When is a planet not a planet? 
When it's a dwarf planet.

The voting is complete. The definition of 'planet' approved today had an additional clause: A planet must have "cleared the neighborhood around its orbit."

This means that Ceres (in the asteroid belt), Pluto, Charon, and Xena (out in the Kuiper belt) do not count as 'planets' simpliciter. Only the eight biggies are 'planets.' However, Ceres et cetera are 'dwarf planets.' This is roughly what I predicted would happen eventually given the broader definition, but it has happened immediately and officially.

The New York Times story on this ends with a comment from Neil deGrasse Tyson, director of the Hayden Planetarium:
Dr. Tyson said the continuing preoccupation with what the public and schoolchildren would think about this was a concern and a troubling precedent. "I don't know any other science that says about its frontier, 'I wonder what the public thinks,' " he said. "The frontier should move in whatever way it needs to move."

To put it bluntly, this is bupkis. Whether or not Pluto counts as a planet is not a matter of what we know about the universe, but a question of how we choose to express our knowledge. There is a class of entities that meet the criteria agreed upon today, and Pluto is not a member. There is a class of entities that meet the criteria proposed earlier in the month, and Pluto is a member of that.

The word 'planet' predates modern astronomy, and it belongs in part to ordinary language. If scientists were simply to ignore ordinary usage in decide which words to use in designating various kinds, then they would not be moving the frontier of science "in whatever way it needs to move"-- they would be deliberately misusing words.

To put the point with a bit more subtlety: There is an advantage to using existing terms for new or more precise concepts, because people will immediately have a rough idea of what you mean. There is the disadvantage that the connection with previous meanings may be confusing or distracting. If that disadvantage is too great, then coin a neologism. The frontier of science would not be stopped by schoolkids, if it needed to say something schoolkids did not want to hear.

Now we get to watch and see whether people actually accept the outcome of today's vote. Ron is already on board.

UPDATE: More news over at space.com. Without quoting anyone verbatim, they say that the decision is "billed as a victory of scientific reasoning over historic and cultural influences." Piffle, I say. [Repeat the above arguments.]

Yet the opposition is already maneuvering for high ground:
"I'm embarassed for astronomy," said Alan Stern, leader of NASA's New Horizon's mission to Pluto and a scientist at the Southwest Research Institute. "Less than 5 percent of the world's astronomers voted. ... This definition stinks, for technical reasons." ... He expects the astronomy community to overturn the decision.

Of course, that is just what the leader of an expensive project studying Pluto would say. Even though the decision might go either way based on the science, there is prestige and possibly even funding at stake.

ANOTHER UPDATE: Janet's kids are happy to infer from Pluto's being a dwarf planet that it is still a planet. This must be taken with a grain of salt, since they are also prepared both to promote Xena and to demote Pluto. If toy stores invest in redesigning plastic orreries, planetariums change their displays, and textbooks are rewritten, then one usage rather than another may become entrenched without any further centralized decisionmaking.

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Planet? I usually make it up as I go along 
The International Astronomical Union (IAU) votes tomorrow in Prague on a proposed definition of the word 'planet.' Space.com has a nice discussion of the proposal here and here.

There are nine canonical planets. The problem begins because one of them, Pluto, is really not up to the measure of the others. It is just an icy rock ball on the periphery of the solar system, but common usage and popular opinion holds that it is a planet. Worse still, there are comparably sized rocks out there; for example, 2003 UB313 aka Xena. Whatever the scientific merits of the case, the guys down at the Dairy Queen won't stand for Pluto being demoted from planet to mere rock. As Janet explains, "Pluto is too a planet! It was a planet when I learned it in school, and it hasn't gotten any smaller since then!" This is a nice example of the way that scientific concerns and common usage together shape how we decide to use words.

The proposed definition is this:
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.

