05 September 2007

* * * * * THE READING ROOM * * * *

 ... The unimaginable scale of our Universe means that astronomy has never really become an experimental science, but has largely remained an observational one, having more in common with, say, archaeology than chemistry or other laboratory-based disciplines. Consequently, even though it is perhaps the oldest science, it is also in some respects the least mature. The absence of the traditional interplay between theory and experiment, the inability to perform repeated experiments under slightly different conditions, and the sheer difficulty of measuring anything at all have stunted its development compared to younger fields.

     For this reason, one often finds in astronomy certain tendencies that other subjects have largely grown out of, such as a mania for classification and nomenclature. Taxonomy has its place within the scientific method: modern chemistry owes much to Mendeleev's periodic table; botany could not have progressed without Linnaeus; and the theory of evolution was founded on Darwin's painstaking studies on the Galapagos Islands. But arranging things in groups and giving them names does not in itself constitute scientific progress, no matter how systematically it is done. The great experimental physicist Lord Rutherford dismissed this kind of activity as not science but "stamp collecting."

     This brings us to the grand debate that took place last summer under the auspices of the International Astronomical Union, and which provides the context for David A. Weintraub's book Is Pluto a Planet? The problem before the IAU General Assembly was what to do about the fact that recent investigations have revealed the presence of a number of objects orbiting the Sun that are ostensibly as worthy of the name "planet" as Pluto, which in our current textbooks is the ninth one out.

     Obviously, which objects should be called planets depends on how you define what a planet is. The solar system contains objects of all shapes and sizes, from tiny asteroids to immense gas giants such as Jupiter and Saturn. Where should one draw the line? The original proposal was to increase the number of planets to twelve by admitting some lowly new members to the club, but in the end the IAU decided to demote Pluto to the status of a "dwarf" planet thus restricting the number of true planets to eight. This was a controversial decision, at least in the United States, because the vital vote was taken on the last day of the meeting when most of the US delegates had to take flights home. Pluto was discovered by an American, Clyde Tombaugh, in 1930, so the decision deprived the nation of its only planet-discoverer.

     The 'no' decision hinged on the adoption of three criteria: that the object be round, i..e., have a shape determined by internal gravitational forces; that it should have cleared its own orbit of debris; and that it should be orbiting our own star, the Sun. None of these has any special scientific value; the resulting decision was therefore pretty arbitrary.

     Moreover, deep-space observations have led to the discovery of literally hundreds of planetlike objects orbiting other stars. These exoplanets offer much greater prospects for scientific progress into the general theory of planet formation than the few objects that happen to have formed in our own particular vicinity, so why are they excluded from the definition? In any case, what have we learned from the new nomenclature? Pluto is still the same object that it was before August 2006, and astronomers still don't understand what one can infer from its own particular properties about the general process of planet formation. Is it a planet? Who cares? In this case there really is nothing in a name."

-- Peter Coles, Professor of Theoretical Astrophysics at the University of Cardiff, in the Times Literary Supplement, 24/31 August 2007.

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