Quaoar (minor-planet number 50000) is a ringed dwarf planet candidate in the Kuiper belt, a band of icy planetesimals beyond Neptune. It has a slightly ellipsoidal shape with an average diameter of 1,100 km (680 mi), about half the size of the dwarf planet Pluto. The object was discovered by American astronomers Chad Trujillo and Michael Brown at Palomar Observatory on 4 June 2002. Quaoar has a reddish surface made of crystalline water ice, tholins, and traces of frozen methane. Quaoar has two thin rings orbiting outside its Roche limit, which defies theoretical expectations that rings outside the Roche limit should be unstable. Quaoar has one moon named Weywot and a potential second moon that has not yet been confirmed. It is believed that Quaoar's elongated shape, gravitational influence of its moon(s), and extremely cold temperature help keep its rings stable.
History
Discovery
Quaoar was discovered on 4 June 2002 by American astronomers Chad Trujillo and Michael Brown at the Palomar Observatory in the Palomar Mountain Range in San Diego County, California. 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. 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. Quaoar appeared relatively bright for a distant object, suggesting that it could have a size comparable to the diameter of Pluto. To ascertain Quaoar's orbit, Brown and Trujillo initiated a search for archival precovery images. They obtained several precovery images taken by the Near-Earth Asteroid Tracking survey from various observatories in 1996 and 2000–2002. In particular, they had also found two archival photographic plates taken by astronomer Charles T. Kowal in May 1983, who at the time was searching for the hypothesized Planet X at the Palomar Observatory. From these precovery images, Brown and Trujillo were able to calculate Quaoar's orbit and distance. Additional precovery images of Quaoar have been later identified, with the earliest known found by Edward Rhoads on a photographic plate imaged on 25 May 1954 from the Palomar Observatory Sky Survey. Before announcing the discovery of Quaoar, Brown had planned to conduct follow-up observations using the Hubble Space Telescope to measure Quaoar's size. He had also planned to announce the discovery as soon as possible and found it necessary to keep the discovery information confidential during the follow-up observations. Rather than submitting his Hubble proposal under peer review, Brown submitted his proposal directly to one of Hubble's operators, who promptly allocated time to Brown. While setting up the observing algorithm for Hubble, Brown had also planned to use one of the Keck telescopes in Mauna Kea, Hawaii, as a part of a study on cryovolcanism on the moons of Uranus. 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. The discovery of Quaoar was formally announced by the Minor Planet Center in a Minor Planet Electronic Circular on 7 October 2002. It was given the provisional designation 2002 LM60, indicating that its discovery took place during the first half of June 2002. Quaoar was the 1,512th object discovered in the first half of June, as indicated by the preceding letter and numbers in its provisional designation. On that same day, Trujillo and Brown reported their scientific results from observations of Quaoar at the 34th annual meeting of the American Astronomical Society's Division for Planetary Sciences in Birmingham, Alabama. They announced Quaoar was the largest Kuiper belt object found yet, surpassing previous record holders 20000 Varuna and 55565 Aya. Quaoar's discovery has been cited by Brown as having contributed to the reclassification of Pluto as a dwarf planet. Since then, Brown has contributed to the discovery of larger trans-Neptunian objects, including Haumea, Eris, Makemake and Gonggong. On 20 November 2002, Quaoar was given the minor planet number 50000, a deliberate choice highlighting its unusual size and the fact that it was discovered during the search for Pluto-sized objects in the Kuiper belt. Likewise, the large Kuiper belt object Varuna had previously received the round number 20000 to mark a similar milestone, and there had even been plans to give a round number to Pluto itself. However, Pluto later received the number 134340, and other large discoveries such as 136199 Eris were assigned numbers in the order in which their orbits were confirmed.
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