It's bigger than Pluto - but is it a planet?

Officially 430 miles wider, 'Xena' heats up debate over how to define heavenly bodies.

If Pluto truly is the ninth planet, then it's official: Our solar system has 10.

Astronomers have sized up an object - known formally as 2003 UB313 - and determined that it really is bigger than Pluto.

This bit of one-upmanship, reported in Thursday's issue of the journal Nature, could play a key role in helping astronomers update their definition of a planet. Currently, the International Astronomical Union, which decides these matters, is weighing possible definitions, researchers say. The outcome could leave textbooks, place mats, and neckties with the current nine, drop the number to eight, or vastly expand the number, depending on which definition the IAU chooses.

But the object, unofficially dubbed Xena, is more than a pawn in a game of solar-system semantics. It's the most distant object yet seen in the solar system and the largest since astronomers first eyed Neptune in 1846. It highlights the surprises scientists are uncovering as they probe a ringlike frontier that reaches from just beyond Neptune to more than 50 times the distance between the Earth and sun. That region is thought to hold at least 100,000 objects.

"We're just on the edge here," says Lowell Observatory astronomer Larry Wasserman in Flagstaff, Ariz. "We're just starting to learn what's going on out there."

The region, known as the Kuiper Belt, contains objects that are thought to be pristine remnants of the early cloud of dust and gas from which the solar system formed some 4.6 billion years ago. Thus, they can open a window on the composition of this cosmic cocoon. The subtleties of their orbits can help trace the history of how the outer planets reached their current locations.

Astronomers announced Xena's discovery last year. The team, led by Caltech planetary scientist Michael Brown, found it during a survey of bright Kuiper Belt objects it is conducting at the Mt. Palomar Observatory near San Diego. Xena's elongated orbit around the sun grazes a spot close to Neptune's orbit, then takes it out to roughly 97 astronomical units - 97 times the distance between Earth and the sun. By contrast, Pluto's average orbital distance is about 39 AU. And Xena's orbit is tilted at a 44 degree angle - extreme compared with the orbits of much of the rest of the solar system. Xena is nearly as bright as Pluto. Like Pluto, it appears to be made of methane ice. And it hosts a tiny moon.

But the best astronomers could do at the time was estimate Xena's size as "certain to be larger than Pluto," based on the amount of light it reflects. Now, a team of European astronomers have used a radio telescope in Spain to give a firmer number to Xena's "waistline." It's only about 1,860 miles across - less than the width of the United States - but still some 430 miles wider than Pluto.

Xena's girth gives the IAU's classification scheme a handful of options, notes Scott Sheppard, an astronomer at the Carnegie Institution of Washington. For example, he explains, the IAU could pick an arbitrary size, say 1,200 miles across, to keep Pluto in the fold. That would yield 10 planets, including Xena. It could bestow the moniker on any object with enough mass to be spherical - which would lead to planet inflation. It might consider tacking the term to any object that has a unique orbit and whose gravity dominates its neighborhood - itself a fuzzy concept. This would demote Pluto, relegating it to nonplanet status.

The debate over planets at the small end of the scale mirrors one that some astronomers have been having over planets at the large end as they detect solar systems beyond our own. Where do gas giant planets leave off and brown-dwarf stars begin? A brown dwarf is a star that failed to gather enough mass to ignite.

"Nature hasn't been very nice to us" as a guide on when to anoint an object as "planet," says Dr. Sheppard, who says he considers Xena the "nail in the coffin" of the Pluto-is-planet argument.