Scattered disc
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The scattered disc (or scattered disk) is a distant
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.[1] The innermost portion of the scattered disc overlaps with a torus-shaped region of orbiting objects traditionally called the Kuiper belt,[2] but its outer limits reach much farther away from the Sun and farther above and below the ecliptic than the Kuiper belt proper.[a]
Because of its unstable nature, astronomers now consider the scattered disc to be the place of origin for most
Discovery
Traditionally, devices like a
The first scattered-disc object (SDO) to be recognised as such was 1996 TL66,[6][7] originally identified in 1996 by astronomers based at Mauna Kea in Hawaii. Three more were identified by the same survey in 1999: 1999 CV118, 1999 CY118, and 1999 CF119.[8] The first object presently classified as an SDO to be discovered was 1995 TL8, found in 1995 by Spacewatch.[9]
As of 2011, over 200 SDOs have been identified,
Subdivisions of trans-Neptunian space
Known trans-Neptunian objects are often divided into two subpopulations: the Kuiper belt and the scattered disc.[16] A third reservoir of trans-Neptunian objects, the Oort cloud, has been hypothesized, although no confirmed direct observations of the Oort cloud have been made.[2] Some researchers further suggest a transitional space between the scattered disc and the inner Oort cloud, populated with "detached objects".[17]
Scattered disc versus Kuiper belt
The Kuiper belt is a relatively thick
In contrast to the Kuiper belt, the scattered-disc population can be disturbed by Neptune.
The MPC, however, 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).[10] However, the difference between the Kuiper belt and the scattered disc is not clear-cut, and many astronomers see the scattered disc not as a separate population but as an outward region of the Kuiper belt. Another term used is "scattered Kuiper-belt object" (or SKBO) for bodies of the scattered disc.[23]
Morbidelli and Brown propose that the difference between objects in the Kuiper belt and scattered-disc objects is that the latter bodies "are transported in semi-major axis by close and distant encounters with Neptune,"[16] but the former experienced no such close encounters. This delineation is inadequate (as they note) over the age of the Solar System, since bodies "trapped in resonances" could "pass from a scattering phase to a non-scattering phase (and vice versa) numerous times."[16] That is, trans-Neptunian objects could travel back and forth between the Kuiper belt and the scattered disc over time. Therefore, they chose instead to define the regions, rather than the objects, defining the scattered disc as "the region of orbital space that can be visited by bodies that have encountered Neptune" within the radius of a Hill sphere, and the Kuiper belt as its "complement ... in the a > 30 AU region"; the region of the Solar System populated by objects with semi-major axes greater than 30 AU.[16]
Detached objects
The Minor Planet Center classifies the trans-Neptunian object
Sedna is not the only such object:
There are no clear boundaries between the scattered and detached regions.[25] Gomes et al. define SDOs as having "highly eccentric orbits, perihelia beyond Neptune, and semi-major axes beyond the 1:2 resonance." By this definition, all distant detached objects are SDOs.[17] Since detached objects' orbits cannot be produced by Neptune scattering, alternative scattering mechanisms have been put forward, including a passing star[29][30] or a distant, planet-sized object.[28] Alternatively, it has been suggested that these objects have been captured from a passing star.[31]
A scheme introduced by a 2005 report from the Deep Ecliptic Survey by J. L. Elliott et al. distinguishes between two categories: scattered-near (i.e. typical SDOs) and scattered-extended (i.e. detached objects).
An alternative classification, introduced by B. J. Gladman, B. G. Marsden and C. Van Laerhoven in 2007, uses 10-million-year orbit integration instead of the Tisserand parameter.[33] An object qualifies as an SDO if its orbit is not resonant, has a semi-major axis no greater than 2000 AU, and, during the integration, its semi-major axis shows an excursion of 1.5 AU or more.[33] Gladman et al. suggest the term scattering disk object to emphasize this present mobility.[33] If the object is not an SDO as per the above definition, but the eccentricity of its orbit is greater than 0.240, it is classified as a detached TNO.[33] (Objects with smaller eccentricity are considered classical.) In this scheme, the disc extends from the orbit of Neptune to 2000 AU, the region referred to as the inner Oort cloud.
