Scattered Disc

Scattered disc is a distant circumstellar disc in the Solar System that is sparsely populated by icy small solar system bodies, which are a subset of the broader family of trans-Neptunian objects.

The scattered-disc objects (SDOs) have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater than 30 astronomical units (4.5×109 km; 2.8×109 mi).

These extreme orbits are thought to be the result of gravitational “scattering” by the gas giants, and the objects continue to be subject to perturbation by the planet Neptune.

Although the closest scattered-disc objects approach the Sun at about 30–35 AU, their orbits can extend well beyond 100 AU. This makes scattered objects among the coldest and most distant objects in the Solar System.

The innermost portion of the scattered disc overlaps with a torus-shaped region of orbiting objects traditionally called the Kuiper belt, but its outer limits reach much farther away from the Sun and farther above and below the ecliptic than the Kuiper belt proper.

Eris, the largest known scattered-disc object (center), and its moon Dysnomia (left of object)

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.

Scattered objects, like other trans-Neptunian objects, have low densities and are composed largely of frozen volatiles such as water and methane. Spectral analysis of selected Kuiper belt and scattered objects has revealed signatures of similar compounds. Both Pluto and Eris, for instance, show signatures for methane

Because of its unstable nature, astronomers now consider the scattered disc to be the place of origin for most periodic comets 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.

The Kuiper belt was initially thought to be the source of the Solar System’s ecliptic comets. However, studies of the region since 1992 have shown that the orbits within the Kuiper belt are relatively stable, and that ecliptic comets originate from the scattered disc, where orbits are generally less stable.

©BBC Sky at Night Magazine

Many objects of the proposed Oort cloud are also thought to have originated in the scattered disc. Detached objects are not sharply distinct from scattered disc objects, and some such as Sedna have sometimes been considered to be included in this group.

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