The whole of science is nothing more than a refinement of everyday thinking. ~Albert Einstein
A while back, I had a little item about the coming meeting of the International Astronomical Union. A major item on their agenda is to determine, supposedly once and for all, what the definition of a “planet” is.
According to my inexpensive dictionary, a planet is “One of the large bodies that orbit the sun and shine by reflected light.” It goes on to list the regular list of nine planets that everyone has grown up knowing: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune – and Pluto. It further defines a planet as “a similar body revolving around another star.” This is very simple except for the word “large.” That is the very crux of the issue.
Perhaps to you, this isn't a big deal. My wife is one of the few people I've ever known who can look through a telescope at some wonderful object in the sky and say, “That's nice. Boy, the mosquitoes are really biting tonight, aren't they?” She summarized her attitude about Pluto succinctly: “Why don't they leave the poor thing alone?” To a lot of people, though, this is a serious issue. When Neil DeGrasse Tyson recently removed Pluto from an exhibit he oversees, he received a lot of negative feedback. Young children wrote him letters asking, in some cases demanding, that he put Pluto back.
To others, demoting Pluto would somehow demean the work of Clyde Tombaugh, discoverer of the ninth planet. I don't think one could ever do that. If you've ever seen the technique he used, you'd realize the patience and skill that it took to locate an object at that distance and identify it as orbiting the Sun. You could declare Pluto to be space flotsam, and you'd still have to credit Tombaugh's dogged pursuit of such a tiny, far-off object.
Pluto wasn't really what Tombaugh was looking for. Based on perturbations in Neptune's orbit, it was expected that there would be a large planet out there. Pluto didn't really fit the bill, but, what the heck, it was round, it was orbiting the Sun, so they called it a planet. Some astronomers were not satisfied even back then, thinking that there must be another, larger planet out there. No one has ever found this large object.
The question is, can Pluto even be considered a planet?
The issue is size partly size. Pluto isn't very big. In fact, there are moons bigger than Pluto. Charon, its moon, is nearly as big as it is. Another issue is the Kuiper Belt.
Way out there past Neptune is the Kuiper Belt, which, according to current theory, is full of left-over solar-system-building materials, a bunch of frozen ice-balls orbiting the faraway Sun. If Pluto is a Kuiper Belt object, it's more kin to a comet than a planet. In fact, as I mentioned in the aforementioned article, based on theory, there ought to be lots of Pluto-sized objects out there, all frozen lumps of ice and dirt.
What got the debate going is that someone finally found one.
About a year ago, astronomer Michael Brown of Cal Tech found something out past Pluto that was just a bit larger than the ninth, erm, thing. The object was very bright and slightly larger than Pluto. Brown has tentatively named it “Xena” (yes, after that Xena). So, is Xena the tenth planet? Or are there more out there?
As I said in the earlier article, definitions are important to science. Keeping an order to information is important to developing theories about such things as the formation of the solar system. So part of what makes a planet a planet is the way it was formed. Kuiper Belt objects are left-over materials, so they play a role in the formation of the other planets, but they aren't part of the resultant evolution of those planets. In fact, they are made of primordial stuff.
It's not that Pluto and Xena, and all their Kuiper Belt buddies, aren't important. It's that we need to understand what they are.
But, even scientists get emotional about things. If something has been called “A” for years, it's hard to start calling it “B” all of a sudden, especially if that change means that the something is being relegated to a lesser position in the scheme of things.
There have been negotiations going on amongst the IAU delegates to try to reach a consensus and to possibly avoid a hard and fast definition of planet. Some people say it's fine to call Pluto a planet, but then you have to extend the definition of planet to include such things as Ganymede or Titan. If you say, no, it has to directly orbit the sun, then what about asteroids like Ceres and Vesta? They're massive enough to be round (at least we think they are), so if size doesn't matter, then they should be added to the list of planets, along with Xena and any other round Kuiper Belt goodies we may find.
Unfortunately, instead of clearing up the definition, it appears that the IAU is quite possibly going to muddy the waters further. A definition to be proposed for a vote next week will set a definition that, in summary, goes like this:
A planet is an object that has enough mass to be round, is in orbit around a star, and is itself a star or a satellite of another planet.
Using this definition will increase the population to 12 by adding Ceres, Xena, and Charon. Whoa, you say, Charon? Isn't it in orbit around Pluto? Well, not exactly. It seems that the definition has a little exception whereby if two objects orbit a common center that is not inside the primary object, they are both going to be planets.
And then there's the plutons. Anything with an orbital period in excess of 200 years is a "pluton", which, frankly, sounds like something from a 1950's "B" sci-fi epic. At amy rate, instead of clarifying planethood, we now create inner planets and plutons, as well as "co-planets", like Charon. In addition, there are definitions for satellites (anything orbiting a planet if it doesn't violate the "Charon clause") and small solar system bodies (everything that's left).
As if all this isn't bad enough, there are "unnofficial" categories of planets: Dwarf planets (things smaller than Mercury, which includes Pluto); giant planets (the big four gas giants); classical planets (the four inner rocky planets plus the four inner planets). So now, Pluto is a planet, a pluton, and a dwarf planet, not to mention being part of a double-planet system.
When did they let lawyers into the IAU?
If you don't like that one, and apparetnly a lot of people don't, there's an alternative proposal already floating around (unfortunately, New Scientist's link is goofy; try pasting this in your browser: http://www.newscientistspace.com/article.ns?id=dn9797&feedId=online-news_rss20 and see if it works). It isn't exactly a different definition; it's a further corollary to the previous mess. The corollary, proposed by an Uruguayan, Julio Fernandez, says that, in addition to all the other stuff, a planet must be "by far the largest body in its local population."
"By far"? "Local population"? I thought the purpose of this excersize was to make the definition more precise, not make it so vague as to be meaningless. Apparently Mr. Fernandez had "dump Pluto" in the pool, because that is about all this definition accomplishes.
The only sane voice in all of this nonsense comes from an astonomer at Johns Hopkins University, who is quoted in the last article: "I think the whole debate is absurd. The fact (in my opinion) that Pluto is in a different class from the eight planets does not make it less interesting."
Well said, but by the time the IAU is finished, we may not even be able to figure out where Pluto is, much less what it is.