Orbits
The scattered disc is a very dynamic environment.[15] 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.[15] For this reason Gladman et al. prefer to refer to the region as the scattering disc, rather than scattered.[33] Unlike Kuiper-belt objects (KBOs), the orbits of scattered-disc objects can be inclined as much as 40° from the ecliptic.[34]
SDOs are typically characterized by orbits with medium and high eccentricities with a
The classical objects (
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 possible resonant orbits within the scattered disc include 1:3, 2:7, 3:11, 5:22 and 4:79.[17]
Formation
The scattered disc is still poorly understood: no model of the formation of the Kuiper belt and the scattered disc has yet been proposed that explains all their observed properties.[16]
According to contemporary models, the scattered disc formed when
Models for a continuous formation throughout the age of the Solar System illustrate that at weak resonances within the Kuiper belt (such as 5:7 or 8:1), or at the boundaries of stronger resonances, objects can develop weak orbital instabilities over millions of years. The 4:7 resonance in particular has large instability. KBOs can also be shifted into unstable orbits by close passage of massive objects, or through collisions. Over time, the scattered disc would gradually form from these isolated events.[17]
Computer simulations have also suggested a more rapid and earlier formation for the scattered disc. Modern theories indicate that neither Uranus nor Neptune could have formed in situ beyond Saturn, as too little primordial matter existed at that range to produce objects of such high mass. Instead, these planets, and Saturn, may have formed closer to Jupiter, but were flung outwards during the early evolution of the Solar System, perhaps through exchanges of angular momentum with scattered objects.[40] Once the orbits of Jupiter and Saturn shifted to a 2:1 resonance (two Jupiter orbits for each orbit of Saturn), their combined gravitational pull disrupted the orbits of Uranus and Neptune, sending Neptune into the temporary "chaos" of the proto-Kuiper belt.[39] As Neptune traveled outward, it scattered many trans-Neptunian objects into higher and more eccentric orbits.[37][41] This model states that 90% or more of the objects in the scattered disc may have been "promoted into these eccentric orbits by Neptune's resonances during the migration epoch...[therefore] the scattered disc might not be so scattered."[40]
Composition
Scattered objects, like other trans-Neptunian objects, have low densities and are composed largely of frozen volatiles such as water and methane.[42] 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.[43]
Astronomers originally supposed that the entire trans-Neptunian population would show a similar red surface colour, as they were thought to have originated in the same region and subjected to the same physical processes.
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.[42] Michael E. 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. Pluto, conversely, being closer to the Sun, would be warm enough that methane would freeze only onto cooler, high-albedo regions, leaving low-albedo tholin-covered regions bare of ice.[43]
Comets
The Kuiper belt was initially thought to be the source of the Solar System's
Comets can loosely be divided into two categories: short-period and long-period—the latter being thought to originate in the Oort cloud. The two major categories of short-period comets are
There are many differences between SDOs and JFCs, even though many of the Jupiter-family comets may have originated in the scattered disc. Although the centaurs share a reddish or neutral coloration with many SDOs, their nuclei are bluer, indicating a fundamental chemical or physical difference.[45] One hypothesis is that comet nuclei are resurfaced as they approach the Sun by subsurface materials which subsequently bury the older material.[45]
See also
Notes
- ^ The literature is inconsistent in the use of the phrases "scattered disc" and "Kuiper belt". For some, they are distinct populations; for others, the scattered disc is part of the Kuiper belt. Authors may even switch between these two uses in a single publication.[3] In this article, the scattered disc will be considered a separate population from the Kuiper belt.
References
- ^ Maggie Masetti. (2007). Cosmic Distance Scales – The Solar System. Website of NASA's High Energy Astrophysics Science Archive Research Center. Retrieved 2008 07-12.
- ^ arXiv:astro-ph/0512256.
