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Pluto: And then there were eight

The solar system's far-flung rock becomes a "dwarf planet." Three other contenders have lost out, too. Images: The planetary losers

Pluto has just been demoted.

The celestial body, long known as one of the nine planets of the solar system, will now be considered a "dwarf planet," the General Assembly of the 2006 International Astronomical Union ruled in a vote Thursday in Prague, Czech Republic.

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune will be defined as "classical planets."


Three other bodies had been contending for planetary status as well: Ceres, the largest-known asteroid; "Xena," the nickname for 2003 UB313; and Charon, which has been considered Pluto's moon.

Ceres and "Xena" will now share "dwarf planet" status with Pluto. Charon, it has been concluded, will be grouped with "small solar-system bodies."

The IAU said in a statement on Thursday that the definition for planet is now officially "a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape and (c) has cleared the neighborhood around its orbit."

"More dwarf planets are expected to be announced by the IAU in the coming months and years,"a ccording to the IAU statement. "Currently, a dozen candidate dwarf planets are listed on IAU's dwarf planet watchlist, which keeps changing as new objects are found and the physics of the existing candidates becomes better-known."

About 2,500 members of the IAU, a community of astronomers from around the world, have been meeting since late last week to debate and vote on a series of resolutions that include definitions of solar-system bodies. The IAU General Assembly is held every three years. This session, four terrestrial bodies in particular have been the focus of the debate--the most prominent being Pluto, which was discovered in 1930.

Varying proposals from IAU members included referring to these smaller terrestrial bodies in different areas of the solar system as "planetoids" and "trans-Neptunian objects." Another proposal referred to the smaller objects as "plutonian objects." Yet another proposed the idea of a hierarchy of "planets," "dwarf planets" and "small solar-system bodies," according to the IAU. Still others wanted to keep Pluto as a planet but come up with alternatives for the other three.

IAU President Ron Ekers and other members of the IAU board are expected to hold a press conference Thursday on the final outcome of all the resolution votes in Prague.