Pluton or Planet?
Ronald Bailey | August 16, 2006, 7:35am
Scientists at the annual meeting of the International Astronomical Union are apparently going to vote to let Pluto remain a planet and promote scores of similar bodies to that status. The proposed definition of a planet is:
"A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet."
However, to make a distinction between the "classical" inner planets like the Earth, Jupiter and so forth, these smaller planets will also be called "plutons" with Pluto having the honor of being the first such celestial object discovered.
Does it make much of a difference calling one object a planet and another a pluton? As philosopher Ludwig Wittgenstein argued in his Philosophical Investigations: "Think of the tools in a tool-box: there is a hammer, pliers, a saw, a screw-driver, a ruler, a glue-pot, glue, nails and screw.---The functions of words are as diverse as the functions of these objects." The just as the word tool encompasses hammers, saws, and so forth, now the word planet encompasses plutons.
It's unlikely that schoolchildren will have to memorize the names of scores of new planets, uh plutons. After all how many of you can name all of the known moons of Saturn and Jupiter off the top of your head right now?
MikeP | August 16, 2006, 2:40pm | #
I must say, this proposed definition bites.
You brought in Wittgenstein, so I'll bring in Rand. She may have been loopy in how much you could derive from philosophy, but her epistimological notion of concepts and concretes was pretty damn good.
When you have a collection of objects that have enough similar characteristics that it is useful to group them into a concept, you make a concept that generalizes the common characteristics. For 2500 years we had, "Planets are the bodies that move in the ecliptic against the stationary star field." We discovered Uranus and Neptune, which looked a lot like the known planets.
We discovered Ceres, which looked a lot like the known planets, though smaller. Then we discovered hundreds of other objects in Ceres' orbit and recognized that Ceres' was simply a large exemplar of a new class of objects that should not be called planets. They are asteroids. Ceres got its planet tag removed.
Then we discovered Pluto, which looked a lot like the known planets, though smaller, somewhat out of the ecliptic, and pretty eccentric. Now we have discovered a lot more Pluto-like objects further out.
The proper next step is to remove the planet tag from Pluto, just as it was removed from Ceres two centuries ago. Pluto is more like the highly eccentric, out of the ecliptic, mostly ice KBOs than it is like the known planets. It belongs in a new concept. KBOs don't belong in the 2500 year old concept "planet", especially based on a distinction as silly as "they are round".
Tombaugh was looking for a planet. He discovered the only KBO so near the sun and so near the ecliptic. Everyone called it a planet. Now that it is evident that it has more in common with KBOs or "plutons" than with planets, Pluto should be called a "pluton". And the useful concept of planet should not be sullied by adding KBOs that happen to be round into it.
thoreau | August 16, 2006, 3:43pm | #
I think that the debate over whether or not to call Pluto a planet illustrates what Feynman said about the difference between knowing the name of something and understanding what it is.
We now realize that our solar system is a layered system: The inner core has 4 solid planets, in many ways very similar to each other. Then there's an asteroid belt. Beyond that are 4 gas giants, sharing certain common features. Beyond the gas giants are a large number of rocky objects referred to as "Kuiper Belt Objects", the largest of which bear some resemblance to the inner planets. These outer objects extend to a considerable distance from the sun.
Given the differences between the inner planets, the gas giants, and the Kuiper Belt Objects (including Pluto), it would be better to stop talking about a single category known as planets, and instead talk about 3 categories.
I realize that people don't want to make such fine distinctions in casual conversation, and that's fine. But the mere fact that there is so much debate should clue us in that the term "planet" is papering over a wide range of objects, and that there are several distinct categories of objects present. And that should clue us in, once again, on the difference between knowing the name of something and knowing what it is, how it works, and how it came about.
Pluto will always be an exceptional object, whatever name they might assign it. It is among the largest of the Kuiper Belt Objects, it is presumably the closest, and it is the only one significant enough to be inferred as a discrete entity by examining the orbits of the other planets (AFAIK).
Once you know that about Pluto, it doesn't really matter what name you give it. It's simply an extraordinary object in its own right.
thoreau | August 16, 2006, 3:52pm | #
I think that the debate over whether or not to call Pluto a planet illustrates what Feynman said about the difference between knowing the name of something and understanding what it is.
We now realize that our solar system is a layered system: The inner core has 4 solid planets, in many ways very similar to each other. Then there's an asteroid belt. Beyond that are 4 gas giants, sharing certain common features. Beyond the gas giants are a large number of rocky objects referred to as "Kuiper Belt Objects", the largest of which bear some resemblance to the inner planets. These outer objects extend to a considerable distance from the sun.
Given the differences between the inner planets, the gas giants, and the Kuiper Belt Objects (including Pluto), it would be better to stop talking about a single category known as planets, and instead talk about 3 categories.
I realize that people don't want to make such fine distinctions in casual conversation, and that's fine. But the mere fact that there is so much debate should clue us in that the term "planet" is papering over a wide range of objects, and that there are several distinct categories of objects present. And that should clue us in, once again, on the difference between knowing the name of something and knowing what it is, how it works, and how it came about.
Pluto will always be an exceptional object, whatever name they might assign it. It is among the largest of the Kuiper Belt Objects, it is presumably the closest, and it is the only one significant enough to be inferred as a discrete entity by examining the orbits of the other planets (AFAIK).
Once you know that about Pluto, it doesn't really matter what name you give it. It's simply an extraordinary object in its own right.
