Classical Kuiper belt object

In astronomy a classical Kuiper Belt object, also called a cubewano 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.

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

Objects identified as cubewanos include:

• Makemake, the largest known cubewano and a dwarf planet
• Haumea, was provisionally listed as a cubewano by the Minor Planet Center in 2006, but is no longer listed as a cubewano.
• (50000) Quaoar and (20000) Varuna, each considered the largest TNO at the time of discovery
• 19521 Chaos, 58534 Logos, 53311 Deucalion, 88611 Teharonhiawako
• , ,

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.

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).

Distribution

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.

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°.

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). The Minor Planet Center (MPC) and the Deep Ecliptic Survey (DES) do not list cubewanos (classical objects) using the same exact criteria. Many TNOs classified as cubewanos by the MPC are classified as ScatNear (possibly scattered by Neptune) by the DES. Dwarf planet Makemake is such a borderline classical cubewano/scatnear object. 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.

Families

The first collisional family, i.e. a group of objects thought to be remnants of a single body has been identified. It includes Haumea, its moons, 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.

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

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

References