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Pluto no longer a planet
Aug. 24, 2006
Special to World Science
Updated Aug. 25
Astronomers from across the globe meeting in Prague adopted a new definition of “planet” Thursday. But the wording excluded Pluto, a member of the club of Solar System planets since its discovery in 1930. And some researchers criticized the definition as
The decision follows years of growing frustration among scientists
over the lack of a definition. New findings have made it increasingly
obvious that traditional notions of “planet”—usually described as a large, round body orbiting a
star—are too fuzzy to be of much use.
Pluto in a computer
reconstruction of Hubble Space Telescope data.
Some asteroids are almost as large and round as planets. And some planets are almost large and hot enough to be considered a type of star called a brown dwarf.
After tumultuous debate in Prague, the prestigious International Astronomical Union
voted to define a planet as a celestial body that orbits the
sun; is massive
enough for its self-gravity to pull it into a ball shape; and “has cleared the neighbourhood around its orbit.”
Advocates of the wording argued that a
planet must be the dominant object in its neighborhood.
This definition leaves eight planets in our Solar System, astronomers said: Mercury, Venus, Earth, Mars, Jupiter, Saturn, uranus and Neptune. Pluto is out because its oblong orbit overlaps with that of larger Neptune.
The decision at a conference of 2,500 astronomers was a stark shift from a week ago, when the organization leaders floated a proposal that would have reaffirmed Pluto’s planetary status and made planets of its largest moon and two other objects. That plan proved unpopular.
The guidelines ultimately adopted also didn’t sit well with researchers who still hold that Pluto is a planet. But they offered some solace by creating a category of “dwarf planet,” which includes Pluto. This classification is similar to the planet category but comprises those objects that have not cleared their orbital neighborhood.
The dwarf category is to include also the asteroid Ceres, considered a planet in the 1800s before it got demoted; and 2003 UB313, nicknamed Xena, an
icy object slightly larger than Pluto.
“The classification doesn’t matter. Pluto—and all Solar System objects—are mysterious and exciting new worlds that need to be explored and better understood,” Louis Friedman, executive director of the Planetary Society in Pasadena, California, told the BBC News.
The decision settles a controversy over whether Xena would rise to planetary status, said Mike Brown of California Institute of Technology. Brown and colleagues announced Xena’s
year. “I’m of course disappointed that Xena will not be the tenth planet, but I definitely support the IAU in this difficult and courageous decision,” said Brown.
“It is scientifically the right thing to do.”
Not all astronomers agreed. Besides those who insist Pluto is a planet, others complain that the new definition is still muddled.
“What exactly is meant by a planet ‘clearing its neighborhood?’ said Hal Weaver of Johns Hopkins university in Maryland.
Debate may also continue because, although the new definition is meant to distinguish planets from smaller bodies, it didn’t take on the question of how to separate super-large planets and stars. That question doesn’t affect the classification of planets in our Solar System, but will be relevant to some others.
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