Scattered disk

The scattered disk is a subset of the population of trans-Neptunian Objects (TNOs) characterized by aphelia beyond the strict limits of the Kuiper belt and often by highly eccentric and highly inclined orbits.

Definitions
By the most commonly accepted definition, a scattered disk object (SDO) is any object having:
 * A perihelion greater than 30 AU (which is the approximate semi-major axis of the orbit of Neptune), and
 * A semi-major axis greater than 50 AU and hence beyond the distance of a 1:2 orbital resonance with Neptune.

The scattered disk, therefore, is the collection of SDOs.

SDOs usually have highly eccentric orbits. But those orbits need not be highly inclined to the ecliptic. The inclination of the orbit of an SDO can vary from 0.2° to 46.8°

Some astronomers object to defining SDOs as a separate class from Kuiper belt objects (KBOs). They have coined the term "scattered Kuiper belt object" (SKBO) to refer to this class of objects.

Population and Examples
The largest SDO found to date is the dwarf planet Eris, shown above with its moon, Dysnomia. As of June 23, 2008, about 220 SDOs have been identified, this although Trujillo, Jewitt, and Luu estimated in 2000 that as many as 31,000 SDOs would be found.

Origin of the Scattered Disk
The term scattered disk arises from the idea that the planet Neptune scattered these objects from relatively stable orbits within or slightly inclined to the plane of the ecliptic into their present unstable and highly inclined and/or eccentric orbits.

Gerard P. Kuiper, in 1951, postulated, from the nebula hypothesis, that the original accretion disk out of which the planets formed left a remnant, beyond 30 AU, of bodies that could not fully coalesce. Different astronomers have proposed two models to explain what happened next to some of these bodies:
 * 1) Neptune, Uranus, and Saturn migrated outward to their present orbits, while Jupiter migrated inward. As Neptune approached 30 AU, its migration slowed to a stop. During this migration, certain bodies passed close to it and then flew away on random paths.
 * 2) SDOs came originally from the Kuiper belt, from which any of them might have escaped following a gravitational interaction with Neptune, an encounter with a Kuiper Belt Object (KBO) 500 km or more in radius, or a collision.

Detached Scattered Disk Objects
Seven TNOs are in orbits around the Sun at perihelia greater than 40 AU and semi-major axes greater than 50 AU. These include Sedna, the one named object among them. They are significantly more distant from the Sun than are other SDOs, and most astronomers consider them too far away for Neptune to have had any effect on them. For that reason, many astronomers do not consider them part of the scattered disk at all, but instead part of an "inner Oort cloud." These are the "detached" SDOs, and are a subject of great debate. Some astronomers suggest that they are part of an "extended" scattered disk.

The presence of these extremely distant objects has led some astronomers to speculate that a star as massive as the sun might have come as close to the Sun as 800 AU and sent these objects into highly eccentric orbits. Others speculate that a gas giant having the mass either of Neptune or Jupiter, located much farther away, could produce the same or similar effect.

Connection with comets
Most astronomers consider the scattered disk, like the Kuiper belt, to be a potential source of short-period comets.

A creationist perspective
The scattered disk shares with the Kuiper belt the same basic problem that militates against its being a source of short-period comets. Quite simply, the scattered disk is far less densely populated than the estimates of eight years ago predicted it to be.

Theories of the origins of the scattered disk often depend on migration of the gas giants from their original orbits to their present orbits. However, given that even the formation of Uranus and Neptune at their present positions is problematic, the formation of Jupiter even further away from the Sun than its present orbit would be even more problematic. A second problem is that no astronomer has shown how the migration of Neptune could have stopped at its present distance from the Sun.

Observation and exploration
The only observations of the scattered disk have been through Earth-based telescopes and the Hubble Space Telescope. The New Horizons mission, now on its way to Pluto, does not yet have a planned rendezvous with any SDO.

Related links

 * Scattered disk by Wikipedia
 * List of Centaurs and Scattered Disk Objects. Harvard University, Cambridge, Massachusetts. Updated daily.

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