The other night I watched a show on PBS about Pluto, which some time back was the center of controversy over whether or not it should be considered a planet. The topic wouldn't have arisen at all had not an astronomer discovered a mess of planet-like lumps in a previously-believed empty region of our solar system far beyond Pluto. The astronomer dubbed his find Eris. Even then, the matter would not have come to a head except one of these lumps turned out to be bigger than Pluto.
You could almost imagine the whisper going round observatory water coolers: "If we let Eris be a planet, next there'll be another, and another, and pretty soon every asteroid and comet will want to be a planet, too."
One scientist proposed the simple definition that if a body orbits the sun and has enough mass to compress itself into a sphere, it's a planet. This, of course, meant Eris would be classified as a planet as well as an indeterminite number of other objects yet to be found. This did not sit well.
The International Astronomical Union considered another definition for planet - in addition to being a spherical orbiting body, a planet had to be massive enough to sweep its region of space clear of debris. Neither Pluto nor Eris does this. The IAU's vote on the new definition was a landslide; Pluto was no longer a planet.
You may remember the uproar when this vote occurred. People were attached to Pluto; they could live in a solar system with an indeterminate number of planets, but they couldn't abide giving up a planet they already had. The final compromise was that Pluto, Eris, et al, were to be called Plutoids, the definition of Plutoid being a dwarf planet. So the IAU grudgingly admitted Pluto as a planet, but insisted on remembering it was very small. "Okey-doke," said the world at large, "we knew it was small. Just so long as you remember it's a planet."
Marvelous how powerful a thing a word is. The IAU did not alter the mass of Pluto, or shift its orbit, or wipe it from the sky. They merely wanted to call it something else. And they couldn't. An international body of renowned astronomers voted overwhelmingly to shift six letters off an icy lump at the edge of the solar system. And they couldn't do it.
Here's to Pluto.
