黃道離散天體
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| 外海王星天體及類似天體 |
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黃道離散天體 （scattered disc objects ）是在我們太陽系最遠的區域（離散盤）內零星散佈著，主要由冰組成的小行星，是範圍更廣闊的外海王星天體（trans-Neptunian objects（TNO））的一部分。離散盤最內側的部分與柯伊伯带重疊，但它的外緣向外伸展並比一般的柯伊伯带天體遠離了黃道的上下方。
目录 |
[编辑] 形成
我們對離散盤的所知非常有限，雖然天文學的主流觀點認為它是太陽系形成的早期過程中，因為海王星向外遷徙造成的引力擾動才被從柯伊伯带散射入高傾斜和高離心率的軌道內。相比之下，柯伊伯带像是一個相對"圓"和"平坦"的甜甜圈，以平和的圓軌道（類QB1天體）和略為橢圓的共振軌道（類冥天體），將天體約束在30至44天文單位的圓環內；離散盤內的黃道離散天體軌道環境就比較怪異了。黃道離散天體，就以矮行星鬩神星為例，在垂直黃道方向上的距離幾乎和平行方向上與太陽的距離一樣遠；軌道模擬也顯示黃道離散天體的軌道是怪異且不穩定的，並且最終會從太陽的核心區域拋至奥尔特云甚至更遙遠的地方。
有些跡象顯示半人馬群只是單純的從柯伊伯带被向內拋射，而不是向外拋射的天體，可以稱為"順海王星" （cis-Neptunian）黃道離散天體。事實上，有些天體，像是1999 TD10已經模糊了這些觀點，因此小行星中心已經將黃道離散天體和半人馬群一併列表。[1]依據這已經混淆不清的範疇，有些科學家已經改用"離散柯伊伯带天體" scattered Kuiper belt object（SKBO）來涵蓋或統稱半人馬群和離散盤內的天體。
雖然外海王星天體Senda在官方的小行星中心被認定是一顆黃道離散天體，但發現者米高·布朗因為它的近日點距離遠達76天文單位，不太會受到行星的引力擾動影響，因此認為他應該是奥尔特云內側的天體，而不該是顆黃道離散天體。[2]
This line of thinking suggests that a lack of gravitational interaction with the outer planets disqualifies a TNO from scattered disc membership, which would create an outer edge somewhere between Sedna and more conventional SDOs like Eris. 這條線認為建議缺乏引力相互作用與行星從疏散圓盤會員資格使TNO不合格，將創造一個外面邊緣某處在Sedna和更加常規的SDOs之間像Eris。 If Sedna is beyond the scattered disk, it may not be unique; Template:Mpl, which was discovered before Sedna, may also be an inner Oort cloud object or (more likely) a transitional object between the scattered disc and the inner Oort cloud. 如果Sedna是在疏散盤之外，它可能不是獨特的; Template:Mpl，在Sedna之前被發現，也許也是一個內在Oort雲彩對象或(更加可能)一個過渡對象在疏散圓盤和內在Oort雲彩之間。
Such objects, referred to as detached, have orbits which cannot be created by Neptune scattering. 這樣對象，被稱為"分開了"，有不可能通過海王星驅散創造的軌道。 Instead, a number of explanations have been put forward including a passing star [3] or a distant, planet-sized object. 反而，一定數量的解釋提出了包括一個通過的星或一個遙遠，行星大小的對象。[4] See Sedna.
[编辑] 軌道
The first SDO to be recognized was Template:Mpl, first identified in 1996 by astronomers based at Mauna Kea. The first object presently classified as an SDO to be discovered was Template:Mpl, found by Spacewatch.
The diagram on the right illustrates the orbits of all known scattered disk objects up to 100AU together with Kuiper belt objects (in grey) and resonant objects (in green). The eccentricity of the orbits is represented by segments (extending from the perihelion to the aphelion) with the inclination represented on Y axis.
[编辑] Perihelia
Typically, the scattered objects are characterised by orbits with medium and high eccentricities but their perihelia bring them no closer than 35AU, clear from direct influence of Neptune (red segments). Plutinos (grey segments for Pluto and Orcus) as well as resonant objects at 2:5 (in green) can approach Neptune closer as their orbits are protected by resonances. This perihelion > 35 AU condition is actually one of the defining characteristics of scattered objects.
