Hefty Discovery: Finding a Kuiper belt king

A newly discovered celestial body appears to be the largest object that scientists have found in the solar system since their detection of Pluto in 1930. Designated 2002 LM60, it’s unofficially known as Quaoar (pronounced Kwa-whar) after a Native American god.

BIG FIND. Quaoar (far right), a newfound giant in the Kuiper belt, alongside (left to right) Earth, Earth’s moon, and Pluto. This newly discovered object is 1,300 kilometers in diameter. A. Feild/ NASA

Residing in the Kuiper belt, the reservoir of comets and other frozen bodies beyond the orbit of Neptune, Quaoar measures 1,300 kilometers in diameter, more than half the width of Pluto. Along with the finding of several other Kuiper belt behemoths over the past 2 years, the discovery suggests the region may harbor even larger bodies.

Michael E. Brown and Chadwick A. Trujillo of the California Institute of Technology in Pasadena reported Quaoar’s discovery this week at the annual meeting of the American Astronomical Society’s Division for Planetary Sciences in Birmingham, Ala.

Brown and Trujillo found Quaoar on an image taken June 4 with a 48-inch telescope at Palomar Observatory near Escondido, Calif. Looking through archival images recorded at Palomar, the scientists also identified Quaoar in images from 1982, 1996, 2000, and 2001. Because they could trace the path of the body over a 20-year period, the astronomers were able to determine Quaoar’s orbit and distance from Earth. The researchers then made further observations with two other detectors, including the Hubble Space Telescope.

Quaoar orbits the sun every 288 years in a near-perfect circle inclined by 7.9 degrees relative to the plane in which every planet but Pluto travels. Many astronomers now assert that Pluto is itself a Kuiper belt object that got knocked into a highly inclined elliptical orbit that crosses the orbit of Neptune (SN: 6/9/01, p. 360: Available to subscribers at Nine Planets, or Eight?.). With several large objects now known to belong to the Kuiper belt, Pluto’s size no longer makes the body an oddity in the belt.

“Quaoar definitely hurts the case for Pluto being a planet,” Brown says.

Some astronomers had calculated that another Kuiper belt object, now known as Ixion (SN: 7/21/01, p. 41: Available to subscribers at A new giant in the Kuiper belt.), is almost as big as Quaoar. But that result is highly uncertain, says Ixion codiscoverer Robert L. Millis, director of the Lowell Observatory in Flagstaff, Ariz. Ixion hasn’t been observed with a telescope, such as Hubble, that could measure its size directly or with a submillimeter telescope, which determines the object’s temperature, information that can be used to calculate size. Quaoar, however, was observed with both Hubble and a submillimeter telescope.

Two other recently discovered Kuiper belt objects, also observed with submillimeter telescopes, each have a diameter of about 900 km.

“It would seem quite likely that there are a few Pluto-sized objects, maybe even

Mars-sized objects” that lie farther out than Quaoar in the Kuiper belt, says Brian G. Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.


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