In The Field

IAU: fight,fight

I’m just back from the open discussion on what makes a planet. It stopped just short of fisticuffs. For people who argue that defining a planet is a meaningless labelling exercise, astronomers seem to care a great deal.

Within seconds of comments being invited, queues formed at the microphones. One by one the waiting astronomers denounced, in tones ranging from offended to furious, the idea of a planet that had been put forward by the IAU. As this went on, the representatives of the planet definition committee sitting at the front slumped into their chairs, heads propped on hands.

The hour allocated for discussion today wasn’t enough for everyone to vent their views. At one point, the meeting chair, president of the IAU Ron Ekers, tried to hurry things along. This prompted someone near the front of the cavernous hall to shout out:

“If this is a democracy, listen to the questions. You don’t have to speak so much, let the people speak!” And that was just for starters.


The session had begun with short presentations on the definition to be discussed, which differs slightly to that put forward on 16 August. You can read Nature’s story on the original definition here. The new resolution was given to delegates as they entered the cavernous hall, rival definitions were handed out inside.

The official resolution has now been divided into three, each of which will be voted on separately on Thursday 24 August (although it may change again – discussions continue). The new resolutions cover the requirements of roundness; the distinction between a binary planet and a planet-moon system; and the naming of Pluto-like objects. There are now alternatives to the problematic “plutons”. For now, they are a class called “XXXXX” – with a set of names to choose from. What do you think of plutonids, plutians, or Tombaugh Planets, after Pluto’s discoverer?

Complaints against this new definition ranged from the way the process had been handled to fundamental differences of opinion. Astronomers divided into tribes to battle the definition – those who count themselves as “dynamicists” want to see planets defined as the leading objects in their orbits, those interested in planetary structure want something else, astrophysicists something else again.

Andrea Milani was first to reach a microphone. He became more incensed as he spoke, ending by saying “your paper is a kind of offence to the entire dynamical community”.

Like many of the others, the comments were not only personal opinions. Milani spoke for one of the groups within the IAU. Another presented the views of the German astronomical community: “the attempt to come up with a physical definition is honourable, but it is bound to fail”.

Other astronomers railed against being kept in the dark. “It is a pity that although I occupy a seemingly high post in the IAU, I only learn about the proposal when I come here, not before.”

Those who work on extrasolar planets felt that their field – full of massive planets, with many times the mass of Jupiter – had been wrongly neglected. Why did the definition not set an upper mass limit?

As this point was raised again and again, Ron Ekers, attempting to herd cats, became more and more frustrated. “We want your input, but not right now. You might think adding one more step is simple. It isn’t,” he eventually snapped.

These astronomers are clearly self-selecting – only the angry ones scrambled out of their seats to speak. But the underlying emotion is clear. As Paul Murdin, a UK professor, remarked to me afterwards: “the comments were intelligent, but they came out with a passion that makes me think this debate has a non-intelligent dimension to it”.

I had to fight the urge to stand up and tell the lot of them to get a grip. Various people within the IAU, and people appointed by them, have spent two years discussing this already. The same arguments are going round and round again. You could argue, as some did, that the best option is not to offer any definition at all. But where does that leave the waiting public? Astronomers realise that this whole debacle, if it fizzles into nothing, is going to make them look stupid. So why not accept some compromise?

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