Now that the New Horizons spacecraft has completed its flyby of Pluto, people are again beginning to question the wisdom of the decision by the International Astronomical Union in 2006 that Pluto, which has been considered the ninth planet of the solar system since its discovery in 1930, was from thereon to be classified as a ‘dwarf planet’, presumably because it didn’t ‘clear its’ orbit’, whatever that means. This decision was made by a small minority of the membership of the IAU, and was questioned almost immediately, not just by nostalgic grown-up children who had been forced to memorize that the solar system had nine planets, and by Sheldon Cooper of The Big Bang Theory, but also by members of the scientific community, such as the Harvard-Smithsonian Center for Astrophysicists. The recent flyby, combined with the stunning images sent back showing Pluto looking very much like a planet, albeit a small one, has reignited the debate. So what are the arguments on each side?
First, let us consider Pluto’s size. It’s true that Pluto, even though New Horizons found it to be slightly larger than previously thought, if it were reinstated would be the smallest planet in the solar system, about half the size of Mercury. But since when has size been more important than anything else. Physics tells us that the most powerful forces reside in the tiniest objects, atoms and the myriad of sub-atomic particles that constitute them. Why should astronomy be any different? Saturn has a moon, Ganymede, that is slightly larger than Mercury. If size is the most important thing, why isn’t Ganymede a planet?
Next, let us consider the matter of good English. Even if Pluto is to be reclassified as a ‘dwarf planet’, how can a dwarf planet not be considered a planet? If you go to a petting zoo and see a dwarf pony, and try to convince the attendant that it wasn’t really a pony, he would probably look at you suspiciously and perhaps even begin to question why you were in a place meant for children.
Finally, there is the matter of popular culture. Even though the scientists who reclassified Pluto would be loath to admit it, their pronouncements, while they may rule in their world, do not necessary rule in the wider world. If something seizes the public imagination, like the fact that Pluto should be a planet, whether that is what they learned when growing up or for any other reason, the mere pronouncement by a group of astronomers that it doesn’t meet their arcane standards will not change that fact in the mind of the public. When Albert Einstein first proposed his theory of relativity, it instantly captured the imagination of the public and other scientists, even before it was formally proven by the observation of the deflection of stars by the Sun’s gravity during the eclipse of 1919, unlike the decision to re-classify Pluto. If most people still prefer to see Pluto as a planet, for whatever reason, should their wishes not automatically be dismissed just because they don’t agree with a decision made by a minority of professional astronomers, one that is questioned even by some of their own.