How many planets?
Whoever it is who said there's nothing new under the sun wasn't paying attention to the field of astronomy. New things under, beyond and around the sun are being discovered all the time, throwing into disarray long-settled understandings of how the solar system is organized. Right now, Pluto is on the bubble - and it seems likely that the bubble will burst, leaving the solar system with only eight officially recognized planets.
Pluto's status is being discussed at a meeting of the International Astronomical Union in Prague. The discussion arises because of the recent discovery of an object bigger and more distant from the sun than Pluto - 2003 UN313, nicknamed Xena by its discoverer, Michael Brown of the California Institute of Tech- nology.
A natural response would be to proclaim Xena to be the 10th planet, the first new one since Clyde Tombaugh spotted Pluto in 1930. But that way lies trouble. Both Xena and Pluto are now understood to be parts of the Kuiper Belt, located beyond the orbit of Neptune. Some 800 Kuiper Belt objects have been discovered, and there may be as many as 70,000 with diameters of 60 kilometers or more. If Pluto is a planet, then so must be Xena - and if Pluto and Xena are planets, then so could be any number of other Kuiper Belt objects.
(The name Xena may not stick. By virtue of his discovery, Brown has the privilege of suggesting a name for the new object for 10 years after its number was assigned. The IAU's and its Committee for Small Body Nomenclature has the authority to approve proposed names. There are thousands of named objects in the sky - including Eugenia, discovered in 1857.)
People have grown comfortable with a nine-planet solar system. Most could get used to 10. But 23, or 39, or 53 - numbers that have been suggested - would be too many. A fourth-grader should be able to memorize the names of the planets, with mental room left over for the states and their capitals.
As the IAU gropes for a scientifically defensible definition of what is and what isn't a planet, some members say it's time to stop thinking in terms of planets as a single category, and instead begin listing them in groups. Earth is part of the group of inner rocky planets that includes Mercury, Venus and Mars. Jupiter, Saturn, Uranus and Neptune are the outer gas planets. Pluto, Xena and probably many others are in a third class of small, distant, icy objects.
Such classifications are more complicated than what we're used to. But then, the solar system is an increasingly complicated place.