50000 Quaoar
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50000 Quaoar

50000 Quaoar
Quaoar PRC2002-17e.jpg
Quaoar imaged by the Hubble Space Telescope in 2002
Discovery [1]
Discovered byC. Trujillo
M. E. Brown
Discovery sitePalomar Obs.
Discovery date5 June 2002
MPC designation(50000) Quaoar
Named after
(deity of the Tongva people)
TNO[3] · cubewano[4][5]
Orbital characteristics[3]
Epoch 27 April 2019 (JD 2458600.5)
Uncertainty parameter 3
Observation arc65.27 yr (105,405 days)
Earliest precovery date25 May 1954
Aphelion45.418 AU
Perihelion41.964 AU
43.692 AU
288.78 yr (105,416 d)
0° 0m 12.287s / day
Known satellites1 (Weywot[6]
Physical characteristics
Mean diameter
Mean radius
Mean density
Equatorial surface gravity
? 0.3 m/s2
Equatorial escape velocity
? 0.58 m/s
h (double-peaked light curve)[11]
h (single-peaked light curve)[11]
IR (moderately red)
19.1 (opposition)[15]
2.4 (assumed)[3][1]

50000 Quaoar ,[a] provisional designation , is a non-resonant trans-Neptunian object (cubewano) and a possible dwarf planet in the Kuiper belt, a region of icy planetesimals beyond Neptune. It measures approximately 1,121 km (697 mi) in diameter, about half the diameter of Pluto. The object was discovered by American astronomers Chad Trujillo and Michael Brown at the Palomar Observatory on 5 June 2002.[1] Signs of water ice on the surface of Quaoar have been found, which suggests that cryovolcanism may be occurring on Quaoar.[16] A small amount of methane is present on its surface, which can only be retained by the largest Kuiper belt objects.[17] In February 2007, Weywot, a synchronous moon in orbit around Quaoar, was discovered by Brown.[6] Weywot is measured to be 80 km (50 mi) across. 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.[2]



Quaoar was discovered by astronomer Chad Trujillo on 5 June 2002, when he identified it in images acquired by the Samuel Oschin Telescope at Palomar Observatory the night before.[1] The discovery was submitted to the Minor Planet Center on 6 June, with Trujillo and his colleague Michael Brown credited for the discovery.[18] At the time of discovery, Quaoar was located in the constellation Ophiuchus, at an apparent magnitude of 18.5. Its discovery was formally announced at a meeting of the American Astronomical Society on 7 October 2002. The earliest prediscovery image of Quaoar was found on a photographic plate imaged on 25 May 1954 from the Palomar Observatory Sky Survey.[3] Quaoar's discovery has been cited as Brown as having contributed to the reclassification of Pluto as a dwarf planet.[19]


Quaoar is named for the Tongva creator god, following the International Astronomical Union (IAU) naming convention for non-resonant Kuiper belt objects after creator deities.[2] The Tongva people are native to area around Los Angeles, the city where Quaoar was discovered.[19] Michael Brown and his team had picked the name with the more intuitive spelling Kwawar, but the preferred spelling among the Tongva was Qua-o-ar.[2] Upon the discovery of Quaoar, prior to IAU approval of its name, Quaoar was temporarily referred as "Object X", after Planet X.[19] At that time, Quaoar was thought to be a possible tenth planet, with a size comparable to that of Pluto.[19] After the announcement of its discovery,[20] Quaoar was provisionally designated , as it was discovered in the year 2002.[3][21] The minor planet number 50000 for Quaoar was not a coincidence, but chosen to commemorate a particularly large object found in the search for a Pluto-sized object in the Kuiper belt, parallel to the similarly numbered 20000 Varuna.[22][23] However, subsequent even larger discoveries such as 136199 Eris were simply numbered according to the order in which their orbits were confirmed.[24]

Physical characteristics

Artist's impression of Quaoar's moderately red surface along with its moon Weywot.

Quaoar's albedo or reflectivity could be as low as 0.1, similar to Varuna's albedo of 0.127.[25] This may indicate that fresh ice has disappeared from Quaoar's surface.[26] The surface is moderately red, meaning that Quaoar is relatively more reflective in the red and near-infrared spectrum than in the blue.[16] The Kuiper belt objects Varuna and Ixion 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.[27] A 2006 model of internal heating via radioactive decay suggested that, unlike 90482 Orcus, Quaoar may not be capable of sustaining an internal ocean of liquid water at the mantle-core boundary.[28]

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.[29] With a measured mean temperature of ~ 44 K (-229.2 °C), the upper limit of Quaoar's atmospheric pressure is expected to be in the range of a few microbars.[29] 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.[29] The possibility of a methane atmosphere, with the upper limit being less than 1 microbar,[7][29] 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 (-231.2 °C) and that its atmosphere consists of mostly methane.[7][29] The upper limit of atmosphere pressure was tightened to 10 nanobars after another stellar occultation in 2019.[8]

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 1,121 km (697 mi) suggests that it is a dwarf planet. American astronomer Michael Brown estimates that rocky bodies around 900 km (560 mi) in diameter relax into hydrostatic equilibrium, and that icy bodies relax into hydrostatic equilibrium somewhere between 200 km (120 mi) and 400 km (250 mi).[30] 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 (5×1020 kg, 800 km),[31] and Brown states that Quaoar "must be" a dwarf planet.[32] Light-curve-amplitude analysis shows only small deviations, suggesting that Quaoar is indeed a spheroid with small albedo spots and hence a dwarf planet.[33]

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 300 km (190 mi) to 500 km (310 mi) 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.[34] 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 more need for the collision theory.[7]


Size estimates for Quaoar
Year Diameter (km) Method Refs
2004 imaging [35]
2007 thermal [36]
2010 thermal/imaging [26]
2013 thermal [10]
2013 occultation [7]
2019 occultation [8]
Hubble photo used to measure size of Quaoar
Quaoar compared to the Earth and the Moon.

