BL: planet, dwarf planet
Posted: 07 August 2015 07:24 AM   [ Ignore ]
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Easy etymology, tricky definition

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Posted: 07 August 2015 12:54 PM   [ Ignore ]   [ # 1 ]
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Even though it’s not posed very well, it’s an overstatement to say that the IAU definition is “a scientifically useless classification”.

Clearing its neighborhood is a too-quick way of saying that a planet must gravitationally dominate its orbit. This is important to orbital dynamics. Something in the neighborhood of a planet either gets shot off out of the area, collides with the planet, gets picked up as a moon, gets shepherded to one of the planet’s Lagrange points, or gets locked into a resonant orbit. These disparate fates, which otherwise are just an odd grab bag, are unified by thinking of the object as a planet in something like the IAU sense.

It’s true (as you note) that the IAU definition is written so that it only applies to our solar system. If we could resolve distant star systems at a finer resolution, though, the category is one we’d care about for other stars as much as for our own sun.

It’s also true (as you note) that if the same object were in a different context, in a different orbit or not orbiting a star at all, then it would not be a planet. But context matters, and you don’t need categories to make sense of orbital dynamics for objects without orbits.

In short: There are definite deficiencies with the IAU definition as formulated. But it readily admits of a more carefully-formulated version that (a) picks out the same objects in our solar system and (b) is scientifically useful.

best,
P.D. Magnus

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Posted: 08 August 2015 05:28 AM   [ Ignore ]   [ # 2 ]
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Welcome, P.D., and thanks for a very illuminating response!

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Posted: 10 August 2015 12:38 AM   [ Ignore ]   [ # 3 ]
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Not everyone thinks the IAU definition is workable. NASA Principal Investigator Alan Stern, for instance, opines that it is “bullshit”.

http://www.techinsider.io/new-horizons-alan-stern-says-pluto-dwarf-planet-bullshit-2015-7

But Stern has a clear opinion about Pluto’s demotion:

“It’s bulls----,” he told Tech Insider (and said we could quote him on that).

The problem, Stern said, is that the reclassification largely stemmed from the opinions of astronomers, not planetary scientists. His beef here is that astronomers study a large variety of celestial objects and cosmic phenomena, while planetary scientists focus solely on planets, moons, and planetary systems.

“Why would you listen to an astronomer about a planet?” Stern said.

He compared it to going to a podiatrist for brain surgery instead of a brain surgeon.

“Even though they’re both doctors, they have different expertise,” Stern said. “You really should listen to planetary scientists that know something about this subject. When we look at an object like Pluto, we don’t know what else to call it.”

It makes sense that a planetary scientist would prefer a different classification scheme from the one an astronomer likes, and it would not be the only case of allied fields having slightly different definitions for technical terms.

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Posted: 10 August 2015 04:14 AM   [ Ignore ]   [ # 4 ]
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The problem stems from the fact that the definition grew out of a non-scientific need. The problem was that of naming convention. Would these new Kuiper Belt objects get names of Roman gods, or should they be named something else?

The solution the IAU came up with was to promulgate a definition of a planet that would exclude the Kuiper Belt objects. As a result, the definition is a poorly cobbled together hodgepodge of characteristics. The limitation to the sun, rather than any star, is a direct outgrowth the nomenclature problem, even though it flies in the face of one of the basic tenets of science—that the laws of nature apply everywhere. An arbitrary regional limit may be useful for many purposes, but it is not a scientifically useful characteristic. The other two characteristics represent different ways of looking at planets and don’t sit well together. The hydrostatic equilibrium characteristic is a nod to the planetary scientists, supposedly giving some indication of mass and size, but ending up doing a bad job at both. The clearing the orbit characteristic, which can be a useful synchronic classification for the orbital mechanics crowd, isn’t diachronically useful as planets shift orbits over time. And it doesn’t tell you anything about the object itself, just something about its mass in relation to its distance from the sun.

The IAU would have been much better served if they just said that Kuiper Belt objects would be named after non-European deities, with Pluto’s name being grandfathered, and just leaving the definition alone. (We’ve gone centuries without an official definition of planet, why start now?) That would have enabled scientists to define the word in different ways according to their particular purposes. While that might seem to be a problem, a cause of confusion, it’s really not; it happens all the time in science. Naming is an arbitrary classification, and the only reason scientists do it is in service to a larger goal. Sometimes it’s useful to break out of the defined categories and look at objects in new ways.

And then when people say “what will we teach the schoolchildren about the number of planets in our solar system?”, it becomes a teaching moment. Teachers can talk about the different kinds of objects and their different sizes. You can ask the kids to come up with their own definitions, exploring the various characteristics that might be useful. They might actually learn something about astronomy and planetary science. It’s a much richer exercise than the rote memorization of eight or nine names.

