Quaoar

Quaoar (minor-planet designation: 50000 Quaoar) is a large, ringed dwarf planet in the Kuiper belt, a region of icy planetesimals beyond Neptune. It has an elongated ellipsoidal shape with an average diameter of 1090 km, about half the size of the dwarf planet Pluto. The object was discovered by American astronomers Chad Trujillo and Michael Brown at the Palomar Observatory on 4 June 2002. Quaoar's surface contains crystalline water ice and ammonia hydrate, which suggests that it might have experienced cryovolcanism. A small amount of methane is present on its surface, which can only be retained by the largest Kuiper belt objects.

Quaoar has one known moon, Weywot, which was discovered by Brown in February 2007. Both objects were named after mythological figures from the Native American Tongva people in Southern California. Quaoar is the Tongva creator deity and Weywot is his son. In 2023, astronomers announced the discovery of two thin rings orbiting Quaoar outside its Roche limit, which defies theoretical expectations that rings outside the Roche limit should not be stable.

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. 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.

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 high brightness led the discovery team to speculate that it may be a possible 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.

In Tongva mythology, Kwawar is the genderless creation force of the universe, singing and dancing deities into existence. He first sings and dances 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.

Quaoar was given the minor planet number 50000, which was not by coincidence but to commemorate its large size, being that it was found in the search for a Pluto-sized object in the Kuiper belt. The large Kuiper belt object 20000 Varuna was similarly numbered for a similar occasion. However, subsequent even larger discoveries such as 136199 Eris were simply numbered according to the order in which their orbits were confirmed.

The usage of planetary symbols is no longer recommended in astronomy, so Quaoar never received a symbol in the astronomical literature. A symbol $6.805 Tm$, 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 43.7 AU, 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 170 m/s. 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 generally classified as a trans-Neptunian object or distant minor planet by the Minor Planet Center since it orbits in the outer Solar System beyond Neptune. Since 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 degrees, 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 degrees, 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.

Size and shape
, measurements of Quaoar's shape from its rotational light curve and stellar occultations show that Quaoar is a triaxial ellipsoid with an average diameter of 1090 km. 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 was also the first trans-Neptunian object to be measured directly from Hubble Space Telescope images.

Quaoar's far-infrared thermal emission and brightness in visible light both vary significantly (visible light curve amplitude 0.12–0.16 magnitudes) as Quaoar rotates every 17.68 hours, which most likely indicates Quaoar is elongated along its equator. A 2024 analysis of Quaoar's visible and far-infrared rotational light curve by Csaba Kiss and collaborators determined that the lengths of Quaoar's equatorial axes differ by 19% (a/b = 1.19) and the lengths of Quaoar's polar and shortest equatorial axis differ by 16% (b/c = 1.16), which corresponds to ellipsoid dimensions of 1286 x 1080 x 932 km. The ellipsoidal shape of Quaoar matches the size and shape measurements from previous stellar occultations, and also explains why the size and shape of Quaoar appeared to change in these occultations.

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 900 km in diameter should relax into hydrostatic equilibrium, whereas icy bodies relax into hydrostatic equilibrium somewhere between 200 km and 400 km. 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, 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 $6.268 Tm$, which was determined from Weywot's orbit using Kepler's third law. Measurements of Quaoar's diameter and mass indicate it has a density between $6.537 Tm$, which suggests its interior is composed of roughly 70% rock and 30% ice with low porosity. Quaoar's density was previously thought to be much higher, between $1,286$, because early measurements inaccurately suggested that Quaoar had a smaller diameter and a higher mass. These early high-density estimates for Quaoar led researchers to hypothesize that the object might be a rocky planetary core exposed by a large impact event, but these hypotheses have since become obsolete as newer estimates indicate a lower density for Quaoar.

Surface
Quaoar has a dark surface that reflects about 12% of the visible light it receives from the Sun. This may indicate that fresh ice has disappeared from Quaoar's surface. The surface is moderately red, meaning that Quaoar reflects longer (redder) wavelengths of light more than shorter (bluer) wavelengths. Many Kuiper belt objects such as 20000 Varuna and 28978 Ixion share a similar moderately red color.

Spectroscopic observations by David Jewitt and Jane Luu in 2004 revealed signs of crystalline water ice and ammonia hydrate on Quaoar's surface. These substances are expected to gradually break down due to solar and cosmic radiation, and crystalline water ice can only form in warm temperatures of at least 110 K, so the presence of crystalline water ice on Quaoar's surface indicates that it was heated to this temperature sometime in the last ten million years. For context, Quaoar's present-day surface temperature is less than 50 K. Jewitt and Luu proposed two hypotheses for Quaoar's heating, which are impact events and radiogenic heating. The latter hypothesis allows for the possibility of cryovolcanism on Quaoar, which is supported by the presence of ammonia hydrate on Quaoar's surface. Ammonia hydrate is believed to be cryovolcanically deposited onto Quaoar's surface. A 2006 study by Hauke Hussmann and collaborators suggested that radiogenic heating alone may not be capable of sustaining an internal ocean of liquid water at Quaoar's mantle–core boundary.

More precise observations of Quaoar's near infrared spectrum in 2007 indicated the presence of small quantities (5%) of solid methane and ethane. Given its boiling point of 112 K, methane is a volatile ice at average surface temperatures of Quaoar, unlike water ice or ethane. Both models and observations suggest that only a few larger bodies (Pluto, and ) can retain the volatile ices whereas the dominant population of small trans-Neptunian objects lost them. Quaoar, with only small amounts of methane, appears to be in an intermediary category.

