Demoting Pluto as a planet

By Fred Watson 7 December 2011
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The controversy over Pluto’s status as a planet continues.

IN AUGUST 2006, in Prague, astronomy’s governing body – the International Astronomical Union (IAU) – made a landmark decision: for a body in the Solar System to be a ‘planet’, it must have cleared its own vicinity of asteroidal debris by its gravitational influence.
The controversy was instant: the ninth planet failed the test and was assigned dwarf planet status. “Pluto dumped by the übernerds of Prague,” raged one headline.

The passage of time has done little to diminish the row. Alan Stern, principal investigator of NASA’s New Horizons spacecraft, currently en route to Pluto, has been among the debate’s saner voices. Alan’s mission is to investigate the surface composition and geology of Pluto and its biggest moon, Charon, and observe Pluto’s rarefied atmosphere. You must sympathise with him.
Just months after New Horizons’ launch, its destination was reclassified. Alan regards the ruling as: “…an awful definition; it’s sloppy science and it would never pass peer review.” However, many scientists believe that the new definition is a step forward in advancing our understanding of planetary systems.

Adding to the debate is suspicion that some in the USA oppose the 2006 definition because it demotes the only planet discovered by one of their fellow citizens: Clyde Tombaugh (1906-1997), of Illinois, who made the breakthrough at Arizona’s Lowell Observatory in 1930.

Perhaps we should not be surprised that early last year the State of Illinois passed a local law to ensure that “Pluto is re-established with full planetary status, and that March 13 will be declared Pluto Day…in honor of the date its discovery was announced in 1930”. If science doesn’t give you the answer you want, it seems, you can always legislate.

Fred Watson is Astronomer-in-Charge of the Anglo-Australian Observatory near Coonabarabran, north-western NSW.  Source: Australian Geographic, Issue 97 (Jan-Mar, 2010)