Slashdot Mirror
Why the Word 'Planet' Will Never Be Defined
eldavojohn writes "What makes a planet a planet? Slashdot covered the great debate about whether or not Pluto qualified and Space.com now has up an article explaining why we'll never have the term 'planet' defined to a point that everyone can agree on. Divisions in the scientific community currently stand over whether or not it has to be in orbit around a star, the dynamics of the body in question and apparently the country you come from plays a part in it too. Some feel the United States is the dominant deciding factor on the definition but the IAU has not turned to democratizing the definition yet." From the article: "In the broadest terms, a planet could be thought of as anything from an 800-kilometer-wide (500-mile-wide) round rock orbiting a dead star to a colossal gas ball floating alone in space."
I don't get the problem? First, start off with the idea that a planet must be orbiting a star... similar to how moons are defined as orbiting a planet. Even if they are orbiting a pulsar (dead star) they are still planets, but not if they are orbiting a failed star (brown dwarf). If you find a brown dwarf with satellites call that something else. Then the article mentions the possibility of having planet sized objects orbiting each other the same way binary stars orbit one another. OK, make that a seperate category. After that just define the mass need to be called a planet and be done with it. I'm sure there are plenty of other scenarios out there that need to be defined, but the basic rules don't seem difficult to set up.
... so why not the IAU. Simply break planets down into different subclasses. Everyone knows that Earth is a Class M planet.
There's already a helpful classification guide to help them get started.
And it has nothing to do with the fact Americans have discovered only one object (in Sol System) that COULD, by much stretching and wigging with the definition classify as planet (Pluto), but shouldn't really.
And the fact that it's bleeding obvious to any European, (or unbiased American) that we have EIGHT planets, while it's (mostly) Americans who seem to be violently opposed to the idea of Pluto not being a planet is a mere coincidence.
Define "on fire". For example, white dwarves that orbit a star (e.g. Type Ia supernova progenitors) are hot, but not "on fire" ("burning" nuclear fuel); are they "planets"?
Language is as exact as needed for everyday interaction. But some disciplines decided that they need a less flexible (and in some respects less effective) but more rigid medium and so they decided to define the terms they use normatively. That's why we have logic, algebra and other formalisms. In jurisprudence and the humanities/arts it works different, but they nonetheless deviate from everyday language.
Now we have to decide whether the term "planet" is needed as a technical term - if so we should better define it (and not as a prototype or radial categorie but as a classical one), if not we really can drop the issue and let lexicographers and lexical semanticist quarrel over how to describe its semantics.
"Hannibal's plans never work right. They just work." Amy/A-Team
Planets are not a classical category, and will be subject to prototype effects.
/. in the past, the bodies that qualified as "planets" originally had a collection of unrelated characteristics: they were close enough to be seen from Earth, but too far away to show a disk, and all happened to orbit the sun. If the Earth had a small, distant moon it would have been counted as a planet as well, and if any of the classical planets had naked-eye visible moons perhaps the concept would have evolved differently.
The basic premise behind classical categories is in any case nonsensical, so is isn't clear what benefit there would be if planets fell into any of them.
Physics has been steadily eroding the Aristotlian world view for centuries now, and the categories died with Einstein's unified description of space and time. Aristotle was an acute observer of the human condition, and his world view accurately captures a vast amount of folk-epistemology and folk-metaphysics, but it simply does not generalize to the modern scientific world-view at all well. It is useful, but profoundly limiting.
The difficulting of defining "planet" is a consequence of the social pressure to preserve an archaic term, as if we insisted on doing thermodynamics in terms of phlogiston or caloric, despite those being exploded concepts. As others here have pointed out, and I've pointed out on
Planet is a concept deeply embedded in the accidents of naked-eye astronomy, and the only reasons anyone wants to retain the word at all are that a) the public is attached to it and b) observers who discover new planets attract the really hot memebers of the complementary sexual orientation.
Blasphemy is a human right. Blasphemophobia kills.