Plotting Pluto's Comeback

To Bob Millis, director of the Lowell Observatory in Flagstaff, Ariz., Pluto is still the ninth planet, no matter what the International Astronomical Union chooses to call it. It was Lowell's founder and namesake, Percival Lowell, who predicted mathematically the existence of a "Planet X" beyond Neptune, and it was at the observatory that it was found and named (for the Roman god of the underworld) in 1930. There have been other significant discoveries at Lowell over the years, but finding Pluto remains its signature public achievement. Which is why Millis thinks it was "premature" of the IAU to vote last summer to define "planet" in a way that excluded Pluto. Now, it turns out, Pluto may be headed for a comeback.

It's not just Millis who feels Pluto got a raw deal. Other astronomers think the new definition is a mistake--not just on Pluto's account, but on behalf of all the rocky, icy or gaseous objects, already known or yet to be discovered, circling the Sun or distant stars. Although the IAU isn't scheduled to meet again until 2009, plans are being laid for an ad hoc conference of astronomers next year to come up with another new definition of "planet"--one that will almost certainly restore Pluto's status. The effort is being led by Mark Sykes, director of the Planetary Science Institute, in Tucson, and Alan Stern, principal investigator of the New Horizons space-probe mission, scheduled to arrive at Pluto in 2015. "The IAU definition is so flawed on so many levels," says Stern, whose Web site still defiantly calls Pluto "the last planet," "that some of us decided we can't wait three years to come up with a better one."

In part, the nomenclature fight represents a difference in outlook among subdisciplines of astronomy--between planetary scientists, like Millis and Stern, who are interested in the composition and geologic processes of the planets, and "dynamicists," who study their mass and orbits. The objects that would be excluded by the IAU definition "are each exciting, unique little worlds," says Millis, who in 1988 was part of the team that discovered Pluto's (tiny) atmosphere by observing the way light from a distant star dimmed gradually, rather than abruptly, when Pluto passed in front of it. But the IAU definition shows the influence of the dynamicists. It requires a planet to have "cleared the neighborhood" of other objects--basically, a test of its gravitational dominance. This phrase excludes Pluto--whose orbit runs through the Kuiper Belt beyond Neptune, containing thousands of mountain- to continent-size rocks--and anything else yet to be discovered in that same region. It would also seem to exclude "planets" circling other stars. About 200 such objects have been discovered, but even the closest stars are so far away that we are unlikely, now or possibly ever, to determine whether an object orbiting them has cleared out its "neighborhood."

In a sense, this is a controversy about not very much, a bit like holding a conference to decide whether tomatoes are a fruit or a vegetable. But there is pride at stake. The effort to restore the status of Pluto, the only planet discovered in America, is not coincidentally being led by Americans; some astronomers privately suspect that the vote to strip Pluto of its status may have been tinged by anti-Americanism. And professional status--or, to put it bluntly, funding--is involved as well. Just 20 years ago, says Millis, it seemed as if the study of the solar system by ground-based telescopes--his own field, and the observatory's specialty--might be rendered obsolete by flotillas of unmanned planetary probes and telescopes orbiting the Earth. The discovery of the Kuiper Belt gave ground telescopes plenty to do for the foreseeable future, in discovering and plotting what could amount to more than 100,000 new bodies. The IAU definition, says Millis, seems to cast "a mantle of insignificance" over that whole region of space, "the hottest topic in planetary studies today."

So Pluto itself, small and remote as it is, has some powerful friends. And it's overcome long odds to even make it into the world's planetariums and classrooms. It managed to get itself discovered (by a young Lowell apprentice named Clyde Tombaugh) even though the calculations Percival Lowell used to deduce its existence and position turned out, decades later, to have been based on erroneous data. The little planet just happened to be in the right place at the right time to be seen, which is something like your long-lost brother turning up in a crowd picture of the Super Bowl. Don't count it out yet.