By 5.30pm local time in Prague (GMT+2), we were on version three of the planet definition. A second discussion had been scheduled, after lunchtime saw vociferous opposition to version two (which I blogged about here). A crowd gathered outside the designated room.
I was expecting to be treated to another lively exhibition of dissent – but it was not to be. Jocelyn Bell Burnell, the astronomer who discovered pulsars and a member of the IAU’s resolution committee, took formidable control of the meeting.
With only 45 minutes set aside, she said, comments were to be no more than “elevator pitches” – an idea sold in the time it takes a lift to travel one floor. “And I will cut you off if you are not brief,” she warned. The astronomers meekly followed orders.
Version three, distributed as we filed in for more drama, was a compromise that also seemed to have dissipated much of the earlier anger. It differed from version two mostly in emphasis.
That earlier definition had required first and foremost that a planet be round, then lumped planets that were not “dominant” in their local population into a subcategory of dwarf planets. The new definition required that a planet be both round and dominant, then put any round objects left over into a “dwarf-planet” category.
The details get confusing, but Bell Burnell spelled out the consequences of shuffling the priorities, “this means that Pluto is a dwarf-planet, but it is not a planet.”
Would that be acceptable to the assembled astronomers?
It seemed so. A quick show of hands suggested that the situation of earlier in the day had been inverted. More arms were raised in favour than against.
Gonzalo Tancredi*, one of the people to oppose the earlier definition, was first to make a comment. “We think this is a very good compromise,” he said. What? No shouting?
Other astronomers joined the line to praise the IAU for listening. It was almost a warm and fuzzy moment. But there were still a few wrinkles.
One concern raised, in particular, struck me as worrisome. Brian Boyle, director of the Australia Telescope National Facility, felt the new definition’s complexity would cause a problem. He’d supported version two – “I could explain [that] to Queenslanders at 6am this morning,” he said – but didn’t care for version three. “It may be untenable for the broader public,” he cautioned.
The IAU had purposefully chosen a planet definition committee with a broad background – scientifically, culturally and geographically. The members of this committee preferred the idea that a planet was something round (as a result of its gravity squeezing its shape into what’s technically known as “hydrostatic equilibrium”). This was their pick, even with its messy consequences at the edge of the solar system, where thousands of such “planets” may lurk.
At a meeting of 2,500 astronomers, it’s easy to lose persective (as the length of this post will testify).
But for the IAU, it’s time to move on, and whatever definition appears likely to get most support here will be put to a vote.
The second part of the resolution may still fail. This declared Pluto the first of a category of “plutonids”– much as before, but with a new name. It came complete with typos “apologies,” said Bell Burnell, “we did this in quite a rush this afternoon”.
Some argued that the category was pointless. Brian Marsden, formerly director of the Minor Planet Center, wondered why, if we were going to have a special class of “plutonids”, we didn’t give the asteroids another category, named after Ceres. He suggested “Cereals” – causing much chuckling.
Owen Gingerich, a historian of science and chair of the Planet Definition Committee, made the case for a Pluto-led class of dwarf-planets. “There is a large Pluto fan club out there which is going to be incensed by our actions,” he warned. Making Pluto the prototype of a new class of object “gave a nod to those people who are Pluto fans, just so they can come away with something.” Not very scientific, perhaps, but as Gingerich pointed out, it could save astronomers a lot of trouble later.
More trivially, plutonids proved an unpopular name. “Plutonian objects” was suggested as an alternative, along the lines of the Jovian moons.
And for the sake of completeness, I’ll tell you about the final shift in the resolution. The distinction made between binary planets and planet satellite systems, based on the position of their centre of mass, was chopped altogether. Oh, and now this applies only to our solar system and objects in orbit around our sun, rather than all planets everywhere. That will be changed, said a representative of the IAU, when extrasolar planets and the upper mass limit have been more carefully thought about.
Bored of this yet? Let’s hope they make a decision in the vote on Thursday. Otherwise you’ll be reading about the arguments again during the next IAU general assembly, three years in the future.
[*An earlier version of this post incorrectly named the first person to speak as Andrea Milani.]