Pluto's Planet Status Up In The Air

Solar system planets from sun: (l-r) Mercury, Venus, Mars, Earth, Jupiter, Saturn, Neptune, Uranus and Pluto, 8-6-00
Nearly 2,500 astronomers from 75 countries were working Monday on a universal definition for what qualifies as a planet, and whether Pluto should keep its status.

For decades, most have defined our solar system as having nine planets, though some have questioned whether Pluto — smaller than Earth's moon — belonged. The discovery last year of an object larger and further away than Pluto has thrown cosmic definitions into chaos.

Scientists, gathered for the 12-day meeting of the International Astronomical Union in Prague, were considering whether to remove Pluto's designation as a planet, or to bring the new one, nicknamed Xena, into the fold, possibly along with dozens more.

"We of course need the definition of a planet first," said Pavel Suchan, one of the conference organizers.

Until now, there has been no definitive criteria, with most simply describing a planet as a large, round object that orbits the Sun. Scientists will work on establishing criteria for planet status — including the object's mass, orbit and distance from the Sun — and then whether Pluto and Xena meet the definition.

"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."

Depending on the results, to be announced at the end of the conference, the solar system could be expanded to include 23, 39 or even 53 planets. If the newly found Xena qualifies as a planet, some argue, so should several other bodies found in the outer reaches of the solar system. Other say, however, that if Xena is ruled out as a planet, Pluto should be as well.

A third group of scientists have suggested planets be classified 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."

Astronomer Michael Brown, of the California Institute of Technology, announced the discovery of Xena in July 2005. Like Pluto, it is located in the Kuiper Belt, a disc-shaped zone beyond Neptune containing thousands of comets and planetary objects.

The Hubble Space Telescope measured the bright, rocky object at about 1,490 miles in diameter, roughly 70 miles larger than Pluto. It is 9 billion miles from the Sun, making it the furthest known object in the solar system.

The discovery stoked the planet debate, which 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.

The conference began Monday with discussions about galaxy evolution, star formation and so-called near Earth objects — such as asteroids — that could threaten the Earth.

"If an object with a diameter of at least 1 kilometer falls into the sea, then it would cause a disaster to the life on Earth," Suchan said. "After all, 65 million years ago, the dinosaurs became extinct most probably after an asteroid hit the earth."

  • Stephen Smith

    Stephen Smith is a senior editor for