Classical Kuiper belt object

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In astronomy a cubewano (pronounced /kjuːbiːwənoʊ/) is a Kuiper belt object that orbits beyond Neptune and is not controlled by an orbital resonance with the giant planet. Cubewanos have semi-major axes in the 40-50 AU range and, unlike Pluto, do not cross Neptune’s orbit. They are also called classical Kuiper Belt objects.

The odd name derives from the first trans-Neptunian object (TNO) found (besides Pluto and Charon), (15760) 1992 QB₁. Later objects were called "QB1-o's", or cubewanos.

Objects identified as cubewanos include:

• (15760) 1992 QB₁
• (136472) 2005 FY₉ the largest known cubewano and one of the largest TNO
• (136108) 2003 EL₆₁, notable for its elongated shape, two moons and rapid rotation (3.9h)
• (50000) Quaoar and (20000) Varuna, each considered the largest TNO at the time of discovery
• 2002 TX₃₀₀, 2002 AW₁₉₇, 2002 UX₂₅

The orbits of the large cubewanos (in blue) with the large plutinos (in red) for comparison (H<4.5). The horizontal axis represents the semi-major axes. The eccentricities of the orbits are represented by segments (extending from perihelion to aphelion) with the inclinations represented on the vertical axis.

[edit] Orbits

Most cubewanos are found between the 2:3 orbital resonance with Neptune (populated by plutinos) and the 1:2 resonance. 50000 Quaoar, for example, has a near circular orbit close to the ecliptic. Plutinos, on the other hand, have more eccentric orbits bringing some of them closer to the Sun than Neptune.

The majority of objects (the so-called 'cold population'), have low inclinations and near circular orbits. A smaller population (the 'hot population') is characterised by highly inclined, more eccentric orbits^[1].

The Deep Ecliptic Survey reports the distributions of the two populations; one with the inclination centered at 4.6° (named Core) and another with inclinations extending beyond 30° (Halo). ^[2]

[edit] Distribution

This diagram plots the distribution and plutinos. Histograms are shown for orbit inclinations, eccentricity, and semi-major axes distribution. Inserts on the left compare the populations of cubewanos and plutinos using eccentricity versus inclination plots.

The vast majority of KBOs (more than two-thirds) have inclinations of less than 5° and eccentricities of less than 0.1. Their semi-major axes show a preference for the middle of the main belt; arguably, smaller objects close to the limiting resonances have been either captured into resonance or have their orbits modified by Neptune.

The 'hot' and 'cold' populations are strikingly different: more than 30% of all cubewanos are in low inclination, near-circular orbits. The parameters of the plutinos’ orbits are more evenly distributed, with a local maximum in moderate eccentricities in 0.15-0.2 range and low inclinations 5-10°. See also the comparison with scattered disk objects.

Polar and ecliptic view of the (aligned) orbits of the classical objects (in blue), together with the plutinos in red, and Neptune (yellow).

When the orbital eccentricities of cubewanos and plutinos are compared, it can be seen that the cubewanos form a clear 'belt' outside Neptune's orbit, whereas the plutinos approach, or even cross Neptune's orbits. When orbital inclinations are compared, 'hot' cubewanos can be easily distinguished by their higher inclinations, as the plutinos typically keep orbits below 20°.

[edit] Toward a formal definition

There is no official definition of 'cubewano' or 'classical KBO'. However, the terms are normally used to refer to objects free from significant perturbation from Neptune, thereby excluding KBOs in orbital resonance with Neptune (Resonant trans-Neptunian objects)). Furthermore, there is evidence that the Kuiper Belt has an 'edge', in that an apparent lack of low inclination objects beyond 47-49 AU was suspected as early as 1998 and shown with more data in 2001.^[3] Consequently, the traditional usage of the terms is based on the orbit’s semi-major axis, and includes objects situated between the 2:3 and 1:2 resonances, that is between 39.4 and 47.8 AU (with exclusion of these resonances and the minor ones in-between). ^[1]

However, these definitions lack precision: in particular the boundary between the classical objects and the scattered disk remains blurred. A recent classification by J. L. Elliott et al uses formal criteria based on the mean orbital parameters instead. Put informally, the definition includes the objects that have never crossed the orbit of Neptune. According to this definition, an object qualifies as a classical KBO if:

• it is not resonant
• it has the average Tisserand's parameter exceeding 3
• its average eccentricity is less than 0.2.

Introduced by the report from the Deep Ecliptic Survey,^[2] this definition appears to be adopted in the most recent literature.^[4]

[edit] Families

The first collisional family, i.e. a group of objects thought to be remnants of a single body has been identified. It includes 2003 EL₆₁, its moons, 2002 TX₃₀₀ and four smaller bodies^†. The objects not only follow similar orbits but also share similar physical characteristics. Unlike many other KBO their surface contains large amounts of ice (H₂O) and no or very little tholins. The surface composition is inferred from their neutral (as opposed to red) colour and deep absorption at 1.5 and 2. μm in infrared spectrum.^[5]

^†The four brightest objects of the family are situated on the graphs inside the circle representing 2003 EL₆₁.

[edit] External links

• David Jewitt's Kuiper Belt site @ University of Hawaii
• The Kuiper Belt Electronic Newsletter
• Minor Planet Center List of Trans-Neptunian objects
• TNO pages at johnstonarchive
• Plot of the current positions of bodies in the Outer Solar System

[edit] References

1. ^ ^{a b} D.Jewitt, A.Delsanti The Solar System Beyond The Planets in Solar System Update : Topical and Timely Reviews in Solar System Sciences , Springer-Praxis Ed., ISBN 3-540-26056-0 (2006). Preprint of the article (pdf)
2. ^ ^{a b} 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
3. ^ Chadwick A. Trujillo and Michael E. Brown The Radial Distribution of the Kuiper Belt, The Astrophysical Journal, 554 (2001), pp. L95–L98 pdf
4. ^ E. Chiang, Y. Lithwick, M. Buie, W. Grundy, M. Holman A Brief History of Trans-Neptunian Space. to appear in Protostars and Planets V (August 2006) Final preprint on arXiv
5. ^ Michael E. Brown, Kristina M. Barkume, Darin Ragozzine & Emily L. Schaller, A collisional family of icy objects in the Kuiper belt, Nature, 446, (March 2007), pp 294-296.