We may learn Pluto's fate- and "Goofy's-" on Wednesday
The International Astronomical Union will announce a "simple and scientific" tentative definition of the word "planet" on Wednesday.
While this may surprise some people, the word has never had an official definition before- leading to controversy about whether or not Pluto (pictured with the largest of its three moons, Charon) and other Kuiper Belt Objects like 2003 UB313 (which I have arbitrarily taken to calling either "Mickey" or- more appropriately- "Goofy") qualify.
The discovery of the slightly-larger 2003 UB313 confronted the IAU with the challenge of deciding whether it- and, by implication, Pluto- should be classifed as planets, or as something else. Both- like Kuiper Belt Objects in general- are essentially icy comet-like bodies, whose orbits lie outside the common plane of the other eight planets in our solar system.
Many astronomers believe that Pluto- by far the smallest of the planets, discovered by Clyde Tombaugh in 1930- should never have been considered a planet in the first place. But if Pluto is a planet, it would be hard to come up with a rationale for not considering "Goofy" to be one, too, since it's slightly larger.
"Goofy," too, has a moon. But then, so do some asteroids, which nobody thinks should be considered planets.
Wednesday's definition will still have to be debated and voted upon, so it will be anything but final. But according to MIT planetary scientist Richard Binzel- a member of the committee charged with coming up with the definition- it will clearly rule Pluto (and by extension, Goofy) either in or out.
My personal candidate for a definition (we'll see how close the IAU committee comes to agreeing with it) would be "Any object with sufficient mass to be formed into a sphere, and in non-mutual orbit around no primary other than the Sun." The qualification would be necessary to include Pluto, because technically it and its largest moon, Charon (the closest moon to its primary in relative size in our solar system) orbit each other- and are therefore sometimes considered a double planet.
Comments
That would still mean a constantly changing roster of "planets," with new ones being added and old ones dropping off the list all the time. The orbits of asteroids are notoriously eccentric. And how visible would the object have to be? Some people can see
things in the heavens with the naked eye others can't.
Some of the asteroids your definition would make "planets" are a great deal smaller and less interesting and significant than others which just don't happen to be advantageously positioned for naked eye observation from Earth.
Your definition would require a rather radical change in the significance of the term "planet," as well. It would no longer be a description of the body itself, but rather of our positioning in space relative to it.
The definition you endorse made sense for the ancients, who had no way of knowing anything about the heavenly bodies themselves and no way other than the naked eye to observe them. Not sure it would make much sense today. It's worth noting, by the way, that there are a great many asteroids are visible to the naked eye which the ancients simply didn't notice, but which fit your definition even then.
By the way, your definition would exclude Pluto, which you need a telescope to see. And if you "grandfather" Pluto in, how to exclude the larger 2003 UB313?
Far too subjective, I'm afraid, to work as a scientific definition. And again, it would be a statement more about our vantage point in space than about the objects themselves.