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Scattered disc

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   The scattered disc (or scattered disk) is a distant region of our 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 above and below the ecliptic than the belt proper.

                                        TNOs and similar bodies
                                 * cis-Neptunian objects
                                      + centaurs
                                      + Neptune Trojan
                                 * trans-Neptunian objects (TNOs)
                                      + Kuiper belt objects (KBOs)
                                           o classical KBOs (cubewanos)
                                           o resonant KBOs
                                                # plutinos (2:3 resonance)
                                      + scattered disc objects (SDOs)
                                      + Oort cloud objects (OCOs)

Formation

   The scattered disk 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 TD[10] blur the distinction, and the Minor Planet
   Centre (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 disk .
   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
   disk, it may not be unique; 2000 CR[105], 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

   Scattered disk and Kuiper Belt objects.
   Enlarge
   Scattered disk and Kuiper Belt objects.

   The first SDO to be recognized was (15874) 1996 TL[66], first
   identified in 1996 by astronomers based at Mauna Kea. The first object
   presently classified as an SDO to be discovered was (48639) 1995 TL[8],
   found by Spacewatch.

   The diagram on the right illustrates the orbits of all known scattered
   disk 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.

Perihelia

   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.

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
     * 1999 TD[10] has an orbit with extreme eccentricity (~0.9), bringing
       its perihelion near Saturn's orbit. This could qualify it as a
       Centaur.
     * 2002 XU[93] is currently the object with the highest inclination
       (~78^o) in the Scattered Disc.
     * 2004 XR[190] has the atypical, near circular (the short yellow
       segment) orbit, but it is highly inclined.

Some order in the chaos?

   Resonant objects (shown in green), are not considered to be members of
   the scattered disk. 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 disk might not be so scattered after
   all.

Scattered objects versus classical objects

   Scattered objects compared with the classical objects.
   Enlarge
   Scattered objects compared with the classical objects.

   The inserts in the diagram on the right compare the eccentricity and
   inclination of the scattered disk population to the cubewanos. Each
   small coloured square represents a given range for both the
   eccentricity e and the inclination i ^1. The relative number of objects
   within the square is represented with cartographic colours^2 (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 15-20^o and e=0.5--0.55, low
   i<10^o 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 XR[190]).

   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

   Orbit projections.
   Enlarge
   Orbit projections.

   More traditional, the graph on the left represents polar and ecliptic
   views of the (aligned) orbits of the scattered disk 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
   grey ^1.

   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 perihelion^2
   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?

   Distribution of scattered and detached objects.
   Enlarge
   Distribution of scattered and detached objects.

   The recently discovered objects 2000 CR[105] 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 OO[67] 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 SDOs

   CAPTION: List of Notable SDOs

   Permanent
   Designation Provisional
   Designation Absolute magnitude Albedo Equatorial diameter
   (km) Semimajor axis
   (AU) Date discovered Discoverer Diameter method
   Eris 2003 UB[313] −1.12 0.86 ± 0.07 2400 ± 100 67.7 2003 M. Brown, C.
   Trujillo & D. Rabinowitz direct
   Sedna 2003 VB[12] 1.6 1180–1800 525.606 2003 M. Brown, C. Trujillo & D.
   Rabinowitz
   84522 2002 TC[302] 3.9 > 0.03 < 1211 55.1 2002 NEAT thermal
   2004 XR[190] 4.5 500-1000 57.5 2004 L. Allen
   15874 1996 TL[66] 5.4 0.10? ~630 82.9 1996 D. Jewitt, J. Luu & J. Chen
   thermal
   48639 1995 TL[8] 5.28 & 7.0 (binary) 0.09 assumed ~350 & ~160 52.2 1995
   Spacewatch ( A. Gleason) assumed albedo
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