Pluto pictures prompt ponderous posturing 
The New Horizons probe has just passed by Pluto, providing the more detailed pictures of it than we've ever had. People are excited about it, because it's cool.

Predictably, however, it's also been an occasion for whinging about the word "planet". The New Horizons team wants to talk up Pluto as a planet.

They are happy to play it up for a reporter. Alan Stern, the mission's principal investigator, complains that the IAU definition is "rather unscientific", that it's "trying to legislate what is and isn't a planet -- to keep the numbers small." His idea, I think, is that the discovery of Eris and other Kuiper belt objects revealed that there were too many planets to make up a countable list for school children. He raises some of the usual objections against the orbit-clearing criterion which figures in the official definition of "planet".

My view on this is that the 2006 decision to regiment the word "planet" in a way that excluded Pluto made the word track a natural kind. There's a broader natural kind which includes the planets, Pluto, Ceres, Eris, and a great many other round solar satellites. Astronomers could have made "planet" pick out that broader category, at the cost of reclassifying the asteroid Ceres. Either way, folk use of the term was going to take some bruising. (I deal elsewhere with the usual objections.)

The New Horizons team has some professional investment in the status of Pluto, of course. And their focus on studying Pluto also means that they are focused on phenomena which make the broader natural kind more salient for them. But it misrepresents the way that scientific language works to pretend that there was only one scientific thing for the IAU to do.


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