[编辑] Extremes
The scattered disc is the place where extreme eccentricity and high inclination appears to be the norm and circular orbits are exceptional. Some exceptional orbits are plotted in yellow
- 1999 TD10
has an orbit with extreme eccentricity (~0.9), bringing its perihelion near Saturn's orbit. This could qualify it as a Centaur.
- 2002 XU93
is currently the object with the highest inclination (~78°) in the Scattered Disc.
- Template:Mpl has the atypical, near circular (the short yellow segment) orbit, but it is highly inclined.
[编辑] Some order in the chaos?
Resonant objects (shown in green), are not considered to be members of the scattered disk. Minor resonances are also populated and some computer simulations show that many objects could be actually on weak, higher order resonances (6:11,4:9,3:7,5:12,3:8,2:7,1:4). Quoting one of the researchers:[5] the scattered disk might not be so scattered after all.
[编辑] Scattered objects versus classical objects
The inserts in the diagram on the right compare the eccentricity and inclination of the scattered disk population to the cubewanos. Each small coloured square represents a given range for both the eccentricity e and the inclination i. [6] The relative number of objects within the square is represented with cartographic colours[7] (from small numbers plotted as green valleys to brown peaks).
The two populations are very different: more than 30% of all cubewanos are on low inclination, near circular orbits (the low bottom corner 'peak') and their eccentricity peaks at 0.25. Scattered objects on the other hand are, well, scattered. The majority of the known population have medium eccentricity in 0.25-0.55. Two local peaks correspond to e in the 0.25--0.35 range, inclination 15-20° and e=0.5--0.55, low i<10° respectively. The extreme orbits show up as outliers in grey. Characteristically, there are no known SDO objects with eccentricity lower than 0.3 (with the exception of Template:Mpl).
It is the eccentricity, more than the orbit's inclination, that is the distinctive attribute of the family of scattered objects.
[编辑] Orbit plots
More traditional, the graph on the left represents polar and ecliptic views of the (aligned) orbits of the scattered disk objects[8] (in black) on the background of cubewanos (in blue) and resonant (2:5) objects (in green). As yet unclassified objects in 50-100AU region are plotted in grey.[9]
The solid blue ring is not an artist's representation but a real plot of hundreds of overlapping orbits of the classical objects, fully deserving the name of the main (classical or cubewanos) belt. The minimum perihelion mentioned above is illustrated by the red circle. Unlike SDOs, the resonant objects approach Neptune’s orbit (in gold) .
On the ecliptic view, the arcs represent the same minimum perihelion[10] of 35AU (red) and Neptune’s orbit (at ~30AU, in yellow). As this view illustrates, the inclinations alone do not really distinguish SDO from the classical objects. Instead, the eccentricity is the distinctive attribute (long aphelion segments).
[编辑] Detached objects, or an extended scattered disc?
The recently discovered objects Template:Mpl with a perihelion too far away from Neptune to be influenced by it, led to a discussion among astronomers about a new minor planet set, called the Extended scattered disc (E-SDO[11]). More recently, these objects are referred to as detached objects.[12] (Preprint version (pdf)) or Distant Detached Objects (DDO[4]).
The classification suggested by Deep Ecliptic Survey team, introduces a formal distinction between Scattered-Near objects (which could be scattered by Neptune) from Scattered-Extended objects (e.g. 90377 Sedna) using Tisserand's parameter value of 3.[13]
The diagram illustrates all known scattered and detached objects together with the largest Kuiper belt objects for reference. The very large eccentricities of Sedna and Template:Mpl are partly shown with the red segments, extending from the perihelion to the aphelion, well outside the diagram (>900AU and >1020AU respectively).
