Can’t … stop … blogging … must … resist …
So you may have heard that Pluto is still a planet, and indeed we have a few new ones as well! Phil Plait, Rob Knop, Clifford, and Steinn have all weighed in. Hey, it’s on the front page of the New York Times, above the fold!
The problem is that Pluto is kind of small, and far away. Those aren’t problems by themselves, but there are lots of similar-sized objects that are also out beyond Neptune, in the Kuiper Belt. As we discover more and more, should they all count as planets? And if not, shouldn’t Pluto be demoted? Nobody wants to lose Pluto among the family of planets — rumors to that effect were previously enough to inspire classrooms around the globe to write pleading letters to the astronomical powers that be, begging them not to discard the plucky ninth planet. But it’s really hard to come up with some objective criteria of planet-ness that would include the canonical nine but not open the doors to all sorts of unwanted interlopers. Now the Planet Definition Committee of the International Astronomical Union has proposed a new definition:
1) 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.
It turns out that, by this proposed definition, there are twelve planets — not just the usual nine, but also Ceres (the largest asteroid, between Mars and Jupiter), and also Charon (Pluto’s moon, but far enough away that apparently it doesn’t count as a “satellite,” but as a double-planet), and UB313, a faraway rock that is even bigger than Pluto. I’m not sure why anyone thinks this is an improvement.
The thing is, it doesn’t matter. Most everyone who writes about it admits that it doesn’t matter, before launching into a passionate defense of what they think the real definition should be. But, seriously: it really doesn’t matter. We are not doing science, or learning anything about the universe here. We’re just making up a definition, and we’re doing so solely for our own convenience. There is no pre-existing Platonic nature of “planet-ness” located out there in the world, which we are trying to discover so that we may bring our nomenclature in line with it. We are not discovering anything new about nature, nor even bringing any reality into existence by our choices.
The Pragmatists figured this out long ago: we get to choose the definition to be whatever we want, and the best criterion by which to make that choice is whatever is most useful and convenient for our purposes. But people have some deep-seated desire to believe that our words should be brought in line with objective criteria, even if it’s dramatically inconvenient. (These are the same people, presumably, who think that spelling reform would be really cool.) But as Rob says, there is no physically reasonable definition that would let us stick with nine planets. That’s okay! We have every right to define “planet” to mean “Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto, plus whatever other large rocky bodies we find orbiting other stars.” Or whatever else we want. It’s completely up to us.
So we really shouldn’t have to tear up a century’s worth of textbooks and illustrations, and start trying to figure out when the shape of some particular body is governed by hydrostatic equilibrium, just to pat ourselves on the back for obeying “physically reasonable” definitions. But it looks like that’s what the IAU Planet Definition Committee wants us to do. Of course that’s what you’d expect a Planet Definition Committee to suggest; otherwise why would we need a Planet Definition Committee?
Now if you’ll excuse me, I have change-of-address forms to fill out.
[And don’t even contemplate accusing me of hypocrisy for dragging myself away from a much-deserved blog-vacation to carry on about something that I claim doesn’t matter. The definition of “planet” doesn’t matter; but appreciating that the choice of definition is a matter of our own convenience, not a matter of necessarily conforming to some objective criteria about the physical world, matters a lot.]
Update: Chris Clarke for the opposition.
Here’s a different definition of a planet from Steven Soter, who disagrees both with Sean and with IAU: astro-ph/0608359. He proposes a planet to be an end product of disk accretion around a primary star or substar.
Oh, and this excludes Pluto…
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He proposes a planet to be an end product of disk accretion around a primary star or substar.
That’s probably a nice theoretical ideal, but it would be difficult to confirm this observationally, at least for planets around other stars.
I suggest a new definition for “planet”:
An astronomical object which is worth spending money to investigate.
So, have all the astronomers in the world vote each year on whether newly-discovered object X is or is not a planet. Then, it will be easy to justify sending a probe to investigate X; “after all, it’s a planet!”
Like saying that Einstein’s theory isn’t a more-absolute reflection of nature, than not, it never ceases to amaze me how so many people can fail to recognize that “convenience” isn’t the least bit “arbitrary”. In fact, it is **most-natural** for the quite obvious good reason that defines the “most absolute” (not perfect) reflection of the basic impetus of our near-flat, (imperfect) universe.
How can so many people work so hard to get the most bang for the buck, in terms of the meaning that the definition is capable of delivering, in the fewest possible number of steps, yet see no correlation between this and the basic structuring of our barely expanding universe.
The effort is toward something absolute, but the sad fact of the matter is that inherent “imbalance” means that this idealization doesn’t and can’t exist, but that does NOT mean that it isn’t more-absolute, than not.
How “round is a planet”… Pretty damned round!… get it?… i didn’ think so.
Convenience isn’t about how easy something is, it is about how practical it is, in terms of how much work can be done for the energy that this requires, yet nobody sees a correlation to the kind of entropic-efficiency that our near-flat univerese demands.
I take it back, they do “see” the correlation in things like the “unreasonable effectiveness of math” and the fact that the environment is highly-conducive, (not perfect) for life, but instead of giving this kind of evidence equal time, they *automatically* resort to “explaining it away” in violation of occam and the scientific method, since this is not what is first most-naturally called for by their recognition.
I’m also reminded of how this just blows right past people that can’t dispute the evidence, opting instead to sit-back and bide their time at all costs for a better way to “explain-away” the evidence, rather than to give it equal time in science.
Einstein would have a few words to say about that too…
http://www.lepp.cornell.edu/spr/2003-10/msg0054859.html
I think if they were going to name one Xena they could have named the others Sheena and maybe Red Sonja. That would have been fun.
I love pragmatism but I don’t think physicists are that pragmatic. Pragmatism implies that if you have an correct but difficult to calculate theory and a somewhat imprecise but easy to calculate theory you might actually prefer the second theory if the cost/benefit ratio (calculation complexity/predictive power) is better. However I see that consistently physcist prefer correct and yet extremely complicated theories. It also appears to me that a nonunified theory like the Standard model is to be preferred over a far more complicated unified theory. Using extremely complicated mathematical formalisms is also not pragmatic since it just makes physics harder to learn and therefore less useful.
An astronomical object which is worth spending money to investigate.
So, Eros is a planet. The moon is a big planet – we spent alot of money on it. I think not.
I like the proposed definition. Not just for our solar system, but for the 200+ objects found around other stars. The definition does require knowing something about the objects. One must know how big they are.
While I agree that Ceres and UB313 should be planets (ub313 needs a name, first), Charon was the big surprise. That Charon is a planet should clarify double planets in the future. So, if, say, in 100,000,000 years, the barycenter for the Earth/Moon is outside of the Earth, the Moon becomes a planet. That’s kind of odd, but i can live with it.
I do not like handwaving. “I’d know a planet if i saw one.” Feh.
I take a bit of sadistic pleasure inflicting two dozen planet names on school children. Perhaps they could learn the ten primary planets, and get extra credit for the dozen-plus plutons. Just as I learned the important presidents, but some got extra credit for knowing them all.
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There is really only one sensible way to distinguish a planet from a non-planet: if an astronaut standing on the object’s surface cannot achieve escape velocity by jumping, the object is a planet.
hi…?/can i ask something??why human beings cannot survive in other planets???can you help me research about it???its our project…we need to research about it…