50000 Quaoar

50000 Quaoar is a rocky trans-Neptunian object in the Kuiper belt with one known moon, Weywot; it is a cubewano. Quaoar is the largest body that is classified as a cubewano by both the Minor Planet Center and the Deep Ecliptic Survey. Discoverer Michael Brown’s website gives a three-syllable pronunciation, /'kwɑ:oʊwɑr/ KWAH-oh-war, but it is usually pronounced “Kwawar”.

Quaoar was discovered on 4^th June 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, located in the constellation Ophiuchus, was announced on 7^th October 2002, at a meeting of the American Astronomical Society. The earliest prediscovery image proved to be a 25^th May 1954 plate from Palomar Observatory.

Quaoar is named after the Tongva creator god, following International Astronomical Union 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. Before 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, later even larger discoveries were simply numbered according to the order in which their orbits were confirmed.

Its aphelion is 45.116 AU (6.749189 Tm), its perihelion is 41.695 AU (6.237516 Tm), its semi-major axis is 43.405 AU (6.493353 Tm), and its orbital eccentricity is 0.0394 (almost circular). At around 43 AU and with a near-circular orbit, Quaoar is not significantly perturbed by Neptune, unlike Pluto which is in 2:3 orbital resonance with Neptune. Pluto is closer to the Sun than Quaoar at some times of its orbit, and farther at others.

Quaoar’s orbital period is 285.97 years (104,451.3 days), and its average orbital speed is 4.52 km/s; the mean anomaly is 280.554°, the orbital inclination is 7.996°, typical for the small classical Kuiper-belt objects (KBO) but exceptional among the large KBO; 20000 Varuna, Haumea, and Makemake are all on highly inclined, more eccentric orbits. The longitude of ascending node is 189.033° and the argument of perihelion is 155.624°.

In 2004, Quaoar was estimated to have a diameter of 1260±190 km, subsequently revised downward (844 km in 2007, 890 km in 2010, 1170 km in 2011 and 1074 km in 2013); at the time of discovery in 2002 it was the largest object found in the Solar System since the discovery of Pluto. Quaoar was later supplanted by Eris, Sedna, Haumea and Makemake. Quaoar is about as massive as (if somewhat smaller than) Pluto’s moon Charon, which is approximately 2½ times as massive as Orcus. Quaoar is roughly one fifteenth the diameter of Earth, one quarter the diameter of the Moon, and a third the size of Pluto.

The mass of Quaoar is 1.4±0.1×10²¹ kg, which is about 0.12 Eris masses. Its mean density is between 4.2±1.3 g/cm³ and 2.8 g/cm³ (assuming the moon has a highly eccentric orbit), though a revised density in 2013 is 2.2±0.4 g/cm³. The equatorial surface gravity is between 0.276 and 0.376 m/s² and its equatorial escape velocity is in the range 0.523 to 0.712 km/s. The sidereal rotation period is 17.6788 hours. The geometric albedo is 0.199+0.13−0.07. With a density around 4.2 g/cm³, Quaoar is believed to be a mixture of mostly rock with some ice and is possibly the densest known object in the Kuiper belt. Even dwarf planet Haumea is only estimated to have a density of 2.6 g/cm³. The 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 the object is relatively more reflective in the red and near-infrared than in the blue. 20000 Varuna and 28978 Ixion are also moderately red. 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. Its spectral type is (moderately red) B−V=0.94, V−R=0.64, its apparent magnitude is 19.3, and its absolute magnitude (H) is 2.48±0.76.

Quaoar’s temperature is about 43 K; 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. 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. 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). More precise (2007) observations of Quaoar’s near infrared spectrum indicate the presence of a small (5%) quantity of (solid) methane and ethane. 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 while the dominant population of small TNOs lost them. Quaoar, with only small amounts of methane, appears to be in an intermediary category. If the New Horizons mission visits several Kuiper-belt objects after visiting Pluto in 2015, our knowledge of the surfaces of small KBOs should improve but encounters with large objects seem unlikely.

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 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 which would give a similar blurred image. This method was recently applied by the same authors to measure the size of Eris.

The uncorrected 2004 HST estimates only marginally agree with the 2007 infrared measurements by the Spitzer Space Telescope (SST) which suggest a brighter albedo (0.19) and consequently a smaller diameter (844.4+206.7−189.6 km). 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. 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. Using a weighted average of the SST and corrected HST estimates, Quaoar, as of 2010, can be estimated at about 890±70 km in diameter.

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

Since 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 should qualify as a dwarf planet if the mass required for hydrostatic equilibrium is proven, enough to be considered one under the 2006 draft proposal of the IAU, though the IAU has not formally recognized it as such. 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. 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), 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 is 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 suggests that Quaoar was originally covered by a mantle of ice, making it 300 to 500 km 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), possibly Pluto itself.