- ^ McFadden, Weissman, & Johnson (2007). Encyclopedia of the Solar System, footnote p. 584
- S2CID 16002759.
- ISBN 1-58381-220-2. Archived from the original(PDF) on 2006-10-12. Retrieved 2008-08-14.
- S2CID 4370529. Archived from the original(PDF) on August 12, 2007. Retrieved 2008-08-02.
- ISBN 978-0-521-80019-8. Retrieved 2008-07-02.
- ^ a b Jewitt, David C. (August 2009). "Scattered Kuiper Belt Objects (SKBOs)". Institute for Astronomy. Retrieved 2010-01-23.
- ^ Schmadel, Lutz D. (2003); Dictionary of Minor Planet Names (5th rev. and enlarged ed. edition). Berlin: Springer. Page 925 (Appendix 10). Also see McFadden, Lucy-Ann; Weissman, Paul & Johnson, Torrence (1999). Encyclopedia of the Solar System. San Diego: Academic Press. Page 218.
- ^ a b c IAU: Minor Planet Center (2011-01-03). "List Of Centaurs and Scattered-Disk Objects". Central Bureau for Astronomical Telegrams, Harvard-Smithsonian Center for Astrophysics. Retrieved 2011-01-03.
- Bibcode:2008MPEC....D...38S.
- ^ Staff (2007-05-01). "Discovery Circumstances: Numbered Minor Planets". Minor Planet Center. Retrieved 2010-10-25.
- ^ "Discovery Circumstances: Numbered Minor Planets (90001)-(95000)". Minor Planet Center. Retrieved 2010-10-25.
- Marc W. Buie (2007-11-08). "Orbit Fit and Astrometric record for 04VN112". SwRI (Space Science Department). Archived from the originalon 2010-08-18. Retrieved 2008-07-17.
- ^ ISBN 978-0-12-088589-3.
- ^ )
- ^ a b c d e Gomes, Rodney S.; Fernandez, Julio A.; Gallardo, Tabare; Brunini, Adrian (2008). "The Scattered Disk: Origins, Dynamics and End States" (PDF). Universidad de la Republica, Uruguay. Retrieved 2008-08-10.
- doi:10.1086/320385.
- ^ ISBN 978-0-12-088589-3.
- ^ S2CID 2822011.
- ^ Remo notes that Cis-Neptunian bodies "include terrestrial and large gaseous planets, planetary moons, asteroids, and main-belt comets within Neptune's orbit." (Remo 2007)
- ^ Silber, Kenneth (1999). "New Object in Solar System Defies Categories". space.com. Archived from the original on September 21, 2005. Retrieved 2008-08-12.
- ^ Jewitt, David C. (2008). "The 1000 km Scale KBOs". Retrieved 2010-01-23.
- ^ Brown, Michael E. "Sedna (The coldest most distant place known in the solar system; possibly the first object in the long-hypothesized Oort cloud)". California Institute of Technology, Department of Geological Sciences. Retrieved 2008-07-02.
- ^ .
- ^ Gladman, Brett J. "Evidence for an Extended Scattered Disk?". Observatoire de la Cote d'Azur. Retrieved 2008-08-02.
- ISBN 978-3-540-26056-1.
- ^ .
- S2CID 119486916.
- S2CID 119197960.
- S2CID 119188358.
- ^ S2CID 19385887.
- ^ ISBN 978-0-8165-2755-7.
- S2CID 4369483.
- S2CID 8240136. Archived from the original(PDF) on August 12, 2007. Retrieved 2008-07-02.
- S2CID 4395099.
- ^ PMID 9180070.
- .
- ^ a b Hansen, Kathryn (2005-06-07). "Orbital shuffle for early solar system". Geotimes. Retrieved 2007-08-26.
- ^ S2CID 14153557.
- S2CID 17510705.
- ^ ISBN 978-0-12-088589-3.
- ^ S2CID 1761936.
- S2CID 33160822.
- ^ S2CID 122240711. Archived from the original(PDF) on 2020-05-03.