50000 Quaoar

50000 Quaoar ("Kwawar") is a Kuiper belt object roughly half the size of Pluto, with a diameter of 7003111000000000000♠1110±5 km. It is large enough and massive enough that it is probably a dwarf planet. Its orbit in the Kuiper belt is not in resonance with Neptune, which means it is a classical Kuiper belt object. Quaoar has one known moon, Weywot.

Quaoar was discovered in 2002 and its moon in 2007. It was named after the Tongva creator deity and its moon after his son.

Signs of water ice have been found, which suggests that cryovolcanism may be occurring. A small amount of methane is present on Quaoar's surface, which can only be retained by the largest Kuiper belt objects.

Discovery

Quaoar was discovered on June 4, 2002, by astronomers Chad Trujillo and Michael Brown at the California Institute of Technology, from images acquired at the Samuel Oschin Telescope at Palomar Observatory. The discovery of this magnitude 18.5 object, at the time located in the constellation Ophiuchus, was announced on October 7, 2002, at a meeting of the American Astronomical Society. The earliest prediscovery image proved to be a May 25, 1954, plate from the Palomar observatory sky survey.^[4]

Name

Quaoar is named for the Tongva creator god, following International Astronomical Union (IAU) naming conventions for non-resonant Kuiper belt objects. The Tongva are the native people of the area around Los Angeles, where the discovery of Quaoar was made. Brown et al. had picked the name with the more intuitive spelling Kwawar, but the preferred spelling among the Tongva was Qua-o-ar.

Prior to IAU approval of the name, Quaoar went by the provisional designation 2002 LM₆₀. The minor planet number 50000 was not 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. However, subsequent even-larger discoveries such as 136199 Eris were simply numbered according to the order in which their orbits were confirmed.

Size

In 2004, Quaoar was estimated to have a diameter of 7006126000000000000♠1260±190 km,^[13] subsequently revised downward, which at the time of discovery in 2002 made it the largest object found in the Solar System since the discovery of Pluto. Quaoar was later supplanted by Eris, Sedna, Haumea, and Makemake, though Sedna was later found to be somewhat smaller than Quaoar. Quaoar is about as massive as (if somewhat smaller than) Pluto's moon Charon, which is approximately  2 ¹⁄₂ times as massive as Orcus. Quaoar is roughly one twelfth the diameter of Earth, one third the diameter of the Moon, and half the size of Pluto.

Quaoar was the first trans-Neptunian object to be measured directly from Hubble Space Telescope (HST) images, using a new, sophisticated method (see Brown’s pages for a non-technical description and his paper^[13] for details). Given its distance Quaoar is on the limit of the HST resolution (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 HST optics (point spread function (PSF)), Brown and Trujillo were able to find the best-fit disk size that would give a similar blurred image. This method was recently applied by the same authors to measure the size of Eris.

Hubble photo used to measure size of Quaoar

The uncorrected 2004 HST estimates only marginally agree with the 2007 infrared measurements by the Spitzer Space Telescope (SST) that suggest a higher albedo (0.19) and consequently a smaller diameter (7005844400000000000♠844.4+206.7
−189.6 km).^[14] During the 2004 HST observations, little was known about the surface properties of Kuiper belt objects, but we now know that the surface of Quaoar is in many ways similar to those of the icy satellites of Uranus and Neptune.^[10] Adopting a Uranian-satellite limb darkening profile suggests that the 2004 HST size estimate for Quaoar was approximately 40% too large, and that a more proper estimate would be about 900 km.^[10] Using a weighted average of the SST and corrected HST estimates, Quaoar, as of 2010, can be estimated at about 7005890000000000000♠890±70 km in diameter.^[10]

On 4 May 2011 Quaoar occulted a 16th-magnitude star, which gave 1170 km as the longest chord and suggested an elongated shape.^[15] New measurement from Herschel Space Observatory with revised data from SST suggested that Quaoar has a diameter of 7006107000000000000♠1070±38 km and its satellite, Weywot, of 7004810000000000000♠81±11 km.^[8]

Classification

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 2.2 g/cm³ and estimated size of 1,100 km suggests that it is a dwarf planet. Mike Brown estimates that rocky bodies around 900 km in diameter relax into hydrostatic equilibrium, and that icy bodies relax into hydrostatic equilibrium somewhere between 200 and 400 km.^[16] With an estimated mass greater than 1.6×10²¹ kg, 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),^[17] and Brown states that Quaoar "must be" a dwarf planet.^[18] Light-curve-amplitude analysis shows only small deviations, suggesting that Quaoar is indeed a spheroid with small albedo spots and hence a dwarf planet.^[19]

Hit-and-run collision

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 envisions that Quaoar was originally covered by a mantle of ice that made it 300 to 500 kilometers bigger than it is today, and that it collided with another Kuiper-belt body about twice its size—an object roughly the diameter of Pluto (or even approaching the size of Mars),^[20] possibly Pluto itself.^[21] 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.

