Quaoar (minor-planet number 50000) is a ringed dwarf planet in the Kuiper belt, a band of icy planetesimals beyond Neptune. It has a slightly ellipsoidal shape with an average diameter of , about half the size of the dwarf planet Pluto. The object was discovered by American astronomers Chad Trujillo and Michael Brown at Palomar Observatory on 4 June 2002. Quaoar has a reddish surface made of crystalline water ice, tholins, and traces of frozen methane.
Quaoar has two thin rings orbiting outside its Roche limit, which defies theoretical expectations that rings outside the Roche limit should be unstable. Quaoar has one moon named Weywot and a potential second moon that has not yet been confirmed. It is believed that Quaoar's elongated shape, gravitational influence of its moon(s), and extremely cold temperature help keep its rings stable.
History Discovery
Quaoar was discovered on 4 June 2002 by American astronomers Chad Trujillo and Michael Brown at the Palomar Observatory in the Palomar Mountain Range in San Diego County, California. The discovery formed part of the Caltech Wide Area Sky Survey, which was designed to search for the brightest Kuiper belt objects using the Palomar Observatory's 1.22-meter Samuel Oschin telescope. Quaoar was first identified in images by Trujillo on 5 June 2002, when he noticed a dim, 18.6-magnitude object slowly moving among the stars of the constellation Ophiuchus. Quaoar appeared relatively bright for a distant object, suggesting that it could have a size comparable to the diameter of Pluto.
To ascertain Quaoar's orbit, Brown and Trujillo initiated a search for archival precovery images. They obtained several precovery images taken by the Near-Earth Asteroid Tracking survey from various observatories in 1996 and 2000–2002. In particular, they had also found two archival photographic plates taken by astronomer Charles T. Kowal in May 1983, who at the time was searching for the hypothesized Planet X at the Palomar Observatory. From these precovery images, Brown and Trujillo were able to calculate Quaoar's orbit and distance. Additional precovery images of Quaoar have been later identified, with the earliest known found by Edward Rhoads on a photographic plate imaged on 25 May 1954 from the Palomar Observatory Sky Survey.
Before announcing the discovery of Quaoar, Brown had planned to conduct follow-up observations using the Hubble Space Telescope to measure Quaoar's size. He had also planned to announce the discovery as soon as possible and found it necessary to keep the discovery information confidential during the follow-up observations. Rather than submitting his Hubble proposal under peer review, Brown submitted his proposal directly to one of Hubble's operators, who promptly allocated time to Brown. While setting up the observing algorithm for Hubble, Brown had also planned to use one of the Keck telescopes in Mauna Kea, Hawaii, as a part of a study on cryovolcanism on the moons of Uranus. This provided him additional time for follow-up observations and took advantage of the whole observing session in July to analyze Quaoar's spectrum and characterize its surface composition.
The discovery of Quaoar was formally announced by the Minor Planet Center in a Minor Planet Electronic Circular on 7 October 2002. It was given the provisional designation , indicating that its discovery took place during the first half of June 2002. Quaoar was the 1,512th object discovered in the first half of June, as indicated by the preceding letter and numbers in its provisional designation. On that same day, Trujillo and Brown reported their scientific results from observations of Quaoar at the 34th annual meeting of the American Astronomical Society's Division for Planetary Sciences in Birmingham, Alabama. They announced Quaoar was the largest Kuiper belt object found yet, surpassing previous record holders 20000 Varuna and 55565 Aya. Quaoar's discovery has been cited by Brown as having contributed to the reclassification of Pluto as a dwarf planet. Since then, Brown has contributed to the discovery of larger trans-Neptunian objects, including Haumea, , Makemake and .
On 20 November 2002, Quaoar was given the minor planet number 50000, a deliberate choice highlighting its unusual size and the fact that it was discovered during the search for Pluto-sized objects in the Kuiper belt. Likewise, the large Kuiper belt object Varuna had previously received the round number 20000 to mark a similar milestone, and there had even been plans to give a round number to Pluto itself. However, Pluto later received the number 134340, and other large discoveries such as 136199 Eris were assigned numbers in the order in which their orbits were confirmed.
Name and symbol Upon Quaoar's discovery, it was initially given the temporary nickname "Object X" as a reference to Planet X, due to its potentially large size and unknown nature. At the time, Quaoar's size was uncertain, and its brightness led the discovery team to speculate that it may be a tenth planet. After measuring Quaoar's size with the Hubble Space Telescope in July, the team began considering names for the object, particularly those from local Native American mythologies. Following the International Astronomical Union's (IAU) naming convention for minor planets, non-resonant Kuiper belt objects are to be named after creation deities. The team settled on the name Kwawar, the creator god of the Tongva people indigenous to the Los Angeles Basin, where Brown's institute, the California Institute of Technology, was located.
According to Brown, the name "Quaoar" is pronounced with three syllables, and Trujillo's website on Quaoar gives a three-syllable pronunciation, , as an approximation of the Tongva pronunciation . The name can be also pronounced as two syllables, , reflecting the usual English spelling and pronunciation of the deity Kwawar.NASA/JHUAPL/SwRI (2016) Quaoar
In Tongva mythology, Kwawar is the genderless creation force of the universe, singing and dancing deities into existence. They first sing and dance to create Weywot (Sky Father), then they together sing Chehooit (Earth Mother) and Tamit (Grandfather Sun) into existence. As they did this, the creation force became more complex as each new deity joined the singing and dancing. Eventually, after reducing chaos to order, they created the seven great giants that upheld the world, then the animals and finally the first man and woman, Tobohar and Pahavit.
