Pluto Impeached

The ninth planet may not be long for this Earth. An august body of astronomers is voting how to demote the distant icy body. Whatever the outcome, the decision could rob Pluto of its title as a planet and leave the sun with only eight such bodies.

Pluto has had an identity crisis practically since its 1930 discovery at Lowell Observatory in Flagstaff, Arizona. The distant object turned out to have a much smaller diameter than all the other planets (only 2200 kilometers, according to recent estimates), and its orbit is strangely elongated. Ever the misfit, it's neither a rocky planet like Earth nor a gas giant like Jupiter.

Now, astronomers think Pluto belongs to the "trans-Neptunian objects" (TNOs), small icy objects beyond Neptune's orbit, more than 70 of which have been discovered recently. Some TNOs circle the sun in very Pluto-like orbits, and apparently, Pluto is just the largest member of the new family. Some months ago, this kinship led Richard Binzel of the Massachusetts Institute of Technology to propose entering Pluto in a new catalog of TNOs. It would then grace the textbooks as something like TN-1--although it's unlikely to go by that name in Disney cartoons anytime soon.

Some astronomers are balking at the idea. Brian Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, argues that a perfectly serviceable list of numbered minor bodies (mostly asteroids) already exists and that the former planet belongs there. But the prospect of lumping Pluto with the solar system's riffraff outrages supporters of a new TNO category. "It's the most idiotic thing," says astronomer Jane Luu, a co-discoverer of the first TNO while at the University of California, Berkeley, in 1992.

To try to settle the issue, Mike A'Hearn of the University of Maryland, College Park, is collecting e-mail votes from 500 or so members of International Astronomical Union divisions on the solar system, comets and asteroids, and other relevant topics. However the debate settles out, Pluto's career as a planet seems to be ending, and even astronomers are wistful at the prospect. "No one likes to lose a planet," says Luu.