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

[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.