There was an article in Nature today that I found rather interesting. Basically it said that there was a recommendation from an IAU panel that the lone term "planet" be eliminated from the astronomer's vocabulary. They suggest always using the term "planet" with an adjective to describe location. They'd separate planets into trans-Uranian, terrestrial, and gas giants for our solar system, and classify everything outside our solar system as extrasolar planets.

In the article, it is suggested (by Alan Stern) that using "planet" but adding an adjective to describe the properties of the object is a better idea. I agree. What possible advantage is there in classifying planets by location rather than by properties? Does this seem more than a little weird to anyone else? Can anyone suggest why they might have chosen this particular recommendation?


The article is Nature 437, 456-457 (22 September 2005)

Location and properties have a lot to do with each other, though. People have been groups Mercury-Mars and Jupiter-Neptune together for a very long time.

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I think they need to leave it alone and say--if its Pluto sized or bigger--its aplanet--kuiper or not. If it is smaller--its a planetoid, so long as it has a spherical shape. Smaller than that--it is an asteroid if lumpy.

Are failed-star gas giants planets?

There has to be some arbitrary decision.

Less is more--don't slap Tombaugh in the face, and keep the system.

So we have 10 planets now.

There is no point in making things harder than they have to be.

I like Alan Stern's idea best so far. The method seems to work well with galaxies and other bodies, does it not?

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"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

To some extent, they do. However, there's nothing saying that you couldn't have an extrasolar system with a group of gas giants, then a group of terrestrial planets, then another group of gas giants (et cetera). For those systems, you'd need a different (more extensible) method of classifying objects. It would quickly become complicated, and probably useless, as more complicated extrasolar systems are discovered.

As George points out, pretty much everything else is classified based on properties alone. You wouldn't classify a star based on its proximity to the centre of a galaxy alone, even if most stars there happen to be similar. And we certainly don't place stars in different galaxies in different classifications than stars in our own galaxy. (That's a rather xenophobic sort of thing, isn't it? Do we want the rest of the universe to think we're racist, or geo-centrist, or whatever the appropriate term is? )

So unless there's a very strong argument that location and properties are very closely correlated in the general case, or an argument that we don't really need to be specific, that explanation doesn't satisfy me.

I liked it until that last sentence. Extrasolar planets as a catch all category is as boneheaded as using "planet" alone as a category. If they want to categorize types of planets, which is supremely admirable, bloody do it right...

Broadly, getting rid of the term planet as having any particular scientific meaning seems like a good idea. It will still mean something in the vernacular, but never get a precise scientific definition. Good plan.

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I agree. A more specific and scientifically useful classification system is about the best possible result.

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Heh, good luck with them trying to change it. I mean we still say "sunset" even though we really should be saying "time of day where the Earth's rotation makes the sun disappear over the horizon." I exaggerate, of course, but you get the idea.

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Need I mention the term planetary nebula?

Dave Mitsky

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What about Pluto?

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That is just a Kieper Belt object, not a planet

Even Patrick Moore seems to agree on this from what I have read in the press

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Patrick Moore's a cool guy, but he won't be the one deciding. The IAU select committe on the issue has been debating it for months; on the one side, Brian Marsden (arch anti-Plutonian) and on the other Alan Stern (the all-embracing inclusivist). Don't expect a resolution any time soon.

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It's gotten to the point where careful investigation is needed just to tell parody from reality. I think that means reality is broken.- Noclevername.

The problem is a classical one of classifying things as exclusively one type, when they are occurring with a multidimentional continuum of properties.

You either end up with a loose historically evolved categorisation that gets increasingly harder to defend on grounds other than "It's a planet because we defined it to be, now stop asking questions about what a planet is".

Or you get a system where all classifications are based on definitions, but those definitions become increasingly convoluted as more items are found.

As I see it, it's the silly need for classifying things by exclusive types that creates the whole problem in the first place.

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Oh, Astronomy deathmatch Just what we need to get more people interested in science (well, the skepchicks may help as well)

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Using orbital characteristics would be even better. No assumptions, arbitrary limits, composition, locus... Mother gravity organizing things for us.

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That's not the IAU's current position. And, let's face it, if it weren't for the controversy over the status of Pluto, there would be no discussion about the definition of planet right now. Any serious redefinition of the concept must deal with the status of Pluto.

The problem with that criterium is that it tells us nothing about the composition or formation of the object.

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Easy, Pluto is a transneptunian object (even if it's orbit slightly dips closer to the sun for a small portion of it's orbit)

Pluto is not a planet, the road ends at Neptune

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