Large object is detected in sun orbit beyond Pluto

Discovery rekindles discussion of what constitutes a planet


On the frozen outskirts of the solar system, astronomers have discovered an orbiting object half the size of Pluto, the biggest find since the ninth planet was discovered in 1930.

They've named it Quaoar (KWAH-oh-war), after a California Indian creation deity. It's about one-tenth the size of Earth and orbits the sun every 288 years.

Quaoar is not a planet - it's a "Kuiper Belt object," a member of a distant realm that's just beginning to be explored.

Besides being nearly unpronounceable, this newcomer is creating an awkward situation in the solar system. Quaoar is remarkably similar to Pluto, strengthening the case against Pluto being considered a planet.

Planetary astronomer Michael Brown, of the California Institute of Technology in Pasadena, and postdoctoral scholar Chadwick Trujillo discovered the object in images taken June 4. They announced their discovery yesterday in Birmingham, Ala., at a meeting of the American Astronomical Society's division of planetary sciences.

Quoaor is 800 miles in diameter, but it remained unseen for so long because it's so far away and its surface doesn't reflect much light, said Neil Tyson, director of the Hayden Planetarium in New York.

"There can be very large objects lurking in the outer solar system awaiting discovery," he said.

The argument against Pluto as a planet has been around for a decade, since scientists began discovering similar, smaller objects and recognized the existence of the Kuiper Belt, a vast ring of icy bodies where some comets originate.

Though Pluto orbits right through the Kuiper Belt, many people objected to the suggestion that Pluto be kicked out of the planet pantheon and reclassified as a Kuiper Belt object.

Now, however, some astronomers say that if Pluto is classified as a planet, they'd feel obligated to admit not only Quaoar but 661 other catalogued Kuiper Belt objects.

Astronomers suspect there are millions of icy, rocky bodies in the ring that begins after the orbit of Neptune and goes out several billion miles. They believe that Kuiper Belt objects are 5 billion-year-old relics left over from the proto-planetary disc that formed the solar system.

Brown said he started his search of this region five years ago, hoping to find objects rivaling the size of Pluto. Exploring this region, he said, will help astronomers better understand how our solar system formed and whether other solar systems around distant stars might look like ours.

Brown and his colleagues scanned the skies with the Oschin telescope, on Mount Palomar in Southern California. After they found Quaoar, they realized it appeared, unnoticed, in photographs taken in the 1980s.

Quaoar appears in a few other existing images, too, allowing the astronomers to track its path around the sun. The astronomers used the Hubble space telescope and the giant Keck telescope in Hawaii to help them determine the size of Quaoar.

Brown said they knew Quaoar was not a star by the way it moved around relative to the much more distant stars. The word "planet" comes from the Greek term for "wanderer" because planets appeared to drift among what seemed to be a fixed backdrop of stars. So, according to the Greek definition, Quaoar and Pluto are both planets, but then so are all the countless asteroids and pieces of assorted debris.

Less than one-thousandth the size of its neighbors, Uranus and Neptune, Pluto has always been a misfit, but Pluto expert Alan Stern of the Southwest Research Institute has long argued that it should remain a planet because it has its own moon and atmosphere and appears spherical.

Brown said it's not clear from the fuzzy Hubble pictures whether Quaoar is spherical.

Brown said he doesn't understand why people seem concerned over whether Pluto is called a planet, which he considers a meaningless question.

"It's like deciding to call Australia a continent and Greenland an island," he said.

In any case, NASA plans to launch a mission to Pluto and out into the Kuiper Belt around 2006, scheduled to arrive around 2016.

It's still quite possible, Tyson said, that astronomers will find something else in the Kuiper Belt that's even bigger than Pluto, making Pluto's planetary status even more tenuous.

Brown and his colleagues didn't have complete freedom in naming their find, he said.

"The International Astronomical Union has rules for what you can name objects," he said. Anything found in Quaoar's particular neighborhood must be named after a creation deity, he said.

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