But it can pretend to be a planet!*

[astra_nomer]

I've been getting kind of tired about all the news stories about thousands of astronomers meeting in Prague to determine whether or not Pluto is a planet, as if that's the only thing that will happen at the IAU meeting this week. So it's kind of a relief to hear what the actual news is, which is that the IAU is proposing to create a new class of planet, "dwarf" planets as they've been dubbed by the media.

To summarize, there would be the 8 "classical" planets.

The dwarf planets would include Pluto, Ceres (the largest asteroid), 2003 UB₃₁₃ (Xena), and Charon (Pluto's moon). Furthermore, there would be a class of object called plutons that include all of the above except asteroids.

Except there's already a name for the class of objects that characterize plutons, which is "Kuiper Belt objects." Is is just an issue of size in that case?

What about the Earth's moon and the Galilean satellites, which are all bigger than Pluto?

Really, I think the only people who care about planet nomenclature are schoolchildren and Michael Brown, the discoverer of 2003 UB₃₁₃, who would much rather be known for discovering a planet rather than a large Kuiper Belt object.

I think this quote by Geoff Marcy sums up my own attitude quite nicely:

“I am not attending the I.A.U. meeting, nor do I care about the outcome of any vote about whether Pluto and Xena are ‘planets.’ ”

*This is what DS1 would tell you about Pluto if you asked him about it, prefaced by the statement, "Pluto is a Kuiper Belt Object."

Comments

[jamess-fox.livejournal.com]

Ceres is not a pluton, or a Kuiper belt object. As for Charon, the reason they're putting it in is that the barycenter of the Pluto-Charon system lies outside Pluto, thus, they argue, Pluto and Charon are 'double planets', rather than Pluto being the planet and Charon the moon.

[astra-nomer.livejournal.com]

Ceres is not a pluton, or a Kuiper belt object.

Yes, that's what I said. Ceres is an asteroid.

thus, they argue, Pluto and Charon are 'double planets'

I'm just saying that I find that definition to be unsatisfying.

[chenoameg.livejournal.com]

I think they just need to get us excited about Kuiper Belt Objects, and we can start naming all of them, so that will make everyone happy.

More planets, bah.

[astra-nomer.livejournal.com]

Some KBOs do have names already -- Quaoar and Sedna, for example. But nobody pays any attention to them.

[rms10.livejournal.com]

I'm really unhappy with including Ceres and the KBOs into one category of dwarf planets. Composition should play a role in the categorization, and not just size.

I like promoting Charon, though, since the center of gravity of the Pluto-Charon system is actually between them. Calling Charon a moon is not quite accurate there.

[astra-nomer.livejournal.com]

Funny, I don't mind so much the promotion of Ceres to dwarf planet status. I think subcategorizing the distant ones as plutons is a bit much.

The promotion of Charon is more problematic -- do we make a distinction between binary stars whose centers of mass are inside or outside the larger body?

[rms10.livejournal.com]

Are there really many binary systems whose center of mass is inside the larger body? I don't have a problem with defining binary systems by where the center of mass is, but then again, I don't study stars or planets.

[astra-nomer.livejournal.com]

I suspect some contact binaries or even very close binaries do.

I'll put it another way. The Sun-Jupiter system's center of mass is outside the sun's radius. Should Jupiter be considered a binary companion to the sun?

[rms10.livejournal.com]

Really? Huh. I didn't know that. But anyway, that could be an interesting way to categorize the system.

[firstfrost.livejournal.com]

People who study Pluto also care about whether Pluto is officially a planet or not. :)

[astra-nomer.livejournal.com]

so that he can still be considered a planetary scientist? :)

[firstfrost.livejournal.com]

I think it's tied to this phenomemon:
Some KBOs do have names already -- Quaoar and Sedna, for example. But nobody pays any attention to them.

Planets are just intrinsically cooler than KBOs.

[gigglefest.livejournal.com]

* aww, that's all cute and impressive! I guess I shouldn't be surprised, but still.

[marcusmarcusrc.livejournal.com]

I second gigglefest's statement.

[marcusmarcusrc.livejournal.com]

I note Michael Brown has an op-ed in the nytimes. 3/4 of the op-ed sounds very reasonable, and seems to be an argument for "Yeah, Pluto might not be a real planet, but heck, maybe Australia isn't a real continent either (or maybe Greenland should be), and we just let some things be because it was that way historically."

Then he ends his op-ed by talking about how 2003 UB313 should totally be a planet, but not any of these other posers, thereby totally undermining the rest of his argument.

(To be honest, I've always kind of felt like gas giants and rocky planets should be split into two separate categories, but I guess that's a totally different discussion)

[astra-nomer.livejournal.com]

Of course Michael Brown has an op-ed about it. I'm mostly amused that he's as critical of the criteria as anyone else.

[arcanology.livejournal.com]

A good spanking for everyone who actually cares will sort this thing right out.

[astra-nomer.livejournal.com]

And I know some people who would love to administer said spankings.

[visage.livejournal.com]

So, is there a non-arbitrary definition that'd include the 8 and exclude the rest?

"Pluto and Charon are a double planet because the center of mass is between them" would mean that Earth/Luna is a double planet system as well, right? (Earth's approximately 100 times more massive than the moon; the distance between the earth and moon is approximately 400 times the earth's radius. Assuming I've got my numbers right...)

[astra-nomer.livejournal.com]

Earth: 6.0e27 g
Moon: 7.3e25 g

Distance between: 3.8e5 km
Earth's radius: 6.4e3 km

Center-of-mass distance between earth center: (3.8e5 km)*7.3e25/(6.0e27+7.3e25) = 4.6e3 km

So no, Earth-Moon doesn't qualify.

[visage.livejournal.com]

Ah, oops, I read the moon's orbit's circumference as its radius. =)

[astra-nomer.livejournal.com]

So, is there a non-arbitrary definition that'd include the 8 and exclude the rest?

See, that's the trouble, it's a false dichotomy, really. There are all these objects that orbit the sun and they have a wide range of sizes and compositions and orbital parameters.

It would make sense to impose some kind of mass threshold, but even that would be arbitrary.

There are all sorts of interesting objects to study in the solar system. Arguing about what to call them seems kind of stupid.

[visage.livejournal.com]

There are all these objects that orbit the sun and they have a wide range of sizes and compositions and orbital parameters.

Well, sure, the question is if there's a description that applies to the eight and only to the eight (or ten... but 53?), and if there's such a description that isn't entirely arbitrary.

It would make sense to impose some kind of mass threshold, but even that would be arbitrary.

I gather that the proposed definition includes "massive enough to be round", which seems like a reasonably non-arbitrary threshold to me. ...but inadequate to cut out enough objects.

[astra-nomer.livejournal.com]

I don't know for sure where the 53 comes from, but I suspect that has to do with the "massive enough to be round" criterion.