August 14, 2006
Monday

If it's one thing we thought we knew, it's that the solar system has nine planets, beginning with closest-in Mercury and extending to farthest-out Pluto. But apparently we got ahead of ourselves, and the astronomers are saying not so fast.

The solar system may instead consist of eight planets, or 10, or 53, or some number in between. An astronomy conference now under way in Prague, Czech Republic, may decide to demote Pluto or promote a 10th, nicknamed Xena, to planethood.

Pluto's planetary status, always shaky since its discovery in 1930, received a blow last summer with the discovery by a Cal Tech astronomer of Xena, or 2003 UB313, farther out and slightly larger and brighter than Pluto. Like Pluto, it has a moon.

Also like Pluto, Xena is in the Kuiper Belt, a vast and unexplored zone beyond Neptune, the last "regular" planet, so speak. The Kuiper Belt is filled with thousands of celestial objects - some of them comets, but some of them potentially meeting the same criteria for planet status as Pluto and Xena.

Astronomers are debating ways of handling this, such as going back to eight planets or listing planets by category - "gas giants," for example. Pluto would be an "ice dwarf." Or, astronomers could say that more information is needed and wait until NASA's New Horizons space probe arrives at Pluto 9-1/2 years from now.

The astronomers could solve the status of Pluto rather quickly except for one thing. Many nonscientists, and especially schoolchildren, are enormously sentimental and protective of this tiny frozen rock several billion miles from Earth, and they go nuts whenever it is suggested that Pluto is not a planet.

As for the rest of us, we would tell the astronomers this: We could get reconciled to eight planets; we could deal with 10; but there's no way we're going to memorize the names of 53 planets.

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