Classical Kuiper belt object

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486958 Arrokoth, the first classical Kuiper belt object visited by a spacecraft. UltimaThule CA06 color 20190516.png
486958 Arrokoth, the first classical Kuiper belt object visited by a spacecraft.
The orbits of various cubewanos compared to the orbit of Neptune (blue) and Pluto (pink) Cubewanos.png
The orbits of various cubewanos compared to the orbit of Neptune (blue) and Pluto (pink)

A classical Kuiper belt object, also called a cubewano ( /ˌkjuːbˈwʌn/ "QB1-o"), [lower-alpha 1] is a low-eccentricity Kuiper belt object (KBO) that orbits beyond Neptune and is not controlled by an orbital resonance with Neptune. Cubewanos have orbits with semi-major axes in the 40–50  AU range and, unlike Pluto, do not cross Neptune's orbit. That is, they have low-eccentricity and sometimes low-inclination orbits like the classical planets.


The name "cubewano" derives from the first trans-Neptunian object (TNO) found after Pluto and Charon, 15760 Albion, which until January 2018 had only the provisional designation (15760) 1992 QB1 . [2] Similar objects found later were often called "QB1-o's", or "cubewanos", after this object, though the term "classical" is much more frequently used in the scientific literature.

Objects identified as cubewanos include:

136108 Haumea was provisionally listed as a cubewano by the Minor Planet Center in 2006, [4] but was later found to be in a resonant orbit. [3]

Orbits: 'hot' and 'cold' populations

Semimajor axis and inclination of cubewanos (blue) compared to resonant TNOs (red). TheKuiperBelt 55AU Classical.svg
Semimajor axis and inclination of cubewanos (blue) compared to resonant TNOs (red).

There are two basic dynamical classes of classical Kuiper-belt bodies: those with relatively unperturbed ('cold') orbits, and those with markedly perturbed ('hot') 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 classical objects, the so-called cold population, have low inclinations (<5°) and near-circular orbits, lying between 42 and 47 AU. A smaller population (the hot population) is characterised by highly inclined, more eccentric orbits. [5] The terms 'hot' and 'cold' has nothing to do with surface or internal temperatures. Instead, the terms 'hot and 'cold' refer to the orbits of the objects, by analogy to particles in a gas, which increase their relative velocity as they become heated up. [6]

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). [7]


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 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. [8] )

KBOs and resonances.png
TheKuiperBelt Projections 55AU Classical Plutinos.svg
Left: TNO distribution of cubewanos (blue), resonant TNOs (red), SDOs (grey) and sednoids (yellow). Right: Comparison of the aligned orbits (polar and ecliptic view) of cubewanos, plutinos, and Neptune (yellow).

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, such as 486958 Arrokoth, and more heterogeneous hot population was observed as early as in 2002. [9] Recent studies, based on a larger data set, indicate the cut-off inclination of 12° (instead of 5°) between the cold and hot populations and confirm the distinction between the homogenous red cold population and the bluish hot population. [10]

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. [11]

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 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 an 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. [12] 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). [5]

These definitions lack precision: in particular the boundary between the classical objects and the scattered disk remains blurred. As of 2020, there are 634 objects with perihelion (q) > 40 AU and aphelion (Q) < 47 AU. [13]

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. [7] 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:

SSBN07 classification

An alternative classification, introduced by B. Gladman, B. Marsden and C. van Laerhoven in 2007, uses a 10-million-year orbit integration instead of the Tisserand's parameter. Classical objects are defined as not resonant and not being currently scattered by Neptune. [14]

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

Unlike other schemes, this definition includes the objects with major semi-axis less than 39.4 AU (2:3 resonance)—termed inner classical belt, or more than 48.7 (1:2 resonance) – termed outer classical belt, and reserves the term main classical belt for the orbits between these two resonances. [14]


The first known collisional family in the classical Kuiper belt—a group of objects thought to be remnants from the breakup of a single body—is the Haumea family. [15] 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. [16] 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. [17] Several other collisional families might reside in the classical Kuiper belt. [18] [19]

As of 2008. The four brightest objects of the family are situated on the graphs inside the circle representing Haumea.