This is not oddly disjunctive; neither does it merely stipulate a list of members. So it surely picks out a natural kind.[1]

This definition includes all nine canonical planets. The trick is that it would include many objects not currently considered planets. Xena, for example. Also Charon, which is typically considered a moon of Pluto.

Ceres, the largest asteroid, would also be a planet. This militates against common usage-- the guy at the Dairy Queen knows that asteroids ain't planets-- but Ceres was classified as a planet when it was first discovered in 1801. Asteroids were, in the early days, called 'planetoids' and 'minor planets.' Moreover, Ceres is not just some Oldsmobile-sized chunk of rock. It is 930 km across and comprises about a quarter of the asteroid belt's total mass.

So: The definition is principled, preserves Pluto's planethood, and satisfies the desideratum that every planet we learned about in school should count as a planet. All the same, I think the definition is tantamount to kicking Pluto out of the planet club.

As a rough approximation, people will remember lists of seven items, plus or minus two. Nine planets, fine planets-- yes. But twelve planets, three too many. Once lists get too long, people cluster the items. Instead of having more items, there is one list of kinds of items and seperate lists for each kind.

This is already happening. The eight planets from Mercury to Uranus are classical planets. Ceres is a dwarf planet. Pluto, Charon, and Xena are plutonic planets; dropping the 'planet' moniker entirely, these are plutons. The club of eight retains its aristocratic standards.

I predict that, if the proposal is accepted, then in two decades 'classical' will be dropped in ordinary conversation. Dwarf planets and plutons will have to be specified as such.


[1] As readers will recall, I am inclined to be ecumenical about natural kinds. I might count a stipulated list of the canonical nine planets as a natural kind. That does not mean that we should use the word 'planet' to pick out just those nine things, however.

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Because I can 
I completed my dissertation after digital technology had overtaken document preparation, but before it overtook the submission and archiving of dissertations. I prepared it in LaTeX, processed it as a PDF, printed it on cotton paper, and submitted it in duplicate to the Office of Graduate Studies and Research. OGSR sent both copies to the library, and one copy was forwarded to University Microfilms, Inc. UMI photographed the pages so as to make a copy on microfilm, scanned the microfilm, and made the scans available as a PDF. It is a grainy, uneven scan. It is not searchable. UMI charges money for it, unless you request it from the UCSD network.

It is the 21st century, and we can do better. I have made the original PDF available via Lulu. The fonts are fonts, and they render as vectors. The contents are fully searchable. It is available free, even if you do not live in La Jolla. I opted for Lulu because it provides a kind of backup, and also because they will print hardcopies on demand for anyone who might care. The thought of someone using up an inkjet cartridge to print it is too much to bear.

I should say, in all honesty, that it probably isn't worth your time to look. I am really only doing this because I can.

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Wikipedia paper: The movie 
To sum up last weekend: The Computing and Philosophy conference had suitable proportions of computing, philosophy, food, wine, and camaraderie. Kudos to the organizers for running a tight ship.

I promoted forall x at every reasonable opportunity. I put out fliers and a sample copy in the book exhibit; someone thought enough of it to walk off with the sample copy at the end of the conference, which is a kind of compliment. I had several conversations about it, some with people who are actively trying to pick a logic textbook.

The presentation of my Wikipedia paper went well. In addition to the discussion immediately after the talk, I had some excellent chats later in the weekend. You can watch the video of my talk online, if you are so inclined. (It's in Windows Media format, and I take up about the first third of the stream.) Watching it makes me cringe at how often I interpose 'um' between otherwise interesting words.

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New versions 
I've filled the lacuna in Epistemology and Wikipedia. The conference starts tomorrow, and I present Friday.

I've also posted the updated version of Tom Reid meets Tom Bayes, which continues its quixotic quest to collect rejection notices from the finest philosophy journals.

Eliminating Induction, ditto.

As always, feedback on the papers is welcome. Anything from 'Hmm' to 'Terrible fallacy on page eight' can either be sent by e-mail or commented here in the blog.

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