So, farewell planet Pluto…

One theological mode of discourse is casuistry, best known for disputing the number of angels who can sit on a pinhead. Amongst astronomers, at least those who meet every three years at the General Assembly of the International Astronomical Union (IAU), this form of sophism crops up from time to time.  It does too among geologists, and probably more often, as they have a many things to argue about. At 13.32 GMT on the 24th of August the 26th GA of the IAU in Prague upset a great many people by casting Pluto, formerly known as Planet Pluto, into the indignity of dwarf-planet status. NASA may be well-miffed, as their New Horizon probe has been on its way there since mid-January 2006.

The issue of Pluto’s status popped up after a larger Sun-orbiting object was announced in 2005 (2003 UB313), which, like Pluto is beyond the orbit of Neptune. That new body is the largest known in the dim and distant Kuiper Belt, and Pluto may well be a stray from that region, having a very odd orbit. IAU decided, somewhat late in its existence, to define ‘planet’. Committees were appointed. The primary criterion decided by the final committee to report to IAU was that planets need to orbit the Sun, not another bigger planet. Second, they have to have sufficient mass for their gravitational force to make them nice and round. Sadly, it seems that the committee made quite a gaffe. In order to distinguish trans-Neptunian planets that take more than 200 years to orbit, they suggested the term ‘pluton’ (oh dear). Whatever, that would give the Solar System 12 planets: trans-Neptunian Pluto, Charon (in binary orbit with Pluto) and 2003 UB313; and Ceres, formerly just the largest asteroid known. But the Kuiper Belt might easily have lots of other massive and round objects in it, awaiting discovery. So, has the old Jesuitical mind-expanding exercise been ‘larged-up’? Probably not, in a strictly scientific sense, because additional criterion for planetary status, added by the 26th GA of the IAU, is that one should be massive enough either to have ‘swept’ its orbit clear of minor bodies early on, or to have flung them far away. Since Pluto and Ceres have done neither, they are officially to be considered ‘of diminished stature’. Some worry that traumatised children, fond of Pluto, will be driven from an interest in science. Who knows? But if IAU persists in the name ‘pluton’ as a sop to public opinion, there will be trouble…

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