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 Palomar Observatory.
Quaoar is named for 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. 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 were simply numbered according to the order in which their orbits were confirmed.
In 2004, Quaoar was estimated to have a diameter of 1260±190 km, 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 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.
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 (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 May 2011 Quaoar occulted a 16th-magnitude star, which gave 1170 km as the longest chord and suggested an elongated shape. New measurement from Herschel Space Observatory with revised data from SST suggested that Quaoar has a diameter of 1070±38 km and its satellite, Weywot, of 81±11 km.
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. 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 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 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), possibly Pluto itself. 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.
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 KBO. 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 and the Deep Ecliptic Survey.
The polar view compares the near-circular Quaoar's orbit to 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, 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 from Pluto, which made it the closest large body to the Pluto–Charon system. By Kuiper belt standards this is very close.
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.
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 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 whereas 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 a small Kuiper-belt object after visiting Pluto in 2015, knowledge of the surfaces of KBOs should improve.
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. The satellite was found at 0.35 arcsec from Quaoar with an apparent magnitude difference of 5.6. It orbits at a distance of 14,500 km from the primary and has an orbital eccentricity of about 0.14. 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). Weywot is estimated to only have 1⁄2000 the mass of Quaoar.
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. The name was made official in MPC #67220 published on October 4, 2009.
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. In July 2016 New Horizons spacecraft took a sequence of four images of Quaoar from a distance of about 14 AU.