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Cubewano is also known as a Kuiper belt object.

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

This diagram illustrates the orbits of the larger or brighter Kuiper belt objects (cubewanos), with the larger plutinos (Pluto, Orcus and Ixion) plotted in grey for comparison. The horizontal axis represents the semi-major axes. The eccentricities of the orbits are represented by red segments (extending from perihelion to aphelion) with the inclinations represented on the vertical axis.

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.

Orbits of Cubewanos.

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.

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

Distribution of Cubewanos.

This diagram plots the distribution of all known cubewanos (532 as of February, 2006) and Plutinos. Inserts on the right show histograms for orbit inclinations, eccentricity, and semi-major axes. 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 5º 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 5-10º. See also the comparison with scattered disk objects.

cubewanos - in blue.
This graph represents polar and ecliptic views of the (aligned) orbits of the classical objects - that is, cubewanos - 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 20º.

Toward a formal definition of Cubewanos.

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. 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).

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, this definition appears to be adopted in the most recent literature.

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