Quaoar is thought to be an oblate spheroid around 1,121 km (697 mi) in diameter, being slightly flattened in shape.[8][7] The estimates come from observations of stellar occultations by Quaoar, in which it passes in front of a star, in 2013 and 2019.[7][8] 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.[7] Quaoar is about as large and massive as (if somewhat smaller than) Pluto's moon Charon.[c] Quaoar is roughly half the size of Pluto.[19]

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).[35] In 2004, Quaoar was estimated to have a diameter of 1,260 km (780 mi) with an uncertainty of 190 km (120 mi), using Hubble's measurements.[35][38] 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.[35] 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.[35] This method was also applied by the same authors to measure the size of the dwarf planet Eris.[36]

At the time of its discovery in 2002, Quaoar was the largest object found in the Solar System since the discovery of Pluto.[19] Quaoar's size was subsequently revised downward and was later superseded in size as larger objects (Eris, Haumea, Makemake 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 .[36] 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.[26] In 2010, Quaoar was estimated to be about in diameter, using a weighted average of Spitzer and corrected Hubble estimates.[26] In observations of the object's shadow as it occulted an unnamed 16th-magnitude star on 4 May 2011, Quaoar was estimated to be 1,170 km (730 mi) in diameter.[38] Measurements from the Herschel Space Observatory in 2013 suggested that Quaoar has a diameter of 1,070 km (660 mi).[10] In that same year, Quaoar occulted a 15.8 magnitude star, yielding a chord length of , consistent with the Herschel estimate.[7] Another occultation by Quaoar in June 2019 also yielded a similar chord length of .[8]


In 2004, signs of crystalline ice were found on Quaoar, indicating that the temperature rose to at least 110 K (-163 °C) sometime in the last ten million years.[16] Speculation began as to what could have caused Quaoar to heat up from its natural temperature of 55 K (-218.2 °C). 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.[16] Since then (2006), crystalline water ice was also found on Haumea, but present in larger quantities and thought to be responsible for the very high albedo of that object (0.7).[39] 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 (-161 °C), 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, Eris, and Makemake) can retain the volatile ices whereas the dominant population of small TNOs lost them. Quaoar, with only small amounts of methane, appears to be in an intermediary category.[17]

Orbit and rotation

Polar and ecliptic view of Quaoar's orbit compared to Pluto and various other cubewanos. Quaoar's orbit is colored yellow in the left image and blue in the right image.

Quaoar orbits at about 43.7 astronomical units (6.54×109 km; 4.06×109 mi) from the Sun with an orbital period of 288.8 years.[3] Quaoar has a low orbital eccentricity of 0.0396, meaning its orbit is nearly circular.[3] Its orbit is moderately inclined to the ecliptic at approximately 8 degrees,[3] typical for the population of small classical Kuiper belt objects (KBOs) but exceptional among large KBOs. Quaoar is not significantly perturbed by Neptune[4] unlike Pluto, which is in 2:3 orbital resonance with Neptune (Pluto orbits the Sun twice for every three orbits completed by Neptune). Quaoar is the largest body that is classified as a cubewano, or classical Kuiper belt object, by both the Minor Planet Center[5] and the Deep Ecliptic Survey (although the dwarf planet Makemake, which is larger, is also classified as a cubewano).[4] Quaoar occasionally moves closer to the Sun than Pluto, as Pluto's aphelion (farthest distance to the Sun) is beyond and below Quaoar's orbit. Quaoar's rotation period is uncertain, and two possible rotation periods of Quaoar are given (8.64 hours or 17.68 hours).[9] Derived from the rotational light curves of Quaoar observed on March through June 2003, its rotation period is measured to be 17.6788 hours.[11]


Quaoar and its moon Weywot imaged by the Hubble Space Telescope in 2006

Quaoar has one known moon, Weywot (full designation (50000) Quaoar I Weywot), discovered in 2006.[6] It is thought to be somewhere under 200 km in diameter.[40]


Quaoar from New Horizons, viewed at a distance of 14 AU.

It was calculated that a flyby mission to Quaoar could take 13.57 years using a Jupiter gravity assist, based on launch dates of 25 December 2016, 22 November 2027, 22 December 2028, 22 January 2030 or 20 December 2040. Quaoar would be 41 to 43 AU from the Sun when the spacecraft arrives.[41] 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.[42] Pontus Brandt at Johns Hopkins Applied Physics Laboratory and his colleagues have studied an interstellar probe that would potentially fly by Quaoar in the 2030s before continuing to the interstellar medium.[43][44] Quaoar has been chosen as a flyby target for such a mission particularly for its escaping methane atmosphere and possible cryovolcanism, as well as its close proximity to the heliospheric nose.[43]


  1. ^ a b Trujillo's website gives a three-syllable pronunciation, , as an approximation of the Tongva pronunciation ['k?a?uwar].[18] However, other astronomers including Brown's student Darin Ragozzine pronounce it with two syllables, , reflecting the usual English spelling and pronunciation of the deity, Kwawar.[45][42]
  2. ^ Polar dimension calculated by multiplying the measured equatorial diameter with the oblateness 0.0897 obtained from the occultation in 2013.[7]
  3. ^ Charon's mass is [37] while Quaoar's mass is .[10] Both values are approximately similar, though Charon is slightly more massive. In a similar case, Charon's diameter is while Quaoar's diameter is , being slightly smaller than Charon.


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