50000 Quaoar

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"Quaoar" redirects here. For the Tongva god after which the object is named, see Chinigchinix. Not to be confused with Quasar.

50000 Quaoar
milliarcseconds[19]

Quaoar (

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 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.[13]

History

Discovery

Quaoar was discovered using the Samuel Oschin telescope at Palomar Observatory
Animation of three discovery images taken over a period of 4.5 hours, showing the slow movement of Quaoar (indicated by the arrow)[21]

Quaoar was discovered on 4 June 2002 by American astronomers

Palomar Mountain Range in San Diego County, California.[1] 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.[22] 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.[23][24] Quaoar appeared relatively bright for a distant object, suggesting that it could have a size comparable to the diameter of the dwarf planet Pluto.[25]

To ascertain Quaoar's orbit, Brown and Trujillo initiated a search for archival

Palomar Observatory Sky Survey.[1][3]

Before announcing the discovery of Quaoar, Brown had planned to conduct follow-up observations using the

cryovolcanism on the moons of Uranus.[29] 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.[31][29]

The discovery of Quaoar was formally announced by the

2002 AW197.[22][28] Quaoar's discovery has been cited by Brown as having contributed to the reclassification of Pluto as a dwarf planet.[29] Since then, Brown has contributed to the discovery of larger trans-Neptunian objects, including Haumea, Eris, Makemake and Gonggong
.

Name and symbol

Upon Quaoar's discovery, it was initially given the temporary nickname "Object X" as a reference to

Tongva people indigenous to the Los Angeles Basin, where Brown's institute, the California Institute of Technology, was located.[26]

According to Brown, the name "Quaoar" is pronounced with three syllables, and Trujillo's website on Quaoar gives a three-syllable pronunciation, /ˈkwɑː.oʊ(w)ɑːr/, as an approximation of the Tongva pronunciation [ˈkʷaʔuwar].[23] The name can be also pronounced as two syllables, /ˈkwɑːwɑːr/, reflecting the usual English spelling and pronunciation of the deity Kwawar.[28][33][34]

In Tongva mythology, Kwawar is the genderless[33] creation force of the universe, singing and dancing deities into existence.[2] 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,[23][28] then the animals and finally the first man and woman, Tobohar and Pahavit.[23]

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.

Minor Planet Circular on 20 November 2002.[36]

Quaoar was given the

136199 Eris were simply numbered according to the order in which their orbits were confirmed.[32]

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,[38] is included in Unicode as U+1F77E.[39] 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.[40]

Orbit and classification

Ecliptic view of Quaoar's orbit (blue) compared to Pluto (red) and Neptune (white). The approximate perihelion (q) and aphelion (Q) dates are marked for their respective orbits.
Polar view of Quaoar's orbit (yellow) along with various other large Kuiper belt objects

Quaoar orbits the

aphelion.[3] At such distances, light from the Sun takes more than 5 hours to reach Quaoar.[23] 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 meters per second (380 mph).[41] Quaoar will reach perihelion around February 2075.[6]

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.[4] 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.[1][4] 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.[4]

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.[1][3] 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.[4][5] 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.[29][42] 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.[42] 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.[43]

Physical characteristics

Size and shape

History of diameter estimates for Quaoar
Year Diameter (km) Method Refs
2004 1,260±190 imaging [19]
2007 844+207
−190 		thermal [44]
2010 890±70 thermal/imaging [45]
2013 1,074±138 thermal [46]
2013 1,110±5 occultation [47]
2023 1,086±4 occultation [13]
2024 1,090±40 thermal/occultation [7]

As of 2024, measurements of Quaoar's shape from its rotational

triaxial ellipsoid with an average diameter of 1,090 km (680 mi).[7] Quaoar's diameter is roughly half that of Pluto and is slightly smaller than Pluto's moon Charon.[29] At the time of its discovery in 2002, Quaoar was the largest object found in the Solar System since the discovery of Pluto.[29] Quaoar was also the first trans-Neptunian object to be measured directly from Hubble Space Telescope images.[19]

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.[7] 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 1,286 km × 1,080 km × 932 km (799 mi × 671 mi × 579 mi).[a][7] 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.[7]
: 6 

Diagram showing three views of Quaoar's ellipsoidal shape

Quaoar's elongated shape contradicts theoretical expectations that it should be in

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).[7]: 10  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.[7]: 10  This would resemble the situation of Saturn's moon Iapetus, which is too oblate for its current rotation rate.[49]

Mass and density

Quaoar compared to the Earth and the Moon

Quaoar has a mass of 1.2×1021 kg, which was determined from Weywot's orbit using Kepler's third law.[13] Measurements of Quaoar's diameter and mass as of 2024 indicate it has a density between 1.66–1.77 g/cm3, which suggests its interior is composed of roughly 70% rock and 30% ice with low porosity.[7]: 10–11  Quaoar's density was previously thought to be much higher, between 2–4 g/cm3, because early measurements inaccurately suggested that Quaoar had a smaller diameter and a higher mass.[7]: 10  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.[45]: 1550 [7]: 10 

Surface

Quaoar has a dark surface that reflects about 12% of the visible light it receives from the Sun.[13] This may indicate that fresh ice has disappeared from Quaoar's surface.[45] The surface is moderately red, meaning that Quaoar reflects longer (redder) wavelengths of light more than shorter (bluer) wavelengths.[50] Many Kuiper belt objects such as 20000 Varuna and 28978 Ixion share a similar moderately red color.

