Largest object in solar system since Pluto discovered

Caltech planetary scientists have discovered a sizable spherical body in the outskirts of the solar system. The object, which circles the sun every 288 years, is half the size of Pluto and larger than all of the objects in the asteroid belt combined.

Named Quaoar (KWAH-o-ar) after the creation force of the Tongva tribe, the original inhabitants of the Los Angeles basin, the object is located about 4 billion miles from Earth in a region called the Kuiper belt. This region beyond Pluto’s orbit is where comets originate and where planetary scientists have long expected to eventually find larger planet-shaped objects. Quaoar is by far the largest object found so far in that search.

Currently detectable a few degrees northwest of the constellation Scorpio, Quaoar demonstrates beyond a doubt that large bodies can be found in the farthest reaches of the solar system. Further, the finding provides hope that additional bodies, perhaps even larger than Pluto, will be discovered in the Kuiper belt. Quaoar and other similar bodies should provide new insights into the primordial materials that formed the solar system some 5 billion years ago.

The discovery further supports the growing opinion that Pluto itself is a Kuiper-belt object. According to recent interpretations, Pluto was the first Kuiper-belt object to be discovered, long before the age of enhanced digital techniques and light-detecting charge-coupled devices (CCDs), because it had been kicked into a Neptune-crossing elliptical orbit eons ago.

“Quaoar definitely hurts the case for Pluto being a planet,” says Caltech associate professor of planetary astronomy Mike Brown. “If Pluto were discovered today, no one would even consider calling it a planet because it’s clearly a Kuiper-belt object.”

Brown and Chad Trujillo, a postdoctoral researcher, first detected Quaoar on a digital sky image taken on June 4 with Palomar Observatory’s 48-inch Oschin Telescope. The researchers looked through archived images taken by a variety of instruments and soon found images taken during the years 1983, 1996, 2000, and 2001, which allowed them to establish Quaoar’s distance and orbital inclination and also to determine that its orbit around the sun is remarkably stable and circular.

“It’s probably been in this same orbit for 4 billion years,” Brown says.

The discovery of Quaoar is not so much a triumph of advanced optics as of modern digital analysis and a deliberate search methodology. In fact, Quaoar apparently was first photographed in 1983 by then-Caltech astronomer Charlie Kowal in a search for the postulated “Planet X.” Kowal unfortunately never found the object on the plate—much less Planet X—but left the image for posterity.

Since the discovery, Brown and Trujillo have also employed other telescopes to study Quaoar, including the Hubble Space Telescope and the Keck Observatory on Mauna Kea, Hawaii. Information from these studies will provide new insights into Quaoar’s precise composition. The two are also continuing their search for other large Kuiper-belt bodies.

Amateur astronomers can get a faint image of Quaoar using precise coordinates and a 16-inch telescope fitted with a CCD, such as advertised in magazines like Sky and Telescope. Images on successive nights will show a faint dot of light in slightly different positions.

Additional information and images are available at www.gps.caltech.edu/~chad/quaoar.
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