Of Cosmic Proportions

At first, no one really knew the size of Pluto--some calculations suggested about as big as Earth--so calling it a planet was uncontroversial. But as more was known, astronomers began to question whether it belonged in the same category as the other eight planets. Some even wanted to call it a "comet"--a class of generally much smaller balls of ice and dust that swoop in and out of the vicinity of the Sun on highly eccentric orbits. When the Rose Center opened in 2000, its solar-system exhibit had only eight planets--provoking a flood of angry letters from second graders, according to Michael Shara, curator of astrophysics. "We're trying not to gloat," he said after the IAU vote, "but it's hard not to say we told you so."

This has nothing to do with Pluto's inherent scientific interest. "Whether you're in favor of Pluto being a planet or not, every astrophysicist is cheering on the New Horizons mission," says Shara. "We've never been to a dwarf planet before. What's it like? Is it smooth, like an ice cube, or does it have cracks, which would indicate radioactive or volcanic activity?" Pluto's orbit runs through the Kuiper Belt, a doughnut-shaped region of rocks at the very edge of the solar system that astronomers have just begun to explore.

What they are finding there are clues to the origins of planets and of life itself. "When I was a kid, astronomy books would talk about planet formation as if it were a miracle," says Richard Terrile, a planetary astronomer at the Jet Propulsion Laboratory. "It was a very unlikely event. The fact that we were on Earth and had life here seemed to be 20 serial miracles that all had to happen one after the other. And now we know that all of these events are fairly common." The question of how solar systems form is particularly interesting in light of the recent discoveries of planets' circling other stars, one of the biggest changes in our mental map of the universe in the last decade. Astronomers have found more than 100, some of them potential candidates to harbor life.

What forced the issue of Pluto's status was a discovery last year by a Caltech astronomer named Mike Brown. Since 1998, Brown and his collaborators, David Rabinowitz at Yale and Chad Trujillo of the Gemini Observatory in Hawaii, have been doing more or less what Tombaugh did back in 1930: taking repeated pictures of the sky and looking for things that move. The pictures are now digital, of course, and computers do the initial screening, flagging as many as 100 objects a night for Brown to look at the next morning. The great majority are meaningless, but every now and then he turns up a new Kuiper Belt object, glinting in the light from the distant Sun. The one he spotted one morning last January, in an image originally recorded in October 2003, was unusual, uncommonly bright and in an orbit that took it far beyond Pluto. No Earth-based telescope can measure the size of a planet at that distance, so Brown had to wait until he could get time on the Hubble Space Telescope. By April of this year, he had his answer: the pinpoint of light--officially designated 2003 UB313, but temporarily nicknamed Xena--was about 2,400 kilometers across, around 5 percent bigger than Pluto.

Suddenly, the cozy certainties about the solar system with which an entire generation had grown up were called into doubt. Was Brown's discovery the 10th planet? It certainly looked that way--and if it weren't, then why should Pluto be one? But would the solar system stop at 10? There was no reason to think that the Kuiper Belt didn't hold more objects that are larger. Would they be planets, too? What if one were just a little smaller than Pluto? The whole episode called attention to the fact that astronomers had never formalized exactly what they meant by "planet." It had always seemed obvious, but the definition that seemed to be emerging by default--"something orbiting the Sun, about the size of Pluto, or bigger"--seemed embarrassingly ad hoc.

So the IAU proposed, at first, to set a threshold for planethood defined by shape. Planets form out of an accumulation of dust and rocks. Below a certain diameter, roughly 1,000 kilometers, they tend to stay in whatever random shape emerges from that process; above that size, their own gravity molds them into a sphere. So the first criterion was that a planet had to be round. But many moons are large enough to be round, so the second part of the definition was that a planet had to orbit the Sun, and not another planet.