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[[Image:Eris and Dysnomia.jpg|thumb|250px|right|[[Eris]], largest of all scattered disk objects found thus far, and its satellite, [[Dysnomia]]]]The '''scattered disk''' is a subset of the population of [[trans-Neptunian Objects]] (TNOs) characterized by [[apsis|aphelia]] beyond the strict limits of the [[Kuiper belt]] and often by highly eccentric and highly inclined [[orbit]]s.
== 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.<ref name=Gomes>Gomes R.S., Fernández J.A., Gallardo T., and Brunini A. "[http://www.fisica.edu.uy/~gallardo/scatdisk.pdf The Scattered Disk: Origins, Dynamics and End States]." In: ''The Solar System Beyond Neptune'', University of Arizona, 2008 (ISBN 9780816527557), pp. 259-273. (Preprint) Accessed June 22, 2008.</ref>
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°<ref name=Gomes/>
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.<ref name=Jewitt>Jewitt, David. "[http://www.ifa.hawaii.edu/~jewitt/kb/kb-scattered.html Scattered Kuiper Belt Objects (SKBOs)]." Institute for Astronomy, July 2000. Accessed June 23, 2008.</ref>
== 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.<ref name=Trujillo>Trujillo C.A., Jewitt D.C., and Luu J.X. "[http://www.ifa.hawaii.edu/faculty/jewitt/papers/SKBO/tjl2000.pdf Population of the Scattered Kuiper Belt]." ''Astrophys. J.'' 529:L103-L106, February 1, 2000. {{doi|10.1086/312467}} Accessed June 23, 2008.</ref>
== 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.<ref name=Gomes/> Different [[astronomy|astronomers]] have proposed two models to explain what happened next to some of these bodies:
# [[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.<ref name=Gomes/><ref name=Hahn>Hahn, J.M., and Malhotra, R. "Neptune's Migration into a Stirred-Up Kuiper Belt: A Detailed Comparison of Simulations to Observations." ''Astron. J. '' 130:2392-2414, November 2005. {{arXiv|astro-ph/0507319}} Accessed June 23, 2008.</ref>
# 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.<ref name=Gomes/>
== 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 astromomers 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.<ref name=Gomes/> Some astronomers suggest that they are part of an "extended" scattered disk.<ref name=Gladman>Gladman B. "[http://www.astro.ubc.ca/people/gladman/cr105.html Evidence for an Extended Scattered Disk]?" University of British Columbia, March 31, 2001. Accessed June 21, 2008.</ref>
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.<ref name=Morbidelli>Morbidelli A., and Levison H. "[http://www.iop.org/EJ/abstract/1538-3881/128/5/2564/ Scenarios for the Origin of the Orbits of the Trans-Neptunian Objects 2000 CR<sub>105</sub> and 2003 VB<sub>12</sub> (Sedna)]." ''Astron. J.'' 128:2564-2576, 2004. {{doi|10.1086/424617}} {{arXiv|astro-ph/0403358}} Accessed June 23, 2008.</ref> 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.<ref name=Gomes2>Gomes R.S., Matese J.J., and Lissauer J.J. "[http://adsabs.harvard.edu/abs/2006Icar..184..589G A distant planetary-mass solar companion may have produced distant detached objects]." ''Icarus'' 184(2):589-601, October 2006. {{doi|10.1016/j.icarus.2006.05.026}} Accessed June 23, 2008.</ref>
== Connection with comets ==
Most astronomers consider the scattered disk, like the Kuiper belt, to be a potential source of short-period [[comet]]s.<ref name=Gomes2/>
== 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,<ref>R.N., Birth of Uranus and Neptune, ''Astronomy'' '28'(4):30, 2000</ref> 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.
== References ==
{{reflist|2}}
== Related link ==
* [http://cfa-www.harvard.edu/iau/lists/Centaurs.html List of Centaurs and Scattered Disk Objects]. [[Harvard University]], Cambridge, [[Massachusetts]]. Updated daily.
{{Solarsystem}}
[[Category:Astronomy]]
== 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.<ref name=Gomes>Gomes R.S., Fernández J.A., Gallardo T., and Brunini A. "[http://www.fisica.edu.uy/~gallardo/scatdisk.pdf The Scattered Disk: Origins, Dynamics and End States]." In: ''The Solar System Beyond Neptune'', University of Arizona, 2008 (ISBN 9780816527557), pp. 259-273. (Preprint) Accessed June 22, 2008.</ref>
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°<ref name=Gomes/>
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.<ref name=Jewitt>Jewitt, David. "[http://www.ifa.hawaii.edu/~jewitt/kb/kb-scattered.html Scattered Kuiper Belt Objects (SKBOs)]." Institute for Astronomy, July 2000. Accessed June 23, 2008.</ref>
== 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.<ref name=Trujillo>Trujillo C.A., Jewitt D.C., and Luu J.X. "[http://www.ifa.hawaii.edu/faculty/jewitt/papers/SKBO/tjl2000.pdf Population of the Scattered Kuiper Belt]." ''Astrophys. J.'' 529:L103-L106, February 1, 2000. {{doi|10.1086/312467}} Accessed June 23, 2008.</ref>
== 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.<ref name=Gomes/> Different [[astronomy|astronomers]] have proposed two models to explain what happened next to some of these bodies:
# [[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.<ref name=Gomes/><ref name=Hahn>Hahn, J.M., and Malhotra, R. "Neptune's Migration into a Stirred-Up Kuiper Belt: A Detailed Comparison of Simulations to Observations." ''Astron. J. '' 130:2392-2414, November 2005. {{arXiv|astro-ph/0507319}} Accessed June 23, 2008.</ref>
# 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.<ref name=Gomes/>
== 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 astromomers 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.<ref name=Gomes/> Some astronomers suggest that they are part of an "extended" scattered disk.<ref name=Gladman>Gladman B. "[http://www.astro.ubc.ca/people/gladman/cr105.html Evidence for an Extended Scattered Disk]?" University of British Columbia, March 31, 2001. Accessed June 21, 2008.</ref>
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.<ref name=Morbidelli>Morbidelli A., and Levison H. "[http://www.iop.org/EJ/abstract/1538-3881/128/5/2564/ Scenarios for the Origin of the Orbits of the Trans-Neptunian Objects 2000 CR<sub>105</sub> and 2003 VB<sub>12</sub> (Sedna)]." ''Astron. J.'' 128:2564-2576, 2004. {{doi|10.1086/424617}} {{arXiv|astro-ph/0403358}} Accessed June 23, 2008.</ref> 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.<ref name=Gomes2>Gomes R.S., Matese J.J., and Lissauer J.J. "[http://adsabs.harvard.edu/abs/2006Icar..184..589G A distant planetary-mass solar companion may have produced distant detached objects]." ''Icarus'' 184(2):589-601, October 2006. {{doi|10.1016/j.icarus.2006.05.026}} Accessed June 23, 2008.</ref>
== Connection with comets ==
Most astronomers consider the scattered disk, like the Kuiper belt, to be a potential source of short-period [[comet]]s.<ref name=Gomes2/>
== 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,<ref>R.N., Birth of Uranus and Neptune, ''Astronomy'' '28'(4):30, 2000</ref> 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.
== References ==
{{reflist|2}}
== Related link ==
* [http://cfa-www.harvard.edu/iau/lists/Centaurs.html List of Centaurs and Scattered Disk Objects]. [[Harvard University]], Cambridge, [[Massachusetts]]. Updated daily.
{{Solarsystem}}
[[Category:Astronomy]]