Finally, astronomers could be homing in on a definition of the word planet. After a day of public bickering in Prague, followed by negotiations behind closed doors, the latest draft resolution was greeted with a broadly friendly reception.
If accepted on Thursday, it would be bad news for Pluto, which would no longer be a full-fledged planet.
The crucial change in "draft c" is that a planet must be the dominant body in its orbital zone, clearing out any little neighbours. Pluto does not qualify because its orbit crosses that of the vastly larger Neptune.
The planet definition committee is also stepping back from trying to define all planets in the universe, and sticking to our solar system – a slightly easier task.
It is still a work in progress, however, and the wording will change by Thursday, in part to simplify it and make the final result more palatable to the public.
Terminology is still controversial. Objects that do not quite qualify as planets – because they are big enough to be round but not big enough to dominate their neighbourhoods – might become "dwarf-planets" or planetoids.
These would include Pluto and Ceres, the largest asteroid. And the small fry of the solar system, such as asteroids, might be called small solar system bodies, or retain their current designation as minor planets.
But a supplementary resolution would at least make Pluto the prototype of a class of icy outer worlds beyond Neptune. "The purpose of this is to give a nod to those people who are great Pluto fans," said Owen Gingerich of Harvard University in Massachusetts, US, who is chairman of the committee.
It is not clear what they would be called, however – most early suggestions were rejected by an informal show of hands. Pluton, plutoid, plutonoid and plutid seem to be out of the running, as are "Tombaugh object" and "Tombaugh planet", which had been proposed in honour of Pluto's discoverer, Clyde Tombaugh. "Plutonian object" was the least unpopular choice.
The planet definition committee's first draft definition, released last Wednesday, had admitted Pluto, Ceres and probably dozens more objects to planethood by virtue of being round objects orbiting the Sun (see Planet debate: Proposed new definitions).
Then another group of astronomers, many of whom study the dynamics of the solar system, responded on Friday by insisting that a planet must dominate its neighbourhood, which would admit only the eight fully formed planets (see Pluto may yet lose planet status).
At a fractious lunchtime meeting on Tuesday, the committee's first attempt at a compromise met a hostile response. Watch a video of the planet discussion (Windows Media Player).
"They have presented practically the same resolution as before," said Julio Fernandez of the University of the Republic in Montevideo, Uruguay, lead author of Friday's proposal.
He was cut off when he tried to read his proposal aloud. When more questions were prevented, there was a cry of: "If there is democracy, listen to the questions. Let the people speak!"
Now, although all is not quite sweetness and light, the main sticking point may have been removed, and there is now hope for a positive result at Thursday's vote.
Andrea Milani of the University of Pisa in Italy had fiercely opposed the planet definition committee at the first meeting on Tuesday. But after participating in the private negotiations that afternoon, he told New Scientist: "I'm very satisfied."
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10:45 10 October 2008
