Everything about Scattered Disc Object totally explained
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 farther above and below the
ecliptic than the belt proper.
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 47 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 blur the distinction, and the
Minor Planet Center (MPC) now lists centaurs and SDOs together.
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 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;, 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 can't be created by Neptune scattering. Instead, a number of explanations have been put forward including a passing star or a distant, planet-sized object. See
Sedna.
Orbits
The first SDO to be recognized was, first identified in 1996 by astronomers based at
Mauna Kea. The first object presently classified as an SDO to be discovered was, 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 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). 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.
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
- has an orbit with extreme eccentricity (~0.9), bringing its perihelion near Saturn's orbit. This could qualify it as a Centaur.
- is currently the object with the highest inclination (~78°) in the Scattered Disc.
- has the atypical, near circular (the short yellow segment) orbit, but it's 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
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. The relative number of objects within the square is represented with
cartographic colours (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° and
e=0.5--0.55, low
i<10° 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 ).
It is the eccentricity, more than the orbit's inclination, that's the distinctive attribute of the family of scattered objects.
Orbit plots
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.
The solid blue ring isn't 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 of 35AU (red) and Neptune’s orbit (at ~30AU, in yellow). As this view illustrates, the inclinations alone don't really distinguish SDO from the classical objects. Instead, the eccentricity is the distinctive attribute (long aphelion segments).
Detached objects, or an extended scattered disc?
The recently discovered objects and 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).
More recently, these objects are referred to as
detached objects. or
Distant Detached Objects (
DDO
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 are partly shown with the red segments, extending from the perihelion to the aphelion, well outside the diagram (>900AU and >1020AU respectively).
Noteworthy SDOs
Further Information
Get more info on 'Scattered Disc Object'.
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