When is a moon not a moon? Well, apparently it's when a bunch of astronomers get together and get so far off in their little ivory tower that they lose all touch with common sense.
The proposal being considered this week at the International Astronomical Union's meeting in Prague has a lot of things wrong with it -- like the fact that it instantly expands the number of planets from 9 to about 53, and tries to conceal this fact from the public by claiming it's only an expansion to 12 (Michael Brown, discoverer of the "tenth planet" temporarily named Xena, explains this all very clearly HERE). This is a surefire way to guarantee that nobody will ever take this loony idea seriously. But the craziest part of the whole proposal is its attempt to reclassify Pluto's moon Charon as a planet.
The overall definition of a planet being offered, that it's anything round that orbits a star, is one of the four main proposals that have been out there, and which I have described elsewhere (HERE is my summary of the proposals, from an artucle I wrote in New Scientist last year. I think it's still about the clearest summary of the different ideas that I've seen anywhere, if I do say so.) The definition they chose is probably not the worst idea that's been floated, but it certainly isn't the best one.
But then, completely out of left field, unrelated to any of the discussion that's been going on relatively soberly for more than a year, the IAU committee just made up a whole new, incredibly esoteric and geeky criterion having to do with whether the center of mass of the system is inside or outside the planet's surface, and thus -- poof! A wave of the wand -- Charon is suddenly a planet, which nobody had ever suggested seriously before. There's simply no way a rational person in touch with the world could have made such an outrageous, nonsensical leap. Nobody wanted it, nobody understands it. There was no reason for it.
So, the upshot is that, as I said before, the whole fiasco is just going to end up being totally ignored by everybody, and rightly so. The astronomers can crawl back to their ivory towers, and the rest of us can go along as though nothing had happened -- except that millions of schoolkids around the world will be confused out of their minds, and many of them will figure out that astronomers are just fools.
What is a continent? It's not something geologists debate about. There is no rational definition. Why is Europe a continent? Because of history. Why is Australia the smallest continent, and Greenland the biggest island? Because we say so -- there is no pretense that there's a rational basis for putting the cutoff in size where we do, we just do. Nobody's going to try to change it, nor is anybody going to try to claim there's a rational basis for it. We just live with it.
The astronomers had a chance to provide a definition that would have made everybody happy -- and I do mean everybody, except perhaps the most pointy-headed of the astronomers themselves. Everybody understands that A) any definition is going to be at least somewhat arbitrary, and B) that astronomers themselves, in their actual work, don't give a hoot about such semantic distinctions, it's really only the public at large who are affected by any of this. So why not act accordingly, by adopting a basic philosophy of trying to screw things up as little as possible? And it turns out there's a very, very easy way to do that, provided as a gift by mother nature.
It so happens that Pluto is a bit more than 1,000 km. in radius (1,150), while the asteroid Ceres, Pluto's moon Charon, and most Kuiper Belt objects are well below this threshold (Ceres, which would be a planet under the IAU's silly scheme, is just 450). The new "tenth planet" discovered by Michael Brown last year, at about 1,500 km radius, is bigger than Pluto.
So since any definition is going to be arbitrary anyway, why not just accept the nice, convenient round number of 1,000 km radius, and declare that anything bigger is a planet, anything smaller is an asteroid or comet? Period, end of story. Simple definition, and very easy to remember, no confusion. And that means the solar system has just grown by one, from nine planets to ten. That's a result that anybody can accept -- we've added new planets several times before as new discoveries were made, most recently with the discovery of Pluto in 1930. It's a normal part of the process, and instead of turning people off about crazy astronomers, would actually make people excited and upbeat about astronomy. There's a new planet! There might be more still to be discovered! (Though not a huge number, at least not within our capacity to discover anytime soon.)
A rational answer, a simple definition that would make everybody happy. But nobody is even seriously considering it. I've loved astronomy all my life, I know and respect many, many astronomers and consider many of them my friends, including most of those involved in this stupid debate. But I think they've totally lost their minds, and if they go ahead with this nonsense they will lose public respect, big time. Too bad for them. I hope they come to their senses, but I think the chances of that are virtually nil.