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50000 Quaoar is a binary trans-Neptunian object and dwarf planet candidate orbiting the Sun in the Kuiper belt. It was discovered on June 4, 2002 by astronomers Chad Trujillo and Michael Brown at the California Institute of Technologymarker from images acquired at the Samuel Oschin Telescope at Palomar Observatorymarker.


The discovery of Quaoar, a magnitude 18.5 object 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. It may qualify as a dwarf planet, given its size inferred from direct observation by the Hubble Space Telescope.


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 Angelesmarker, where the discovery of Quaoar was made. Prior to IAU approval of the name, Quaoar went by the provisional designation . 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.


In 2004, Quaoar was estimated to have a diameter of 1260 ± 190 km, 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 , , , and . In addition, it is likely that the subsequently discovered plutino is also larger than Quaoar. It is roughly one tenth the diameter of Earth, one third the diameter of the Moon or 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 which would give a similar blurred image. This method was recently applied by the same authors to measure the size of .

The 2004 Hubble estimates only marginally agree with the 2007 infrared measurements by the Spitzer Space Telescope which suggest a brighter albedo (0.19) and consequently a smaller diameter (844.4 km). Hubble Space Telescope estimates in 2009 suggest a diameter less than 1100 km.

Dwarf planet?

Since Quaoar is a binary object, the mass of primary can be calculated from the orbit of the secondary. Quaoar's estimated density of ~3 g/cm³ and estimated size of about 1000 km suggests that it should qualify as a dwarf planet if the mass required for hydrostatic equilibrium is proven. Mike Brown estimates that at a diameter somewhere between 200 and 400 km, an icy body relaxes into hydrostatic equilibrium.

Planetary scientist, Erik Asphaug, has suggested that Quaoar may have collided with a dwarf planet up to the size of Mars, stripping the lower density mantle from Quaoar, and leaving behind the denser core.


Orbits of Quaoar and Pluto - ecliptic view.
Orbits of Quaoar (blue) and Pluto (red) - polar view.
Quaoar orbits at about 6 billion kilometres (3.7 billion miles) from the Sun with an orbital period of 287 years.

The orbit is near-circular and moderately-inclined (~8°), typical for the population of small classical Kuiper Belt objects (KBO) but exceptional among the large KBO. Varuna, , and 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 (Quaoar’s orbit in blue, Pluto’s in red, Neptune in grey). The spheres illustrate the current (April 2006) positions, 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 is currently only 14 AU from Pluto making it the closest large body to the Pluto-Charon system. By Kuiper Belt standards this is very close.

Physical characteristics

Quaoar is believed to be a mixture of rock and ice, like other Kuiper Belt Objects (KBOs); however its albedo could be as low as ~0.1, which would still be much higher than the lower estimate of 0.04 for . 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 in the spectral class. Larger KBOs are often much brighter because they are covered in more ice and have a higher albedo, and thus they present a neutral colour (see colour comparison).

Hubble photo used to measure size of Quaoar.


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 , 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, , ) 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 has one known satellite, Weywot, formally (50000) Quaoar I Weywot. Its discovery was reported in IAUC 8812 on 22 February 2007. The orbit of this satellite has yet to be determined.The satellite was found at 0.35 arcsec from Quaoar with magnitude difference of 5.6.Assuming an albedo similar to that of the primary the magnitude suggests a diameter of 95 ± 24 km. Brown believes it is likely to be a collisional fragment of Quaoar, which he speculates lost much of its ice mantle in the process.

Brown left the choice of a name up to the Tongva, who chose the sky god Weywot, son of Quaoar.. The name was made official in MPC #67220 published on October 4, 2009.


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