Upon their investigation of names from Tongva mythology, Brown and Trujillo realized that there were contemporary members of the Tongva people, whom they contacted for permission to use the name. They consulted tribal historian Marc Acuña, who confirmed that the name Kwawar would indeed be an appropriate name for the newly discovered object. However, the Tongva preferred the spelling Qua-o-ar, which Brown and Trujillo adopted, though with the hyphens omitted. The name and discovery of Quaoar were publicly announced in October, though Brown had not sought approval of the name by the IAU's Committee on Small Body Nomenclature (CSBN). Indeed, Quaoar's name was announced before the official numbering of the object, which Brian Marsden—the head of the Minor Planet Center—remarked in 2004 to be a violation of the protocol. Despite this, the name was approved by the CSBN, and the naming citation, along with Quaoar's official numbering, was published in a Minor Planet Circular on 20 November 2002.
The usage of planetary symbols is no longer recommended in astronomy, so Quaoar never received a symbol in the astronomical literature. A symbol , used mostly among astrologers, is included in Unicode as U+1F77E. The symbol was designed by Denis Moskowitz, a software engineer in Massachusetts; it combines the letter Q (for 'Quaoar') with a canoe, and is stylized to recall angular Tongva rock art.
Orbit and classification
Quaoar orbits the Sun at an average distance of , taking 288.8 years to complete one full orbit around the Sun. With an orbital eccentricity of 0.04, Quaoar follows a nearly circular orbit, only slightly varying in distance from 42 AU at perihelion to 45 AU at aphelion. At such distances, light from the Sun takes more than 5 hours to reach Quaoar. Quaoar has last passed aphelion in late 1932 and is currently approaching the Sun at a rate of 0.035 AU per year, or about . Quaoar will reach perihelion around February 2075.
Because Quaoar has a nearly circular orbit, it does not approach close to Neptune such that its orbit can become significantly perturbed under the gravitational influence of Neptune. Quaoar's minimum orbit intersection distance from Neptune is only 12.3 AU—it does not approach Neptune within this distance over the course of its orbit, as it is not in a mean-motion orbital resonance with Neptune. Simulations by the Deep Ecliptic Survey show that the perihelion and aphelion distances of Quaoar's orbit do not change significantly over the next ten million years; Quaoar's orbit appears to be stable over the long term.
Quaoar is a trans-Neptunian object. It is classified as a distant minor planet by the Minor Planet Center. Because Quaoar is not in a mean-motion resonance with Neptune, it is also classified as a classical Kuiper belt object (cubewano) by the Minor Planet Center and Deep Ecliptic Survey. Quaoar's orbit is moderately inclined to the ecliptic plane by 8°, relatively high when compared to the inclinations of Kuiper belt objects within the dynamically cold population. Because Quaoar's orbital inclination is greater than 4°, it is part of the dynamically hot population of high-inclination classical Kuiper belt objects. The high inclinations of hot classical Kuiper belt objects such as Quaoar are thought to have resulted from gravitational scattering by Neptune during its outward migration in the early Solar System.
Physical characteristics Size and shape + History of diameter estimates for QuaoarYearDiameter (km)Method References2004imaging2007thermal2010thermal/imaging2013thermal2013occultation2023occultation2024thermal/occultation2025occultation
Analysis of stellar occultation observations from 2011–2024 has shown that Quaoar is a slightly elongated triaxial ellipsoid with dimensions , corresponding to a volume equivalent diameter of . Quaoar's diameter is roughly half that of Pluto and is slightly smaller than Pluto's moon Charon. At the time of its discovery in 2002, Quaoar was the largest object found in the Solar System since the discovery of Pluto.
Quaoar's elongated shape contradicts theoretical expectations that it should be in hydrostatic equilibrium, because of its large size and slow rotation. According to Michael Brown, rocky bodies around in diameter should relax into hydrostatic equilibrium, whereas icy bodies relax into hydrostatic equilibrium somewhere between and . Slowly-rotating objects in hydrostatic equilibrium are expected to be oblate spheroids (Maclaurin spheroids), whereas rapidly-rotating objects in hydrostatic equilibrium, such as Haumea which rotates in nearly 4 hours, are expected to be flattened and elongated ellipsoids (Jacobi ellipsoids). To explain Quaoar's non-equilibrium shape, Csaba Kiss and collaborators hypothesized that Quaoar originally had a rapid rotation and was in hydrostatic equilibrium, but its shape became "frozen in" and did not change as Quaoar spun down due to tidal forces from its moon Weywot. This would resemble the situation of Saturn's moon Iapetus, which is too oblate for its current rotation rate.
Mass and density
Quaoar has a mass of , which was determined from the orbit of its moon Weywot. Measurements of Quaoar's diameter and mass indicate it has a density of , which suggests it has a differentiated and compacted interior consisting of a rocky core surrounded by an icy shell.