The effort to define the term "planet" took a fresh twist today as two competing proposals were put forth at a meeting of astronomers in Prague.
In one case, Pluto would be demoted to "dwarf planet" status, which would mean it would not be a real planet at all.
Astronomers are split down the middle on the issue.
On Wednesday, officials with the International Astronomical Union proposed a planet definition that would make Pluto's moon Charon a planet. Several astronomers criticized the overall proposal as being vague and the Charon aspect specifically for going too far in essentially recasting too many small round objects as full-fledged planets. Eventually, with new discoveries, there would likely be hundreds.
They also were critical of the proposed term "pluton" to describe Pluto, Charon and other small round objects in the outer solar system that would be planets under the new definition.
Today, a subgroup of the IAU met to discuss the proposal. A straw vote was held in which only about 18 astronomers favored the proposal, according to Alan Boss, a planet-formation theorist at the Carnegie Institution of Washington. Another 20 or so said it should be reworked. And about 50 favored an alternate proposal.
"Most of the speakers during the discussion favored the competing proposal, which inserts the criterion that a planet must be 'by far the largest body in its population of bodies,'" Boss told SPACE.com.
That means Pluto and Charon, being no larger than other objects in the sea of rocks beyond Neptune, would not be planets. Pluto would be called a "dwarf planet" rather than a pluton. That would be in keeping with terminology used to describe small stars. For example, brown dwarfs are low-mass stars that fail to produce the thermonuclear fusion that powers real stars.
"The group loudly applauded that description of Pluto," said Boss, who has been critical of the original IAU proposal.
But some astronomers — perhaps about half of those at the meeting — are still rallying for Pluto to remain a planet.
"There is a very large community out there defending keeping Pluto in the list," said Owen Gingerich, an historian and astronomer emeritus at Harvard who led the seven-member IAU committee that generated the original definition. Gingerich said correspondence on the issue has been half in favor of the original definition and half against.
In a telephone interview from Prague, Gingerich acknowledged that in today's meeting astronomers "seemed to be overwhelmingly opposed" to the term plutons, but said "it's not clear why."
Gingerich said calling Pluto a dwarf planet but having it not really be a planet is "almost self-contradictory and linguistically objectionable."
Calling Pluto a dwarf would be a demotion that makes sense to many astronomers who say it was a mistake in the first place to call Pluto a planet when it was discovered in 1930. The dwarf category would essentially give higher status to the eight other planets in our solar system, and it would open up a new category to be populated by dozens of round objects already discovered out beyond Neptune and hundreds more that are expected to be found.
People on the street were far less interested in the whole debate than are the astronomers.
"I guess astronomers must be getting bored and running out of things to do," said 22-year-old college student Mark Ramos.
But in general, people favor keeping Pluto as a planet. "It's my favorite planet," said Emika Watanabe, a preschool teacher from Tokyo.
That is a sentiment that astronomers have been wrestling with for about seven years now. Most astronomers agree it would be scientifically convenient to demote Pluto, but they're well aware of the potential outcry from school children. That "cultural clash," as Gingerich put it, has been one of the biggest stumbling blocks in the ongoing debate over a planet definition.
Boss said the IAU has the authority to handle the debate however it wishes. It could either amend the existing proposal or adopt the competing proposal. The ultimate plan is to put something before the IAU membership for a vote on Thursday, Aug. 24.
Gingerich said he would be meeting today with the IAU Executive Committee as that group ponders the next move. He said the Executive Committee "will undoubtedly come before the membership with a single resolution. They may make some adjustments."