 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. The scattered disc's outer limits extend much farther away from the Sun and above and below the ecliptic than the belt proper.
Formation of the scattered disc.
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 AU to 44 AU 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. SDOs 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 SDO orbits may well be erratic and unstable and that the ultimate fate of these objects is to be permanently ejected from the core of the solar system into the Oort cloud or beyond.
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" SDOs. Indeed, some objects like (29981) 1999 TD10 blur the distinction, and the Minor Planet Center (MPC) now lists centaurs and SDOs together. In recognition of this blurring of categorization, 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 TNO 90377 Sedna is officially considered an SDO 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 . This line of thinking suggests that a lack of gravitational interaction with the outer planets disqualifies a TNO 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 (Morbidelli 2004) or a distant, planet-sized object (Gomes 2006) See Sedna.
Orbits of the scattered disc.
The first SDO to be recognized was (15874) 1996 TL66, first identified in 1996 by astronomers based at Mauna Kea. The first object presently classified as an SDO to be discovered was (48639) 1995 TL8, found by Spacewatch.
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 objects are characterised by orbits with medium and high eccentricities but their perihelia bring them no closer than 35AU, clear from direct influence of Neptune (red segments). Plutinos (grey segments for Pluto and Orcus) as well as resonant object 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.
Scattered disc: Extremes.
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
scattered disc: Some order in the chaos?
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 : the scattered disc might not be so scattered after all.
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 i1. The relative number of objects within the square is represented with cartographic colours2 (from small numbers plotted as green valleys to brown peaks).
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 objects on the other hand are, well, 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 15-20o and e=0.5--0.55, low i<10o respectively. The extreme orbits show up as outliers in grey. Characteristically, there are no known SDO objects with eccentricity lower than 0.3 (with the exception of 2004 XR190).
It is the eccentricity, more than the orbit's inclination, that is the distinctive attribute of the family of scattered objects.
1As near-circular orbits occupy the first column (e<0.05) and the orbits with the lowest inclination (i<5 degrees) occupy the lowest row, the square in the bottom left corner represents the number of near circular, very lowly inclined orbits.
2A grey square represents a single object (an outlier) in this range.
Orbit plots of the scattered disc.
More traditional, the graph on the left represents polar and ecliptic views of the (aligned) orbits of the scattered disc objects (in black) on the background of cubewanos (in blue) and resonant (2:5) objects (in green). As yet unclassified objects in 50-100AU region are plotted in grey1.
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 SDOs, the resonant objects approach Neptune’s orbit (in gold) .
On the ecliptic view, the arcs represent the same minimum perihelion2 of 35AU (red) and Neptune’s orbit (at ~30AU, in yellow). As this view illustrates, the inclinations alone do not really distinguish SDO from the classical objects. Instead, the eccentricity is the distinctive attribute (long aphelion segments).
1For roughly a half of known TNO the orbits are not yet known with the precision sufficient for the classification (a particularly delicate task for resonant objects).
2The precise value is not too important; the value of 35 AU is quoted for coherence with Jewitt. Other authors prefer to use 30AU instead while the data used here appear to fit 34AU.
Detached objects, or an extended scattered disc?
The recently discovered objects 2000 CR105 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, Gladman). More recently, these objects are referred to as detached objects (Jewitt,Delsanti) or Distant Detached Objects (DDO, Gomes et al..
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.
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).
1Note that the positions on the diagram represent semi-major axis (mean distance to the Sun) and not the current positions of the objects. Sedna is currently actually closer than Eris.
Noteworthy scattered disc objects.
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