Orbit

Quaoar orbits at about 43.3 astronomical units (6.48×10⁹ km; 4.02×10⁹ mi) from the Sun with an orbital period of 284.5 years. Its orbit is nearly circular and moderately inclined at approximately 8°, typical for the population of small classical Kuiper-belt objects (KBO) but exceptional among the large KBOs. Pluto, Makemake, Haumea, Orcus, Varuna, and Salacia are all on highly inclined, more eccentric orbits.

Quaoar is the largest body that is classified as a cubewano by both the Minor Planet Center^[3] and the Deep Ecliptic Survey.^[2][22]

The polar view compares the near-circular orbit of Quaoar to the highly eccentric (e=0.25) orbit of Pluto (Quaoar’s orbit in blue, Pluto’s in red, Neptune in grey). The circles illustrate the positions in April 2006, relative sizes, and colours. The perihelia (q), aphelia (Q) and the dates of passage are also marked.

At 43 AU and a near-circular orbit, Quaoar is not significantly perturbed by Neptune,^[2] unlike Pluto, which is in 2:3 orbital resonance with Neptune. The ecliptic view illustrates the relative inclinations of the orbits of Quaoar and Pluto. Note that Pluto's aphelion is beyond (and below) Quaoar's orbit, so that Pluto is closer to the Sun than Quaoar at some times of its orbit, and farther at others.

As of 2008, Quaoar was only 14 AU^[23] from Pluto, which made it the closest large body to the Pluto–Charon system. By Kuiper belt standards this is very close.

Orbit of Quaoar
The orbit of Quaoar (yellow) and various other cubewanos compared to the orbit of Neptune (blue) and Pluto (pink)  Orbits of Quaoar and Pluto – ecliptic view  Orbits of Quaoar (blue) and Pluto (red) – polar view

Physical characteristics

Artistic comparison of Pluto, Eris, Makemake, Haumea, Sedna, 2007 OR₁₀, Quaoar, Orcus, and Earth along with the Moon.

Quaoar's albedo could be as low as 0.1, which would still be much higher than the lower estimate of 0.04 for Varuna. 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 than in the blue. 20000 Varuna and 28978 Ixion are also moderately red in the spectral class. Larger KBOs are often much brighter because they are covered in more fresh ice and have a higher albedo, and thus they present a neutral colour (see colour comparison).

A 2006 model of internal heating via radioactive decay suggested that, unlike Orcus, Quaoar may not be capable of sustaining an internal ocean of liquid water at the mantle–core boundary.^[24]

Cryovolcanism

In 2004, scientists were surprised to find signs of crystalline ice on Quaoar, indicating that the temperature rose to at least −160 °C (110 K or −260 °F) sometime in the last ten million years.^[25]

Speculation began as to what could have caused Quaoar to heat up from its natural temperature of −220 °C (55 K or −360 °F). 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.^[26] 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).^[27]

More precise (2007) observations of Quaoar's near infrared spectrum indicate the presence of small (5%) quantity of (solid) methane and ethane.^[28] Given its boiling point (112 K), methane is a volatile ice at average Quaoar surface temperatures, unlike water ice or ethane (boiling point 185 K). Both models and observations suggest that only a few larger bodies (Pluto, Eris, 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.^[28]

In 2019, when the New Horizons mission visits the small Kuiper-belt object (486958) 2014 MU69 after having visited Pluto in 2015, knowledge of the surfaces of KBOs should improve.

