Miércoles 16 de Agosto de 2006, Ip nº 166

What, Exactly, Is a Planet?
Nearly 2,500 astronomers from 75 countries gathered here Monday to come up with a universal definition of what qualifies as a planet and possibly decide whether Pluto should keep its planet status.

For decades, the solar system has consisted of nine planets, even as scientists debated whether Pluto really belonged. Then the recent discovery of an object larger and farther away than Pluto threatened to throw this slice of the cosmos into chaos.

Among the possibilities at the 12-day meeting of the International Astronomical Union in the Czech Republic capital: Subtract Pluto or christen one more planet, and possibly dozens more.

But the decision won't be an easy one. Scientists attending the conference are split over whether Pluto should be excluded from the list of planets, said Pavel Suchan of the meeting's local organization committee.

"So far it looks like a stalemate," Suchan said. "One half wants Pluto to remain a planet, the other half says Pluto is not worth being called a planet."

Participants hope to set scientific criteria for what qualifies as a planet. 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.

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.

The Hubble Space Telescope measured the bright, rocky object at about 1,490 miles in diameter, roughly 70 miles longer than Pluto. At 9 billion miles from the sun, it is the farthest known object in the solar system.

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

For years, Pluto's inclusion in the solar system has been controversial. Astronomers thought it was the same size as Earth, but later found it was smaller than Earth's moon. Pluto is also odd in other ways: With its elongated orbit and funky orbital plane, it acts more like other Kuiper Belt objects than traditional planets.

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.

Still 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 labeled a "gas giant planet," while Pluto and Xena could be "ice dwarf planets."

A decision on whether Pluto should be excluded or if "Xena" should be included on the list of planets will not be known before the end of the conference, Suchan said.

"We of course need the definition of a planet first."


  14/08/2006. Wired Magazine.