[编辑] Noteworthy SDOs
| Permanent Designation |
Provisional Designation |
Absolute magnitude | Albedo | Equatorial diameter (km) |
Semimajor axis (AU) |
Date discovered | Discoverer | Diameter method |
|---|---|---|---|---|---|---|---|---|
| Eris | 2003 UB313 | −1.12 | 0.86 ± 0.07 | 2400 ± 100 | 67.7 | 2003 | M. Brown, C. Trujillo & D. Rabinowitz | direct[14] |
| Sedna | 2003 VB12 | 1.6 | 1180–1800 | 525.606 | 2003 | M. Brown, C. Trujillo & D. Rabinowitz | ||
| Template:Mpl | 4.5 | 500-1000 | 57.5 | 2004 | L. Allen | |||
| 15874 | Template:Mpl- | 5.4 | 0.10? | ~630 | 82.9 | 1996 | D. Jewitt, J. Luu & J. Chen | thermal |
| 48639 | Template:Mpl- | 5.28 & 7.0 (binary) | 0.09 assumed | ~350 & ~160 | 52.2 | 1995 | Spacewatch (A. Gleason) | assumed albedo |
[编辑] 參考資料和註解
- ↑ List Of Centaurs and Scattered-Disk Objects at the IAU: Minor Planet Center
- ↑ Sedna at www.gps.caltech.edu
- ↑ Alessandro Morbidelli and Harold F. Levison Scenarios for the Origin of the Orbits of the Trans-Neptunian Objects 2000 CR105 and 2003 VB12 (Sedna) The Astronomical Journal, (2004) 128, pp 2564-2576. Preprint
- ^ 4.0 4.1 Rodney S. Gomes, John J. Matese, and Jack J. Lissauer A Distant Planetary-Mass Solar Companion May Have Produced Distant Detached Objects To appear in Icarus (2006). Preprint
- ↑ Hahn J. Malhotra R.Neptune's migration into a stirred-up Kuiper Belt The Astronomical Journal, 130, pp.2392-2414, Nov.2005.Full text on arXiv.
- ↑ As near-circular orbits occupy the first column (e<0.05) and the orbits with the lowest inclination (i<5 degrees) occupy the lowest row, the square in the bottom left corner represents the number of near circular, very lowly inclined orbits.
- ↑ A grey square represents a single object (an outlier) in this range.
- ↑ Minor Planet Circular 2005-X77 Distant Minor planets was used for orbit classification. The updated data can be found in MPC 2006-D28.
- ↑ For roughly a half of known TNO the orbits are not yet known with the precision sufficient for the classification (a particularly delicate task for resonant objects).
- ↑ The precise value is not too important; the value of 35 AU is quoted for coherence with Jewitt 2006. Other authors prefer to use 30AU instead while the data used here appear to fit 34AU.
- ↑ Evidence for an Extended Scattered Disk? at Observatoire de la Cote d'Azur
- ↑ Jewitt, David C.; A. Delsanti (2006). "The Solar System Beyond The Planets", Solar System Update : Topical and Timely Reviews in Solar System Sciences. Springer-Praxis Ed.. ISBN 3-540-26056-0.
- ↑ J. L. Elliot, S. D. Kern, K. B. Clancy, A. A. S. Gulbis, R. L. Millis, M. W. Buie, L. H. Wasserman, E. I. Chiang, A. B. Jordan, D. E. Trilling, and K. J. Meech The Deep Ecliptic Survey: A Search for Kuiper Belt Objects and Centaurs. II. Dynamical Classification, the Kuiper Belt Plane, and the Core Population. The Astronomical Journal, 129 (2006), pp. preprint
- ↑ http://www.gps.caltech.edu/~mbrown/papers/ps/xsize.pdf
[编辑] 相關條目
[编辑] 外部連結
| 太陽系 |
|---|
 |
| 太陽 · 水星 · 金星 · 地球 · 火星 · 穀神星 · 木星 · 土星 · 天王星 · 海王星 · 冥王星 · 鬩神星 |
| 行星 · 矮行星 · 衛星：月球 · 火星的卫星 · 小行星衛星 · 木星的衛星 · 土星的衛星 · 天王星的衛星 · 海王星的卫星 · 冥王星的卫星 · 鬩衛一 |
| 小太陽系：流星體 · 小行星（小行星帶） · 半人马小行星 · 外海王星天體（柯伊伯带 / 黃道離散天體） · 彗星（奥尔特云） |
| 參見天体、太陽系的天體列表，以及天文學主題頁 |