Satellite

Quaoar has one known moon, Weywot (full designation (50000) Quaoar I Weywot). Its discovery by Michael E. Brown was reported in IAUC 8812 on 22 February 2007, based on imagery taken on 14 February 2006.^[6][29] The satellite was found at 0.35 arcsec from Quaoar with an apparent magnitude difference of 5.6.^[30]

Two possible orbits have been determined from the observations: the first is a prograde orbit with an inclination of 14°, the second a retrograde orbit with an inclination of 30° (150°); the other parameters are very similar between the two orbits. It orbits at a distance of 14,500 km from the primary and has an eccentricity of about 0.14, it completes one orbit in about 12.5 days.^[10]

From the surface of Quaoar and at a point where Weywot is at zenith it would have an angular diameter of 15.9 arcminutes at apoapsis and 21.4 arcminutes at periapsis, in comparison the Moon's size varies between 29.4 and 33.5 arcminutes.^{[lower-alpha 3]} Its apparent magnitude at full phase would be about -3, comparable to Jupiter at maximum brightness.^{[lower-alpha 2]}

Assuming an equal albedo and density to the primary, the apparent magnitude suggests that the moon has a diameter of about 74 km ( ¹⁄₁₂ of Quaoar).^[10] Weywot is estimated to only have  ¹⁄₂₀₀₀ the mass of Quaoar.^[10]

Name

Upon discovery, Weywot was issued a provisional designation, S/2006 (50000) 1. Brown left the choice of a name up to the Tongva (whose creator god Quaoar had been named after), who chose the sky god Weywot, son of Quaoar.^[31] The name was made official in MPC #67220 published on October 4, 2009.^[32]

Exploration

New Horizons image sequence of Quaoar

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.^[33] In July 2016 New Horizons spacecraft took a sequence of four images of Quaoar from a distance of about 14 AU.^[34]

Notes

References

External links

Wikimedia Commons has media related to Quaoar.

• Quaoar discoverers' webpage
• Quaoar could have hit a bigger Pluto-sized body at high speeds (Video Credit: Craig Agnor, E. Asphaug)
• Data at Johnston's Archive
• Chilly Quaoar had a warmer past – Nature.com article
• Cryovolcanism on Charon and other Kuiper Belt Objects
• 50000 Quaoar at the JPL Small-Body Database
  • Discovery · Ephemeris · Orbit diagram · Orbital elements · Physical parameters

Dwarf planets
Asteroid belt	IAU: Ceres No longer rounded and therefore not dwarf planets: Vesta
Centaurs	Possibly: Chariklo Chiron
Centaurs (extended)	Possibly: 1995 SN55 (lost) Ceto 2010 TY53
Plutinos	IAU: Pluto Additional objects proposed by Brown and Tancredi: Orcus Ixion 2003 AZ84 2004 UX10 Huya Possibly: 2003 UZ413 2003 VS2 2002 VR128 2004 PF115 2002 XV93 2007 JH43 2001 QF298
Twotinos	Possibly: 2002 WC19
Other resonances/ unknown resonances:	IAU: Haumea Possibly: 2002 XW93 1999 DE9 1999 KR16 2001 YH140 2011 GM27 2010 VK201 2013 FZ27 2014 UM33 1999 CD158
Cubewanos:	IAU: Makemake Additional objects proposed by Brown and Tancredi: 2002 MS4 Salacia Quaoar Varuna Varda 2004 GV9 2002 AW197 2002 TX300 2005 RR43 2003 OP32 2005 RN43 Possibly: Chaos Altjira Sila-Nunam 2002 UX25 2007 JJ43 2005 UQ513 2004 TY364 2003 QW90 1998 SN165 2002 KX14 Praamzius 2004 NT33 2010 FX86 2002 CY248 2010 VR11 Rhadamanthus
Scattered disc	IAU: Eris Additional objects proposed by Brown and Tancredi: 2007 OR10 1996 TL66 2001 UR163 2005 RM43 Possibly: 2002 TC302 2004 XA192 2005 QU182 2006 QH181 2007 UK126 2010 EK139 2010 KZ39 2010 RE64 2010 RF43 2010 TJ 2013 FY27 2008 OG19 1996 GQ21 2012 HH2 1999 CC158 2010 VZ98
Area uncertain	Captured satellites that were once dwarf planets: Triton (captured by Neptune) Phoebe (captured by Saturn, and no longer rounded) Possibly: 2011 FW62 (lost) 2000 YW134 V774104
Detached objects	Possibly: 2004 XR190 2010 GB174 2008 ST291 1995 TL8 2005 TB190 2003 QX113 2003 FY128 2004 VN112 2000 CR105
Sednoids	Objects proposed by Brown and Tancredi: Sedna Possibly: 2012 VP113
Former candidates	Pallas Vesta Hygiea
See also: Charon Vanth Dysnomia Hiʻiaka Mesoplanet Planemo List of trans-Neptunian objects List of possible dwarf planets List of Solar System objects List of Solar System objects by size