New Horizons trajectory and the orbits of Pluto and 486958 Arrokoth 2014 MU69 orbit.jpg
New Horizons trajectory and the orbits of Pluto and 486958 Arrokoth

As of January 2019, only one classical Kuiper belt object has been observed up close by spacecraft. Both Voyager spacecraft have passed through the region before the discovery of the Kuiper belt. [20] New Horizons was the first mission to visit a classical KBO. After its successful exploration of the Pluto system in 2015, the NASA spacecraft has visited the small KBO 486958 Arrokoth at a distance of 3,500 kilometres (2,200 mi) on 1 January 2019. [21]


Here is a very generic list of classical Kuiper belt objects. As of October 2020, there are about 779 objects with q > 40 AU and Q < 48 AU. [22]

See also


  1. Somewhat old-fashioned, but “cubewano” is still used by the Minor Planet Center for their list of Distant Minor Planets. [1]

Related Research Articles

Kuiper belt Area of the Solar System beyond the planets, comprising small bodies

The Kuiper belt is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times as wide and 20–200 times as massive. Like the asteroid belt, it consists mainly of small bodies or remnants from when the Solar System formed. While many asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles, such as methane, ammonia, and water. The Kuiper belt is home to most of the objects that astronomers generally accept as dwarf planets: Orcus, Pluto, Haumea, Quaoar, and Makemake. Some of the Solar System's moons, such as Neptune's Triton and Saturn's Phoebe, may have originated in the region.

In astronomy, the plutinos are a dynamical group of trans-Neptunian objects that orbit in 2:3 mean-motion resonance with Neptune. This means that for every two orbits a plutino makes, Neptune orbits three times. The dwarf planet Pluto is the largest member as well as the namesake of this group. The next largest members are Orcus, (208996) 2003 AZ84, and Ixion. Plutinos are named after mythological creatures associated with the underworld.

Trans-Neptunian object Solar system objects beyond Neptune

A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (AU).

The Deep Ecliptic Survey (DES) is a project to find Kuiper belt objects (KBOs), using the facilities of the National Optical Astronomy Observatory (NOAO). The principal investigator is Robert L. Millis.

Makemake Dwarf planet in the Outer Solar System

Makemake is a dwarf planet and perhaps the second-largest Kuiper belt object in the classical population, with a diameter approximately two-thirds that of Pluto. It has one known satellite. Its extremely low average temperature, about 40 K (−230 °C), means its surface is covered with methane, ethane, and possibly nitrogen ices.

<span class="nowrap">(19308) 1996 TO<sub>66</sub></span> Trans-Neptunian object in the Kuiper belt

(19308) 1996 TO66 (also written (19308) 1996 TO66) is a trans-Neptunian object that was discovered in 1996 by Chadwick Trujillo, David Jewitt and Jane Luu. Until 20000 Varuna was discovered, it was the second-largest known object in the Kuiper belt, after Pluto.

In astronomy, a resonant trans-Neptunian object is a trans-Neptunian object (TNO) in mean-motion orbital resonance with Neptune. The orbital periods of the resonant objects are in a simple integer relations with the period of Neptune, e.g. 1:2, 2:3, etc. Resonant TNOs can be either part of the main Kuiper belt population, or the more distant scattered disc population.

Scattered disc Collection of bodies in the extreme Solar System

The scattered disc (or scattered disk) is a distant circumstellar disc in the Solar System that is sparsely populated by icy small solar system bodies, which are a subset of the broader family of trans-Neptunian objects. The scattered-disc objects (SDOs) have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater than 30 astronomical units (4.5×109 km; 2.8×109 mi). These extreme orbits are thought to be the result of gravitational "scattering" by the gas giants, and the objects continue to be subject to perturbation by the planet Neptune.

<span class="nowrap">(208996) 2003 AZ<sub>84</sub></span> Plutino

(208996) 2003 AZ84 is a trans-Neptunian object with a possible moon from the outer regions of the Solar System. It is approximately 940 kilometers across its longest axis, as it has an elongated shape. It belongs to the plutinos – a group of minor planets named after its largest member Pluto – as it orbits in a 2:3 resonance with Neptune in the Kuiper belt. It is the third-largest known plutino, after Pluto and Orcus. It was discovered on 13 January 2003, by American astronomers Chad Trujillo and Michael Brown during the NEAT survey using the Samuel Oschin telescope at Palomar Observatory.

