Ball of ice heightens debate on 'what is a planet?'

A ball of ice and dust discovered last year in the outskirts of the solar system is 30 percent wider than Pluto, a team of German astronomers is reporting today.

The finding definitively makes the icy ball -- temporarily labeled 2003 UB313 and nicknamed Xena -- the largest known object to be discovered orbiting Earth's sun since Neptune was identified in 1846, and adds to the debate over what should be considered a planet.

The diameter of 2003 UB313, the astronomers found, is 1,860 miles, give or take 250 miles. Like Pluto, which has a diameter of 1,400 miles, 2003 UB313 is a member of the Kuiper Belt, a ring of icy debris that orbits beyond Neptune.

"This is the first measurement of the size," said Frank Bertoldi, a professor of astronomy at the University of Bonn and the lead author of an article that appears in Thursday's issue of the journal Nature.

When the discovery of 2003 UB313 was announced in July 2005, astronomers knew that it was almost certainly larger than Pluto, because it appears surprisingly bright in the sky, even though it is 9 billion miles from the Sun, or about three times as far as Pluto. Even if it had a perfectly reflective surface, 2003 UB313 would have to have the surface area of Pluto to achieve that brightness.

If the surface turned out to be dark, reflecting only a small fraction of sunlight, 2003 UB313 could conceivably have been the size of Mercury, the next in size with a diameter of 3,000 miles.

To determine how large the object is, Bertoldi and colleagues at the Max Planck Institute for Radio Astronomy measured the heat emanating from it. That enabled them to calculate a surface temperature of about minus-418 degrees Fahrenheit.

Combined with the previous optical observations, the data led to the finding that 2003 UB313 reflects about 60 percent of the light that hits it, a reflectivity similar to that of Pluto. That, in turn, allowed the scientists to calculate the diameter at 1,860 miles.

The similarities between 2003 UB313 and Pluto extend beyond size and reflectivity. Like Pluto, 2003 UB313 also has a moon and has methane ice on its surface.

The discovery of 2003 UB313 has intensified a debate over the definition of a planet, which has swirled about Pluto since the late 1990s. Pluto is by far the smallest of the nine planets, and 2003 UB313, while larger than Pluto, is still smaller than Earth's moon.

According to one definition, a planet has to gravitationally dominate its surroundings. That would exclude both Pluto and 2003 UB313.

Some astronomers contend that any object in the solar system large enough that gravity has shaped it into a sphere should be called a planet. But that would also add quite a few asteroids and other Kuiper Belt objects.

Another possibility is to arbitrarily call anything larger than Pluto a planet.

The debate has kept astronomers from bestowing a formal, more memorable name for 2003 UB313. If planet status is bestowed, 2003 UB313 will probably be named after a Roman or Greek deity, like the other planets except Earth.

"There is still a stalemate on the meaning of 'planet,"' said Brian G. Marsden, director of the Minor Planet Center of the International Astronomical Union.

Michael E. Brown of the California Institute of Technology, who discovered 2003 UB313, nicknamed it Xena, after the title character of the television series "Xena: Warrior Princess." (When he later discovered a small moon around 2003 UB313, Brown nicknamed the moon Gabrielle, the name of Xena's sidekick in the series.)

Brown has a formal name for 2003 UB313, but he has not revealed it publicly.

Bertoldi said he preferred to keep Pluto as a planet and to bestow planethood on 2003 UB313.

"As a personal opinion, I don't want to downgrade Pluto as not a planet," he said. "It would be impolite."

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