After a week of contentious public and private debate, a small cluster of astronomers has voted to demote Pluto from its planetary status. Rejecting an expansive definition proposed by a special committee, the astronomers of the International Astronomical Union (IAU) defined a planet as: a celestial body that orbits around the sun; has sufficient mass to become round; and has "cleared the neighborhood around its orbit." On the strength of puny Pluto's inability to dominate nearby Neptune, whose orbit it crosses, as well as to clear out the Kuiper belt of many Pluto-size objects, it fails to qualify as a planet under the new definition.
"We made a mistake 76 years ago," says Mike Brown, an astronomer at the California Institute of Technology who created the need for a stricter definition through his discovery of several massive objects in orbits beyond Pluto's. "I thought that people would be too scared to demote Pluto. It is the right scientific decision."
The IAU has designated Pluto as the first of a new category of objects still searching for a name, with ¿pluton¿ rejected and ¿plutonian¿--defined by the dictionary as relating to Pluto or an inhabitant of the netherworld--struggling for acceptance. The icy globe has been given, for now at least, the designation "dwarf planet," which is basically any round object that is not a planetary satellite, and has failed to clean up its orbital path. But not all astronomers are happy with that term; "there's a word to describe precisely these things: planetoid," Brown notes. "I'm surprised they felt the need to make up a word no one is using rather than employ a word everyone is using."
And some have noted that other planets fail to clear their neighborhoods as well: Jupiter moves in lockstep with thousands of Trojan asteroids and Earth hasn't exactly eliminated the possibility of being struck by one of many NEOs (near-Earth objects) lurking about its orbital track, notes Alan Stern, a planetary scientist at the Southwest Research Institute and principal investigator of NASA's recently launched New Horizons mission to Pluto. "I think it's embarrassing. Less than 5 percent of the world's astronomers voted in a split vote for a definition that is sloppy and technically incorrect," he complains. "In the end, this is the kind of definition that doesn't pass the smell test."
The ultimate decision may not lie with the IAU anyway. "What will make it stick is what textbook writers decide to say," notes Owen Gingerich, emeritus professor of astronomy and science history at the Harvard-Smithsonian Center for Astrophysics. "I think it's a linguistic disaster. You still can't answer the question is Pluto a planet?" Regardless, the bar is now considerably higher for would-be planet finders such as Brown. "It's going to be very tough," he says, speculating that there might be a large enough object in the reaches of space beyond the Kuiper belt. "I find this a fun challenge that I'd like to rise to."