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Originally posted August 22 2006 at 22:08 under Physics. 0 Comments. 1 Trackback (now closed). Last modified: 22 August 2006 at 22:36

OMG! Still Planets!

Still bemused
A Classical, rocky, major planet

There’s still argument about what a planet is. Things aren’t really progressing either. Rather than one proposal there are now three separate, but not alternate, proposals which, as I’ll go into, haven’t really changed anything. Anyone interested might want to watch the video of the latest meeting. Judging from that things are far from in large agreement with the leadership…

Firstly from that meeting it is obvous there are some serious points being glossed over. The idea that we’re dealing with extra-solar planets in this definition is being dismissed, yet the wording of the draft resolutions would seem to clearly imply planets in systems other than our own (certainly I’d assumed it was referring to such things). It clearly says star, not Sun. If we ignore extra-solar planets then we’re just going to have to do things all over again (apparently the extra-solar planet people have their own definition which may or may not be compatible).

The committee also seemed at one point eager to stress that the “teachability” of the new definition isn’t really important (i.e. we shouldn’t worry about having to try to explain all these planets, or why they’re so different). Yet at another point it was stressed how simple this makes it to explain what a planet is (if it’s round it’s a planet; if it’s lumpy then it’s not, never mind if it’s almost the same size and mass in a very similar orbit). This is indicative of the inconsistency, stubbornness and self-contradiction which is becoming entrenched in the debate.

The stubbornness is more evident in the fact that the new draft of the proposal(s) whilst managing to play with words (oh the irony) don’t actually change anything. The much touted “dominant object in their local population” enters but only to refine the definition of classical planets in our system, something that was pretty clear anyway. The new draft doesn’t change the proposed definition of an actual planet at all. A note of what will constitute a Small Solar System Body (most asteroids etc) has appeared, one can only assume to try and counter the obvious fact that so many things become planets.

The whole debate over “Plutons” (a word which rightly has been dropped in favour of a yet to be determined alternate) has been moved to a separate proposal. I don’t see why the issues need to be muddied with that debate at all. However, to my mind the fact that the “classical planets” are explicitly distinguished from “Plutons” is an admittance that there are two (at least) classes of object. The term planet is historically much closer to meaning one of the “classical planets” than it is to the new proposed definition.

The problem of the difference between a satellite and a planet also gets its own proposal. I’m not too sure of the wisdom of that. If the first (resolution 5) gets passed and this (resolution 7) doesn’t then it’s going to leave quite a large problem (not that resolution 7 would particularly solve all the problems. At one point it was almost joked about how long it will take the Moon to drift out far enough to become a planet, which was completely side stepping the point).

In short the newer proposal change nothing and still leave all the problems I previously discussed.


It’s not much good saying the proposals are no good if no one can suggest something better (or maybe, see below, it is). Mind, it seems that the idea of a competing proposal being put up for vote isn’t going to fly—which somewhat brings into question the whole openness and democracy of the process. So what are the options?

Do Nothing

It’s been rightly pointed at that as long as a way to name and classify the interesting objects we find there’s no real need to answer the question at all. We can let common culture decide if Pluto’s companions should be planets or not. Whilst this may have been a valid route at the start it seems that the interest and demand now produced (whipped up in part by the press release style of the initial proposal) won’t allow this as a get out, or even a delay for more debate. The IAU have been backed into a corner where they must now at least propose something for the vote, even if it might not pass (which would probably make the whole community look foolish). This is all a shame as obviously in the end it doesn’t matter what we call the things, they’re still as interesting and as important.

Dominant Objects

Stop taking half measures and introduce the “dominant objects in their local population” clause properly, to define a planet not just some made up sub-section. This gets us closest to what culturally is thought of as a planet. It does relegate Pluto but the committee shouldn’t be worried about that by their own admission, as it’s still physics based. I think all things considered this is my preferred option. It incorporates the two sets of physics being debated (keeping hydrostatic equilibrium—roundness—and introducing orbital mechanics), keeps the number down to something that feels right for the word planet, and matches cultural expectations.

Find a Third Way

Sometimes sides become so entrenched that a compromise is only possible by looking for something neither is for or against. Plucked more or less from thin air (or empty space) how about mass compared to the star. Anything (round) with a mass greater than about 1×10-5% of the star’s mass counts (that puts Mercury in but leaves Pluto at about half the required mass if I’ve roughly calculated right). It’s still physics based… There are options, but it would certainly require time to consider them. Time which, as I said above, seems to not be available

Argue On

There’s more to come on this. Last I heard there was to be yet further meetings and negotiations. One must hope it actually changes something this time, rather than the executive sticking to their guns. The show of hand isn’t actually shown on the video, but accounts say it wasn’t exactly supportive…

Update: Of course I’m running late again; I said there were more meetings. Now it’s reported that we might be getting dominant body in where it should have been put earlier today (the second solution I listed above). It’s a solution, not an ideal one, but a better one than we started with. Of course, it might change again by Thursday’s vote!

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Trackbacks (1):

OMG! Less Planets!

I've been away, so no comment until now (I'd only vaguely heard the resolution had passed and Pluto demoted---only just read the actual resolution). First the good bit: They've seen sense and demoted Pluto to something which isn't a planet.... [Read More]

Tracked from IMS_Blog on Aug 27, 2006 at 19:51

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This Crazy Fool

Dr Ian Scott
Croydon (and Gateshead), United Kingdom
Bullding Services Engineer (EngDesign), PhD in Physics (University of York), football fanatic (Newcastle United), open source enthusiast (mainly Mozilla)

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