This is an artist's impression of the icy Kuiper belt object 2002 LM60, dubbed "Quaoar" by its discoverers. Illustration Credit: NASA and G. Bacon (STScI); Science Credit: NASA and M. Brown (Caltech)

Astronomers have discovered a distant body that appears to be the largest object in the Kuiper Belt, a body half the size of Pluto that raises new questions about the definition of a planet.

Caltech astronomers Michael Brown and Chad Trujillo announced the discovery of 2002 LM60, unofficially called Quaoar, during a session of the annual meeting of the American Astronomical Society's Division for Planetary Sciences in Birmingham, Alabama. The object was discovered in June in images from the 1.2-meter Oschin Telescope at Palomar Observatory in California.

Brown and Trujillo checked the archives of several observatories and found Quaoar in images dating back to 1982, allowing them to calculate the object's orbit. They found that Quaoar was in a near-circular orbit about 43 astronomical units (6.5 billion kilometers) from the Sun, more than 1.5 billion kilometers farther away than the current position of Pluto.

The Caltech astronomers followed up their groundbased observations with images from the Hubble Space Telescope. Those observations, carried out in July and August using Hubble's new Advanced Camera for Surveys, found that the object was 40 milliarcseconds across, corresponding to a diameter of 1,300 kilometers. This makes Quaoar the largest object found in the Kuiper Belt, a field of icy objects that extends for billions of kilometers beyond the orbit of Neptune. The largest Kuiper Belt object (KBO) previously found had been 2001 KX76, with a diameter estimated to be between 960 and 1,270 km. Two other KBOs, Varuna and 2002 AW197, are each estimated to be 900 km across.

The discovery of large KBOs like Quaoar comes at a time when astronomers are reconsidering calling Pluto a planet. The existence of several large KBOs, part of a population of over 500 KBOs discovered in the last decade, has caused some astronomers to question whether Pluto - still larger than any KBO - should itself be considered a KBO in addition to, or instead of, a planet. "Quaoar definitely hurts the case for Pluto being a planet," said Brown. "If Pluto were discovered today, no one would even consider calling it a planet because it's clearly a Kuiper Belt object."

Other astronomers point out that Pluto is still quite different than other KBOs. It is still significantly larger than KBO, with a diameter of approximately 2,300 km. This makes Quaoar closer in size to Ceres, at 933 kilometers across the largest asteroid, than Pluto. While several KBOs appear to have moons, only Pluto has a large moon, Charon, which is larger than all but a few other KBOs. Pluto has has a much higher albedo, or reflectivity, than Quaoar or other KBOs, suggesting that the planet undergoes processes that keep the ices on its surface fresh.

The flurry of large KBO discoveries in recent years does suggest that there may be a broad continuum of sizes of solar system objects, rather than a sharp break between larger planets and smaller asteroids and KBOs. The International Astronomical Union (IAU) has declined in the last few years to take up the issue of reclassifying Pluto or better defining what a planet is.

Because Quaoar was only recently discovered, the IAU has not formally approved its name, and the object is still officially called of 2002 LM60. The name is taken from the Tongva tribe that originally lived in the Los Angeles area; it refers to a creation god who "came down from heaven; and, after reducing chaos to order, laid out the world on the back of seven giants. He then created the lower animals, and then mankind."