50000 Quaoar

50000 Quaoar
Quaoar (minor-planet designation 50000 Quaoar, provisional designation ) is a dwarf planet in the Kuiper belt, a region of icy planetesimals beyond Neptune. A non-resonant object (cubewano), it measures approximately in diameter, about the size of Saturn's moon Dione or half the size of Pluto. The object was discovered by American astronomers Chad Trujillo and Michael Brown at the Palomar Observatory on 4 June 2002. Signs of water ice on the surface of Quaoar have been found, which suggests that cryovolcanism may be occurring on Quaoar. 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 rings orbiting Quaoar outside its Roche limit, which defies theoretical expectations that these rings should not be 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 the dwarf planet 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.
Planetary symbols are no longer much used 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 10 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.
Physical characteristics
Quaoar's albedo or reflectivity could be as low as 0.1, similar to 's albedo of 0.127. This may indicate that fresh ice has disappeared from Quaoar's surface. The surface is moderately red, meaning that Quaoar is relatively more reflective in the red and near-infrared spectrum than in the blue. The Kuiper belt objects Varuna and are also moderately red in the spectral class. Larger Kuiper belt objects are often much brighter because they are covered in more fresh ice and have a higher albedo, and thus they present a neutral color. A 2006 model of internal heating via radioactive decay suggested that Quaoar may not be capable of sustaining an internal ocean of liquid water at the mantle–core boundary.
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 ~ , 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 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.
Mass and density
Because Quaoar is a binary object, the mass of the system can be calculated from the orbit of the secondary. Quaoar's estimated density of around and estimated size of suggests that it is a dwarf planet. American astronomer Michael Brown estimates that rocky bodies around in diameter relax into hydrostatic equilibrium, and that icy bodies relax into hydrostatic equilibrium somewhere between and . With an estimated mass greater than , Quaoar has the mass and diameter "usually" required for being in hydrostatic equilibrium according to the 2006 IAU draft definition of a planet (5 kg, 800 km), and Brown states that Quaoar "must be" a dwarf planet. Light-curve-amplitude analysis shows only small deviations, suggesting that Quaoar is indeed a spheroid with small albedo spots and hence a dwarf planet.
Planetary scientist Erik Asphaug has suggested that Quaoar may have collided with a much larger body, stripping the lower-density mantle from Quaoar, and leaving behind the denser core. He envisioned that Quaoar was originally covered by a mantle of ice that made it to bigger than its present size, and that it collided with another Kuiper belt object about twice its size—an object roughly the diameter of Pluto, or even approaching the size of Mars. This model was made assuming Quaoar actually had a density of 4.2 g/cm3, but more recent estimates have given it a more Pluto-like density of only 2 g/cm3, with no further need for the collision theory.
Size
Quaoar is thought to be an oblate spheroid around in diameter, being slightly flattened in shape. The estimates come from observations of stellar occultations by Quaoar, in which it passes in front of a star, in 2013 and 2019. Given that Quaoar has an estimated oblateness of and a measured equatorial diameter of , Quaoar is believed to be in hydrostatic equilibrium, being described as a Maclaurin spheroid. Quaoar is about as large and massive as (if somewhat smaller than) Pluto's moon Charon. Quaoar is roughly half the size of Pluto.
Quaoar was the first trans-Neptunian object to be measured directly from Hubble Space Telescope images, using a method comparing images with the Hubble point spread function (PSF). In 2004, Quaoar was estimated to have a diameter of with an uncertainty of , using Hubble's measurements. Given its distance Quaoar is on the limit of Hubble's resolution of 40 milliarcseconds and its image is consequently "smeared" on a few adjacent pixels. By comparing carefully this image with the images of stars in the background and using a sophisticated model of Hubble optics (PSF), Brown and Trujillo were able to find the best-fit disk size that would give a similar blurred image. This method was also applied by the same authors to measure the size of the dwarf planet Eris.
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 size was subsequently revised downward and was later superseded in size as larger objects (and ) were discovered. The uncorrected 2004 Hubble estimates only marginally agree with the 2007 infrared measurements by the Spitzer Space Telescope that suggest a higher albedo (0.19) and consequently a smaller diameter. Adopting a Uranian satellite limb darkening profile suggests that the 2004 Hubble size estimate for Quaoar was approximately 40 percent too large, and that a more proper estimate would be about 900 km. In 2010, Quaoar was estimated to be about in diameter, using a weighted average of Spitzer and corrected Hubble estimates. In observations of the object's shadow as it occulted an unnamed 16th-magnitude star on 4 May 2011, Quaoar was estimated to be in diameter. Measurements from the Herschel Space Observatory in 2013 suggested that Quaoar has a diameter of . In that same year, Quaoar occulted a 15.8-magnitude star, with multiple positive detections yielding a mean diameter of , consistent with the Herschel estimate. Another occultation by Quaoar in June 2019 yielded a similar chord length of .
Rotation and shape
Rotational light curves of Quaoar observed on March through June 2003 gave two possible rotation periods: 8.64 hours for a single-peaked light curve by a spheroidal body, or 17.68 hours for a double-peaked light curve by an elongated ellipsoidal body. The discovery of Quaoar's two rings in 2023 showed that its true rotation period should most likely be 17.68 hours in order to explain the rings' locations in stable spin-orbit resonances. This in turn implies Quaoar should be somewhat elongated to induce such a resonance. Various occultation measurements of Quaoar's dimensions show that its diameter slightly varies at different epochs, further supporting the possibility of a triaxial ellipsoid shape for Quaoar.
Composition and cryovolcanism
In 2004, signs of crystalline water ice were found on Quaoar, indicating that the temperature rose to at least sometime in the last ten million years. Speculation began as to what could have caused Quaoar to heat up from its natural temperature of . Some have theorized that a barrage of mini-meteors may have raised the temperature, but the most discussed theory speculates that cryovolcanism may be occurring, spurred by the decay of radioactive elements within Quaoar's core. Crystalline water ice was also found on in 2006, but it is present in larger quantities and is thought to have originated from an impact. 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 , 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, near-infrared spectroscopic observations by the James Webb Space Telescope revealed the presence of methanol (CH3OH), ethane (C2H6) and butane (C3H8) in Quaoar's surface.
Rings
Properties
Quaoar is the third minor planet known and confirmed to have a ring system, after 10199 Chariklo and 136108 Haumea. It 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 , 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 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 and Quaoar's 1:3 spin-orbit resonance at . 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 , 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 . Compared to Q1R, the Q2R ring appears relatively uniform with a radial width of . 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.
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.
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 is thought to be approximately in diameter.
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 Interstellar Express 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.
Notes
References
External links
Frequently Asked Questions About Quaoar
Quaoar could have hit a bigger Pluto-sized body at high speeds (Video Credit: Craig Agnor, E. Asphaug)
Chilly Quaoar had a warmer past – Nature.com article
Quaoar: Planetoid Beyond Pluto – SPACE.com article by Elizabeth Howell
Beyond Jupiter – (50000) Quaoar
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Discoveries by Michael E. Brown
Discoveries by Chad Trujillo
Named minor planets
Dwarf planets
Binary trans-Neptunian objects
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