Spectroscopic observations by

internal ocean of liquid water at Quaoar's mantle–core boundary.[51]

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 trans-Neptunian objects lost them. Quaoar, with only small amounts of methane, appears to be in an intermediary category.[31]

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.[52]: 4  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.[52]: 10 

Possible atmosphere

The presence of methane and other

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.[47][14] The upper limit of atmosphere pressure was tightened to 10 nanobars after another stellar occultation in 2019.[53]

Satellite

Artist's impression of Quaoar with its ring and its moon Weywot
Main article: Weywot

Quaoar has one known moon,

Weywot, son of Quaoar.[20][54] It orbits Quaoar at distance of about 13,300 km and is thought to be approximately 170 km (110 mi) in diameter.[55]

Rings

Discovery

Light curve graph of a star's brightness as seen by the Gemini North Observatory during the 9 August 2022 occultation by Quaoar and its two rings. The asymmetry of the outer Q1R ring's opacity is apparent from its differing brightness dips before and after the occultation by Quaoar at the center.

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.[10] 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.[10][56][57] 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.[10][56]

In April 2023, astronomers of the Lucky Star project published the discovery of another ring of Quaoar, Q2R.

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.[13] Quaoar is the fourth minor planet known and confirmed to have a ring system, after 10199 Chariklo, 2060 Chiron, and Haumea.[10][e]

Properties

Orbit diagrams of the Quaoar–Weywot system
Viewed from Earth
Viewed top-down over Quaoar's north pole

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.[13]

Ring–moon system data[13]
Rings
Ring
designation 		Radius
(km) 		Width
(km) 		Optical depth
(τ)
Q2R 2520±20 10 ≈0.004
Q1R 4057±6 5–300 0.004–0.7
Moon
Name Semi-major axis
(km) 		Diameter
(km) 		Period
(days)
Weywot 13289±189 170 12.4311±0.0015

The outer ring, Q1R, orbits Quaoar at a distance of 4,057 ± 6 km (2,521 ± 4 mi), over seven times the radius of Quaoar and more than double the theoretical maximum distance of the

accreting into a larger mass.[10]

Q1R is located in between the 6:1

mean-motion orbital resonance with Quaoar's moon Weywot at 4,021 ± 57 km (2,499 ± 35 mi) and Quaoar's 1:3 spin-orbit resonance at 4,197 ± 58 km (2,608 ± 36 mi). 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.[10] 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.[10]

The inner ring, Q2R, orbits Quaoar at a distance of 2,520 ± 20 km (1,566 ± 12 mi), about four and a half times Quaoar's radius and also outside Quaoar's Roche limit.[13] The Q2R ring's location coincides with Quaoar's 5:7 spin-orbit resonance at 2,525 ± 58 km (1,569 ± 36 mi). Compared to Q1R, the Q2R ring appears relatively uniform with a radial width of 10 km (6.2 mi). 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.[13]

Exploration

Quaoar from New Horizons viewed at a distance of 14 AU

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.

Shensuo probe designed to explore the heliosphere has it considered as a potential flyby target.[60][61][62] 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.[60]

Notes

  1. ^ a b Ellipsoidal dimensions in km is calculated from the volume equivalent diameter of 1,090 km, axial ratios of a/b = 1.19 and b/c = 1.16 given by Kiss et al. (2024),[7] and the formula for the volume of an ellipsoid, .
  2. ecliptic north pole at β = +90° ; i with respect to the ecliptic would be the complement
    of β, which is expressed by the difference i = 90° – β. Thus, the axial tilt of Quaoar's outer ring is 13.62° with respect to the ecliptic. If the outer ring is coplanar to Quaoar's equator (having the same north pole orientation), then Quaoar would have the same axial tilt with respect to the ecliptic.
  3. ecliptic latitude).[12]
    Subtracting this value of β from +90° gives the inclination of Quaoar's outer ring with respect to the ecliptic: i = 90° – β ≈ 14.02°. If the outer ring is coplanar to Quaoar's equator (having the same north pole orientation), then Quaoar would have the same axial tilt with respect to the ecliptic.
  4. ^ In the convention for minor planet provisional designations, the first letter represents the half-month of the year of discovery while the second letter and numbers indicate the order of discovery within that half-month. In the case for 2002 LM60, the first letter 'L' corresponds to the first half-month of June 2002 while the preceding letter 'M' indicates that it is the 12th object discovered on the 61st cycle of discoveries (with 60 cycles completed). Each completed cycle consists of 25 letters representing discoveries, hence 12 + (60 completed cycles × 25 letters) = 1,512.[32]
  5. ^ 2060 Chiron's rings were initially observed in 2011, and were confirmed by 2022

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