The scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy minor planets. It is 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.
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 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 belt proper.
As of 2011, over 200 SDOs have been identified.
Orbits
The scattered disc is a very dynamic environment. Because they are still capable of being perturbed by Neptune, SDOs' 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.
SDOs are typically characterized by orbits with medium and high eccentricities with a semi-major axis greater than 50 AU, but their perihelia bring them within influence of Neptune.
Having a perihelion of roughly 30 AU is one of the defining characteristics of scattered objects, as it allows Neptune to exert its gravitational influence.
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.
The Minor Planet Center (MPC) makes a clear distinction between the Kuiper belt and the scattered disc; separating those objects in stable orbits (the Kuiper belt) from those in scattered orbits (the scattered disc and the centaurs). However, the difference between the Kuiper belt and the scattered disc is not clearcut, and many astronomers see the scattered disc not as a separate population but as an outward region of the Kuiper belt.
Composition
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.
Most SDOs have a white or greyish appearance. One explanation is the exposure of whiter subsurface layers by impacts; another is that the scattered objects' greater distance from the Sun creates a composition gradient, analogous to the composition gradient of the terrestrial and gas giant planets.
Mike Brown, discoverer of the scattered object Eris, suggests that its paler colour could be because, at its current distance from the Sun, its atmosphere of methane is frozen over its entire surface, creating an inches-thick layer of bright white ice.
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