What, exactly, is a "planet"?
The International Astronomical Union thought it had settled the matter when its members voted, two years ago, for a first-ever scientific definition of the term. Boy, were they wrong!
Taken in Prague during the final day of the group's triennial General Assembly, the vote was deemed necessary because an object larger than Pluto (now called Eris) had been discovered in the distant Kuiper Belt. Was it a planet, a giant comet, an asteroid? If Pluto is a planet, then isn't Eris one too? Or if Eris was just the newly crowned "King of the Kuiper Belt," then where did that leave Pluto? Naming rights were at stake!
A "planet," the IAU decided, must circle the Sun (it's not a satellite), has enough mass for gravity to have drawn it into a round shape, and has enough mass to have cleared out everything else in its orbital neighborhood through impact or scattering. This left Pluto and Eris, literally and formally, out in the cold.
Few planetary scientists like the IAU's definition, which is confusing and vague. Within days of the IAU's vote, some of them started a recall petition.
"Clearing" is dynamicist-speak for a gravitationally dominant object, such as Jupiter. Except that Jupiter has thousands of asteroids, called Trojans, that share its orbit — so maybe Jupiter and, likewise, Saturn aren't planets. Earth still has to fend off stray asteroids that pass its way, so is Earth a planet? And if Earth were out at the orbit of Neptune, it wouldn't have the gravitational chops to dominate much of anything. No clearing, no planethood.
In its haste to get something on the books, the IAU failed to define what the maximum size for a planet should be, or to deal with the growing count of planets known to circle other stars.
Oh, and did I mention that the IAU also decided to make Pluto, Eris, and Ceres charter members of a new class of "dwarf planets" that aren't technically planets? To add to the confusion, two months ago the IAU made good on a action item left over from Prague that Pluto and bodies like it would henceforth be called Plutoids.
To try to bring some sanity to this mess, Pluto petitioners Alan Stern (Southwest Research Institute) and Mark Sykes (Planetary Science Institute) felt that it would be in science's best interest to hold a meeting to explore what "planet" really means. That "Great Planet Debate" is under way at Johns Hopkins University's Applied Physics Laboratory in Laurel, Maryland.
Although it's been billed as a scientific meeting, the mix of 150 attendees is skewed toward educators, students, and reporters interested in the outcome. Still, the few dozen scientists here have had plenty of give and take. Some attendees, like Hal Levison (Southwest Research Institute) maintain that the IAU basically got it right, that gravitational dominance is the single best truth-test for planethood.
But others (and probably most others, I sense), think "roundness" is a better metric. As Sykes points out, an object becomes round once its own gravity wins out over the material strength of whatever it's made of. Geology happens. But how round is "round"? Where do you draw the line between "round enough" and "a little too bumpy"?
Capping off the first day's activities was a debate of sorts between Sykes and Neil deGrasse Tyson, who directs the Hayden Planetarium in New York City. Tyson's planetary preferences became clear when the Rose Center opened in 2000. Pluto was conspicuously missing from its "Walk of the Planets."
Ira Flatow, host of National Public Radio's "Talk of the Nation," gamely tried to moderate the discussion. But he could do little to contain the flamboyant and sometimes animated verbal sparring that ensued.
Sykes, a "roundness" devotee, argued that the IAU should have been more, not less, inclusive in its definition. By his count, the solar system now boasts 13 bodies that qualify as planets: the usual eight plus Ceres, Eris, Pluto, Charon (which he claims isn't really Pluto's satellite because their combined center of mass lies between them), and the recently discovered Makemake.
But Tyson countered that "The word planet has lost all scientific value." Instead, he said, come up with a lexicon that does a better job of characterizing the objects — define it however you want, he stressed, but make it useful.
At least they agree that science shouldn't be legislated by a vote. It's an often-messy enterprise whose outcome often falls outside neat categorizations. And that's part of what makes science so exciting.