Of Cosmic Proportions

That at least had the virtue of intellectual consistency. But it led to a complicated, 12-planet solar system, not counting all the Kuiper Belt objects that could qualify once their sizes are determined. The definition would include Pluto and UB313, but also Ceres, an asteroid just under 1,000 kilometers across, which in fact had been considered a planet when it was first discovered, in 1801. And, confusingly, it would also include Pluto's own moon , Charon. All other moons in the solar system are much smaller than the planets they orbit, so there's no question about which is which. But Charon is almost half the size of Pluto itself, so in the mathematical description of their orbits, the two objects actually appear to be circling each other. If you wanted to keep Pluto, you'd get all these others as well.

So the IAU--perhaps anticipating endless future wrangling over which objects would make the cut, who would get credit for the discovery and what to name them--decided that the solar system had enough planets already. More than enough: it added a third condition that a planet had to dominate its own orbit, clearing the immediate region of smaller objects, that in effect draws the line at eight. No asteroids can qualify, no Kuiper Belt objects. And no Pluto. They are now "dwarf planets."

Was this fair? Was it just? It depends on whom you ask. Shara, of the Rose Center, seems happy to be rid of Pluto. "Pluto is a chunk of ice which controls nothing," he says. "Its orbit is a slave to Neptune's orbit." Brown, who lost out on the honor of finding the first new planet of the 21st century, says he's saddened personally, but that it was the correct scientific choice. To single out the object he found among everything else flying around the Kuiper Belt "would be like saying you found the biggest piece of gravel in the pile. It would feel like cheating."

But others considered it an outrage. Some had a vested interest in Pluto, like the researchers involved in the New Horizons spacecraft project, who will now be devoting the next 10 years of their lives to a mission to a "dwarf" planet. "I'm troubled by the possibility that people will think that objects smaller than the eight planets are less interesting in some sense, and that's not true," says David Stevenson of Cornell, an authority on planet formation. "Pluto is a very interesting object, and so are the others. Some have atmospheres, there are fluids or gases that leak out from the interiors. It's not just size that matters."

Unfortunately, it is size that matters for some things, like getting money from Congress; at least one prominent astronomer said it was a lucky thing that the New Horizons mission got funded years ago, while Pluto was still a planet. Joel Parker of the Southwest Research Institute, one of the New Horizons' lead institutions, said he didn't think American astronomers would take the vote lying down, and predicted there might be a move to revise the definitions when the IAU meets again in 2009. His preferred solution would be to give Pluto "special dual citizenship" as both a Kuiper Belt object and a planet, in recognition of its special cultural status.

In other words, we are fond of it. "A lot of kids like Pluto because it has a cute name," says Parker, and if even one of those kids grows up to be the next Einstein--or, almost as good, the head of the House appropriations committee--shouldn't that be reason enough to keep it? "I have a 6-year-old and a 9-year-old, and it was embarrassing to explain it to them, this definition of a planet that sounds like it was written by a lawyer," says Terrile. But Louis Friedman, executive director of the Planetary Society, doesn't think kids will mind memorizing the name of one fewer planet. "It won't upset the schoolchildren," he predicts. "It's those of us who used to be schoolchildren."