In 2022, low-resolution near-infrared (0.7–5 μm) spectroscopic observations by the James Webb Space Telescope (JWST) revealed the presence of carbon dioxide ice, complex organics, and significant amounts of ethane ice on Quaoar's surface. Other possible chemical compounds include hydrogen cyanide and carbon monoxide. JWST also took medium-resolution near-infrared spectra of Quaoar and found evidence of small amounts of methane on Quaoar's surface. However, both JWST's low- and medium-resolution spectra of Quaoar did not show conclusive signs of ammonia hydrates.

Possible atmosphere
The presence of methane and other volatiles on Quaoar's surface suggest that it may support a tenuous atmosphere produced from the sublimation of volatiles. With a measured mean temperature of approximately 44 K, the upper limit of Quaoar's atmospheric pressure is expected to be in the range of a few microbars. Due to Quaoar's small size and mass, the possibility of Quaoar having an atmosphere of nitrogen and carbon monoxide has been ruled out, since the gases would escape from Quaoar. The possibility of a methane atmosphere, with the upper limit being less than 1 microbar, was considered until 2013, when Quaoar occulted a 15.8-magnitude star and revealed no sign of a substantial atmosphere, placing an upper limit to at least 20 nanobars, under the assumption that Quaoar's mean temperature is 42 K and that its atmosphere consists of mostly methane. The upper limit of atmosphere pressure was tightened to 10 nanobars after another stellar occultation in 2019.

Satellite


Quaoar has one known moon, Weywot (full designation (50000) Quaoar I Weywot), discovered in 2006 and named after the sky god Weywot, son of Quaoar. It orbits Quaoar at distance of about 13,300 km and is thought to be approximately 170 km in diameter.

Discovery
Besides accurately determining sizes and shapes, stellar occultation campaigns were planned on a long-term basis to search for rings and/or atmospheres around small bodies of the outer solar system. These campaigns agglomerated efforts of various teams in France, Spain and Brazil and were conducted under the umbrella of the European Research Council project Lucky Star. The discovery of Quaoar's first known ring, Q1R, involved various instruments used during stellar occultations observed between 2018 and 2021: the robotic ATOM telescope of the High Energy Stereoscopic System (HESS) in Namibia, the 10.4-m Gran Telescopio Canarias (La Palma Island, Spain); the ESA CHEOPS space telescope, and several stations run by citizen astronomers in Australia where a report of a Neptune-like ring originated and a dense arc in Q1R was first observed. Taken together, these observations reveal the presence of a partly dense, mostly tenuous and uniquely distant ring around Quaoar, a discovery announced in February 2023.

In April 2023, astronomers of the Lucky Star project published the discovery of another ring of Quaoar, Q2R. The Q2R ring was detected by the highly-sensitive 8.2-m Gemini North and the 4.0-m Canada-France-Hawaii Telescope in Mauna Kea, Hawaii, during an observing campaign to confirm Quaoar's Q1R ring in a stellar occultation on 9 August 2022. Quaoar is the fourth minor planet known and confirmed to have a ring system, after 10199 Chariklo, 2060 Chiron, and Haumea.

Properties
Quaoar possesses two narrow rings, provisionally named Q1R and Q2R by order of discovery, which are confined at radial distances where their orbital periods are integer ratios of Quaoar's rotational period. That is, the rings of Quaoar are in spin-orbit resonances.

The outer ring, Q1R, orbits Quaoar at a distance of 4057 ± 6 km, over seven times the radius of Quaoar and more than double the theoretical maximum distance of the Roche limit. The Q1R ring is not uniform and is strongly irregular around its circumference, being more opaque (and denser) where it is narrow and less opaque where it is broader. The Q1R ring's radial width ranges from 5 to 300 km while its optical depth ranges from 0.004 to 0.7. The irregular width of the Q1R ring resembles Saturn's frequently-perturbed F ring or Neptune's ring arcs, which may imply the presence of small, kilometer-sized moonlets embedded within the Q1R ring and gravitationally perturbing the material. The Q1R ring likely consists of icy particles that elastically collide with each other without accreting into a larger mass.

Q1R is located in between the 6:1 mean-motion orbital resonance with Quaoar's moon Weywot at 4021 ± 57 km and Quaoar's 1:3 spin-orbit resonance at 4197 ± 58 km. The Q1R ring's coincidental location at these resonances implies they play a key role in maintaining the ring without having it accrete into a single moon. In particular, the confinement of rings to the 1:3 spin-orbit resonance may be common among ringed small Solar System bodies, as it has been previously seen in Chariklo and Haumea.

The inner ring, Q2R, orbits Quaoar at a distance of 2520 ± 20 km, about four and a half times Quaoar's radius and also outside Quaoar's Roche limit. The Q2R ring's location coincides with Quaoar's 5:7 spin-orbit resonance at 2525 ± 58 km. Compared to Q1R, the Q2R ring appears relatively uniform with a radial width of 10 km. With an optical depth of 0.004, the Q2R ring is very tenuous and its opacity is comparable to the least dense part of the Q1R ring.

Exploration


It has been calculated that a flyby mission to Quaoar using a Jupiter gravity assist would take 13.6 years, for launch dates of 25 December 2026, 22 November 2027, 22 December 2028, 22 January 2030 and 20 December 2040. Quaoar would be 41 to 43 AU from the Sun when the spacecraft arrived. In July 2016, the Long Range Reconnaissance Imager (LORRI) aboard the New Horizons spacecraft took a sequence of four images of Quaoar from a distance of about 14 AU. Interstellar Probe, a concept by Pontus Brandt and his colleagues at Johns Hopkins Applied Physics Laboratory would potentially fly by Quaoar in the 2030s before continuing to the interstellar medium, and the first of China National Space Administration's proposed Shensuo probe designed to explore the heliosphere has it considered as a potential flyby target. Quaoar has been chosen as a flyby target for missions like these particularly for its escaping methane atmosphere and possible cryovolcanism, as well as its close proximity to the heliospheric nose.