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Posted: 10 August 2015 05:38 AM   [ Ignore ]   [ # 5 ]
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The limitation to the sun, rather than any star, is a direct outgrowth the nomenclature problem, even though it flies in the face of one of the basic tenets of science—that the laws of nature apply everywhere.

This is surely a trivial objection with a trivial solution: “sun” = “star.” I can’t believe any astronomers would refuse to talk about planets circling other stars.  It reminds me of the ruckus over the phrase “by the state” in the health care act (which the Supreme Court properly quashed).

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Posted: 10 August 2015 10:59 AM   [ Ignore ]   [ # 6 ]
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It reminds me of the ruckus over the phrase “by the state” in the health care act (which the Supreme Court properly quashed).

FWIW the straw that the anti-ACA folks were grasping at in King v. Burwell was “by the states” (plural).

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Posted: 10 August 2015 11:26 AM   [ Ignore ]   [ # 7 ]
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This is surely a trivial objection with a trivial solution: “sun” = “star.” I can’t believe any astronomers would refuse to talk about planets circling other stars.

The wording of the IAU resolution is “in orbit around the Sun.” That’s unmistakably the star Sol, and not any other.

But in actual practice astronomers wouldn’t and don’t exclude extrasolar planets when talking about planets. It’s just an example of how poorly worded the definition is. It doesn’t even match how astronomers commonly use the word. In this particular case, the bad wording is a result of the original intent of the exercise—to determine a nomenclature system for the objects in the solar system.

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Posted: 10 August 2015 09:41 PM   [ Ignore ]   [ # 8 ]
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I usually say that this issue doesn’t matter, but i’truth I would have preferred a different ruling from the IAU, for three reasons:

a) The third criterion means that you can’t determine whether something is a planet until you have thoroughly observed, not just the planet, but its orbital environment. For some scattered belt objects this could take decades.

b) The third criterion is not actually all that clear cut. Earth, for instance, has not completely swept its orbital area clear of planetesimals, and every few million years one of them collides with Earth. It is possible to parameterise this property statistically but you still end up having to make a call on how clear is clear enough.

c) Unless it is unavoidable, I think that scientific jargon should avoid using common words in a way that is at odds with their common usage. It would have been better to reframe the definition so that the category “planet” at least included all of the objects that were commonly called planets (and you could do this by leaving out criterion III), or (probably better) just give up on the use of the term planet as an astronomical term of art.

It is similar to the discussion regarding birds and dinosaurs.

Some background: Cladistics is an important trend in modern zoological nomenclature and categorisation. A clade is a group of animals defined as a particular form plus all of its descendants. It is very useful for zoologists to deal with clades, rather than non-clade categories, particularly since the rise of techniques for genetic comparison. It has long been known that all modern birds descend from therapod dinosaurs: for a zoologist, there can be no sensible category that includes all of the dinosaurs but not modern birds. For instance, velociraptor is closer to a modern bird than it is to most dinosaurs: in terms of ancestry, in terms of morphology (feathers and hip-shape), even in terms of chronology (it is 75 million years from modern birds, but lived more than 100 million years after many dinosaur groups died out.) And so it is that the clade Dinosauria includes all dinosaurs and birds.

Which is all well and good, but this is now also reflected in works aimed at a general audience, as a change in meaning of the word dinosaur. The Wikipedia article on dinosaurs has an info box showing dinosaurs living from the Triassic up to the Present, which could baffle or even mislead a casual reader. If they go on to read the article they’ll find out that birds are dinosaurs, which might make them thing zoologists are in the category “weirdos” and science is stupid. Some authors fastidiously use the term “non-avian dinosaur” if they want to refer to ye olde timey dinosaurs.

My view is that it would be better to leave the ordinary word “dinosaur” as it is, and use the jargon term Dinosauria if you want to refer to the clade: accept that zoological category words used in general communication need not be clades. It seems my view is not the common one, though, with specific regard to dinosaur/bird. It is odd because in other cases, authors are happy to let it slide. It is known that “reptile” is not a category that represents a clade, and there are various discussions about how to redefine (or abandon) the category Reptilia, but technical authors seem quite happy to use the term reptile in its normal range of meanings when writing for a general audience.

EDIT: I have made an additions, shown in bold. For some reason, I neglected to finish that sentence, previously. Also, culled a needless ‘either’. Try saying that: “needless either”. There’s some cheap fun.

[ Edited: 11 August 2015 02:21 PM by OP Tipping ]
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Posted: 11 August 2015 08:14 AM   [ Ignore ]   [ # 9 ]
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My view is that it would be better to leave the ordinary word “dinosaur” as it is, and use the jargon term Dinosauria if you want to refer to the clade

I agree, and I thank you for that extremely illuminating comment.

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