(119951) 2002 KX<sub>14</sub>

(119951) 2002 KX14, also written as 2002 KX14, is a medium sized trans-Neptunian object (TNO) residing within the Kuiper belt. It was discovered on 17 May 2002 by Michael E. Brown and Chad Trujillo.

(35671) 1998 SN165, prov. designation: 1998 SN165, is a trans-Neptunian object from the Kuiper belt located in the outermost region of the Solar System. It was discovered on 23 September 1998, by American astronomer Arianna Gleason at the Kitt Peak National Observatory near Tucson, Arizona. The cold classical Kuiper belt object is a dwarf planet candidate, as it measures approximately 400 kilometers (250 miles) in diameter. It has a grey-blue color (BB) and a rotation period of 8.8 hours. As of 2021, it has not been named.

Detached object Dynamical class of minor planets

Detached objects are a dynamical class of minor planets in the outer reaches of the Solar System and belong to the broader family of trans-Neptunian objects (TNOs). These objects have orbits whose points of closest approach to the Sun (perihelion) are sufficiently distant from the gravitational influence of Neptune that they are only moderately affected by Neptune and the other known planets: This makes them appear to be "detached" from the rest of the Solar System, except for their attraction to the Sun.

Haumea family

The Haumea or Haumean family is the only identified trans-Neptunian collisional family; that is, the only group of trans-Neptunian objects (TNOs) with similar orbital parameters and spectra that suggest they originated in the disruptive impact of a progenitor body. Calculations indicate that it is probably the only trans-Neptunian collisional family. Members are known as Haumeids.

(55638) 2002 VE95, prov. designation: 2002 VE95, is a trans-Neptunian object from the outermost region of the Solar System. It was discovered on 14 November 2002, by astronomers with the Near-Earth Asteroid Tracking program at the Palomar Observatory in California, United States. This resonant trans-Neptunian object is a member of the plutino population, locked in a 2:3 resonance with Neptune. The object is likely of primordial origin with a heterogeneous surface and a notably reddish color (RR) attributed to the presence of methanol and tholins. It has a poorly defined rotation period of 6.8 hours and measures approximately 250 kilometers (160 miles) in diameter, too small to be a dwarf planet candidate. As of 2021, it has not yet been named.

<span class="nowrap">(416400) 2003 UZ<sub>117</sub></span>

(416400) 2003 UZ117 is a trans-Neptunian object and suspected member of the Haumea family, located in the Kuiper belt in the outermost region of the Solar System. It was discovered on 24 October 2003, by astronomers of the Spacewatch survey project at Kitt Peak Observatory, Arizona. The object may also be a non-resonant cubewano.

(505448) 2013 SA100, provisional designation 2013 SA100 and also known as o3l79, is a trans-Neptunian object from the classical Kuiper belt in the outermost region of the Solar System. It was discovered on 5 August 2013, by astronomer with the Outer Solar System Origins Survey at the Mauna Kea Observatories, Hawaii, in the United States. The classical Kuiper belt object belongs to the hot population and is a weak dwarf planet candidate, approximately 260 kilometers (160 miles) in diameter.

<span class="nowrap">(516977) 2012 HZ<sub>84</sub></span>

(516977) 2012 HZ84, provisional designation 2012 HZ84, is a small trans-Neptunian object from the Kuiper belt located in the outermost region of the Solar System, approximately 74 kilometers (46 miles) in diameter. It was discovered on 17 April 2012, by a team of astronomers using one of the Magellan Telescopes in Chile during the New Horizons KBO Search in order to find a potential flyby target for the New Horizons spacecraft. In December 2017, this classical Kuiper belt object was imaged by the spacecraft from afar at a record distance from Earth.

(523764) 2014 WC510, is a binary trans-Neptunian object discovered on 8 September 2011 by the Pan-STARRS survey at the Haleakalā Observatory in Hawaii. It was found by Pan-STARRS on 20 November 2014 and was announced later in July 2016 after additional observations and precovery identifications. It is in the Kuiper belt, a region of icy objects orbiting beyond Neptune in the outer Solar System. It is classified as a plutino, a dynamical class of objects in a 2:3 orbital resonance with Neptune. On 1 December 2018, a team of astronomers observed a stellar occultation by the object, which revealed that it is a compact binary system consisting of two separate components in close orbit around each other. The primary and secondary components are estimated to have diameters of around 180 km (110 mi) and 140 km (87 mi), respectively.


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