Classical Kuiper belt object

The orbits of various cubewanos compared to the orbit of Neptune (blue) and Pluto (pink).

In astronomy a classical Kuiper Belt object, also called a cubewano,^[1] is a Kuiper belt object (KBO) that orbits beyond Neptune and is not controlled by an orbital resonance with the gas giant. Cubewanos have semi-major axes in the 40-50 AU range and, unlike Pluto, do not cross Neptune’s orbit.

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

Objects identified as cubewanos include:

• (15760) 1992 QB₁^[2]
• Makemake,^[2] the largest known cubewano and a dwarf planet
• (50000) Quaoar^[2] and (20000) Varuna,^[2] each considered the largest TNO at the time of discovery
• 19521 Chaos, 58534 Logos, 53311 Deucalion, 66652 Borasisi, 88611 Teharonhiawako
• (33001) 1997 CU₂₉, (55636) 2002 TX₃₀₀, (55565) 2002 AW₁₉₇, (55637) 2002 UX₂₅

Haumea (2003 EL₆₁) was provisionally listed as a cubewano by the Minor Planet Center in 2006,^[3] but is no longer listed as one.^[2]

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.

Orbits: 'hot' and 'cold' populations

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.^[4]

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).^[5]

Distribution

This diagram plots the distribution of cubewanos 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 orbit. When orbital inclinations are compared, 'hot' cubewanos can be easily distinguished by their higher inclinations, as the plutinos typically keep orbits below 20°. (No clear explanation currently exists for the inclinations of 'hot' cubewanos.^[6])

Cold and hot populations: physical characteristics

In addition to the distinct orbital characteristics the two populations display different physical characteristics.

The difference in colour between the red cold population and more heterogeneous hot population was observed as early as in 2002.^[7] Recent studies, based on a larger data set, indicate the cut-off inclination of 12° (instead of 5°) between the cold and hot populations while confirming the distinction between the homogenous red cold population and the bluish hot population.^[8]

Another difference between the low inclination (cold) and high inclination (hot) classical objects is the observed number of binary objects. Binaries are quite common on low-inclination orbits and are typically similar-brightness systems. Binaries are less common on high-inclination orbits and their components typically differ in brightness. This correlation, together with the differences in colour, support further the suggestion that the currently observed classical objects belong to at least two different overlapping populations, with different physical properties and orbital history.^[9]

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. (119951) 2002 KX₁₄ may be a inner cubewano near the plutinos. 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.^[10] 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).^[4]

These definitions lack precision: in particular the boundary between the classical objects and the scattered disk remains blurred.

DES classification

Introduced by the report from the Deep Ecliptic Survey by J. L. Elliott et al. in 2005 uses formal criteria based on the mean orbital parameters.^[5] 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.

SSBN07 classification

An alternative classification, introduced by B. Gladman, B. Marsden and C. VanLaerhoven in 2007, uses 10 million years orbit integration instead of the Tisserand parameter. Classical objects are defined as not resonant and not being currently scattered by Neptune.^[11]

Formally, this definition includes as classical all objects with the current orbits that

• are non-resonant (see the definition of the method)
• have the semi-major axis greater than that of Neptune (i.e. excluding Centaurs) but less than 2000 AU (to exclude Inner Oort Cloud Objects)
• have the eccentricity e < 0.240 (to exclude detached objects)

Unlike other schemes, this definition includes the objects with major semi-axis less than 39.4 AU (2:3 resonance) - named Inner classical belt, or more than 48.7 (1:2 resonance) - named Outer classical belt while reserving the term Main classical belt for the orbits between these two resonances.^[11]

Families

The first collisional family—a group of objects thought to be remnants from the breakup of a single body—is the Haumea family.^[12] It includes Haumea, its moons, 2002 TX₃₀₀ and seven 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.^[13] 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^[14].

^†As of 2008. 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
• 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