CALIFORNIA: The solar system is suffering an identity crisis. For decades, Earth has had eight neighbours, with scientists debating whether Pluto really counted.

Then the recent discovery of an object larger and farther away than Pluto threatened to throw this slice of the cosmos into chaos. Should the newly found icy rock known as "2003 UB313" become the solar system's 10th planet? Should Pluto be demoted? And what exactly is a planet, anyway?

Ancient cultures regularly revised their answer to the last question and present-day scientists aren't much better off: There is still no universal definition of "planet." That all could change, and with it science textbooks, thanks to 2003 UB313.

At a 12-day conference starting today, scientists will conduct a galactic census of sorts. Among the possibilities at the meeting of the International Astronomical Union in the Czech Republic capital of Prague are subtracting Pluto or christening the new planet, and possibly dozens more after that.

"It's time we have a definition," said Alan Stern, who heads the Colorado-based space science division of the Southwest Research Institute of San Antonio. "It's embarrassing to the public that we as astronomers don't have one."

The debate intensified last summer when astronomer Michael Brown of the California Institute of Technology announced the discovery of a celestial object larger than Pluto. Like Pluto, it is a member of the Kuiper Belt, a mysterious disc-shaped zone beyond Neptune containing thousands of comets and planetary objects. (Brown nicknamed his find "Xena" after the warrior heroine of a cheesy TV series. Pending a formal name, it officially remains 2003 UB313.)

The Hubble Space Telescope measured the bright, rocky object at about 2,400 kilometres in diameter, roughly 113 kilometres longer than Pluto. At 14.5 billion kilometres from the sun, it is the farthest known object in the solar system.

The discovery stoked the planet debate that has been simmering since Pluto was spotted in 1930.

Some argue that if Pluto kept its crown, Xena should be the 10th planet by default it is, after all, bigger. Purists maintain that there are only eight traditional planets, and insist Pluto and Xena are poseurs.

Others suggest a compromise that would divide planets into categories based on composition, similar to the way stars and galaxies are classified. Jupiter could be labelled a "gas giant planet," while Pluto and Xena could be "ice dwarf planets."

Despite the lack of scientific consensus on what makes a planet, the current nine and Xena share common traits: they orbit the sun, Gravity is responsible for their round shape and they were not formed by the same process that created stars.

The trick for the Prague meeting is to set a criterion that makes sense scientifically. Should planets be grouped by location, size or another marker? If planets are defined by their size, should they be bigger than Pluto or another arbitrary size? The latter could expand the solar system to 23, 39 or even 53 planets.

Source: China Daily