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What is a Planet?

The major news sources are reporting that the Planet Definiton Committee, meeting at the XXVIth General Assembly of the IAU, is recommending a draft defintion of a planet that will include Pluto. (The New York Times article can be found here.)

How do I feel about this? Well, it'll keep Pluto listed as a planet, which I support, but it may very well open up the question of whether Charon, Ceres, and a whole lot of other bodies in our solar system are planets.

So...if you want to know more about my feelings on this matter, watch this space. I'm hoping to have an announcment to make later today. (And that's all I'll say for the moment.)

Comments

Take a look at this: http://aa.usno.navy.mil/hilton/AsteroidHistory/minorplanets.html

Details about the planethood of Ceres & friends, as well as symbols for many early asteroids.
Gotta say, I think Mike Brown is right and this is a mess. How is Charon a planet, er I mean puton, I mean dwarf planet? This feels arbitrary. I think I'd rather see Pluto demoted than see this possibly needless multiplication of entities.

I also think that for a story in the NY Times, the article is rather light-hearted and flippant, but that's another matter entirely.
yeah. there's a line drawn inside my head in that a body orbiting another planet is a moon, not a planet. only bodies orbiting suns should be considered planets. IMAO i think they need to reword it so that charon stays a moon.
Pluto : planets :: indigo : ROYGBIV
That made me laugh. Thanks.
Apparently, it does make Charon a planet as well as Ceres (the latter seems stranger to me). But, on the other hand, "Go Giuseppe Piazzi!" his planet is a planet again.
I think we should go to Ceres.

Cereously.
According to the new draft definition, two conditions must be satisfied for an object to be called a "planet." First, the object must be in orbit around a star, while not being itself a star.

So the next controversy is how brown a dwarf has to be to be called a star.
Look, no contemporary astronomer can hope to be as great as Clyde Tombaugh, who lived much closer to Galileo than we did. It is time to declare the canon of planets (at least around our sun) to be closed, and for the acharonim astronomers to be content with extending the works of the previous giants. Contemporary astronomers are astronomers, but the science they are practicing with respect to choosing what is and isn't a planet is not astronomy. :>)
Further posts on the topic can be found at making light.
It seems pretty vague all around. The debate seems to be all about fondness.

I say, pick a reasonable definition and let in everything that qualifies. But it had better be a tight definition, or lots of things are going to qualify and we'll have to rewrite the definition later. "It's got things orbiting it" for instance. It seems like we'd probably find all kinds of asteroids going around each other. This rule they've made up makes sense to me, although to my intuition it should also have a rule about the type of orbit a round mass has.

The only thing I'm really disappointed about is the IAU spending a lot of time debating the cultural implications. Like there's not enough to fight about if you keep it just to the *science*, now you have to fight about Thinking Of The Children!? While science has implications for everyone, it should be done right, and hang the irate whinings of a world that wants to believe that some things are Settled Forever TM.

Pluto isn't gonna care what we call it.
I like how the "in orbit around a star" is in there, in order to make Ceres and Charon planets, yet somehow leave Moon (y'know, Luna) out of the equation. The damn thing's bigger than Pluto, Charon, Ceres, heck, it's one big sucker. And just because the center of the orbits of this double planet system happens to be inside the Earth doesn't mean that Luna isn't orbiting the Sun, too.
is charon in orbit around the sun? i thought it orbited pluto. huh. but then why has it been called pluto's moon all this time?

clearly my knowledge of "things astronomical" is lacking.
As I recall, Pluto and Charon are a double-whatever system, orbiting about a common center of mass that isn't within either body, thus putting them both in orbit around the Sun. As I understand it (and I may be wrong), the fact that the Earth-Moon cg is within the Earth is what they're using to keep from defining our Moon as a planet, but allowing Pluto's.
I've been kind of torn about this, too... it could get very complicated.
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