The IAU draft definition of "planet" and "plutons"

[Blue Fire]

From: http://www.iau2006.org/mirror/www.ia...1_release.html

The IAU draft definition of "planet" and "plutons"

16. August 2006, Prague

The world's astronomers, under the auspices of the International Astronomical Union (IAU), have concluded two years of work defining the difference between "planets" and the smaller "solar system bodies" such as comets and asteroids. If the definition is approved by the astronomers gathered 14-25 August 2006 at the IAU General Assembly in Prague, our Solar System will include 12 planets, with more to come: eight classical planets that dominate the system, three planets in a new and growing category of "plutons" - Pluto-like objects - and Ceres. Pluto remains a planet and is the prototype for the new category of "plutons."

This was a bit of a surprise for me. Even tho I had been in favor of keeping Pluto a planet, I fully expected that it would suffer a full demotion.

So now we have Plutons, or probably will have after the vote, eh? It will be interesting to see how the public reacts to all this!

[stuart]

Boooo. I suppose this is the compromise solution between those who want to keep it a planet (you for example) and those who want it demoted to a KBO (me for example).

I guess they're adopting a classification system used on galaxies where there are all sorts of galaxies but they're all "galaxies." This might just be me being bitter on a lack of sleep, but to me, this is one of those compromises that I don't think will really make anyone happy. Assuming it gets adopted.

[Dragon Star]

Well, the reason Pluto actually is going to probably stay a planet is because of the public's reaction. Children allover the world have sent hate-mail to NASA and other organizations begging them to keep it a planet...and that has had the largest effect. Pluto is the most famous of the planets among children because it's a mystery, if you suddenly told a child that his favorite planet is a large bolder... it kinda kills the spirit.

I don't care either way, it's not that important to me, but I can see why it would stay a planet, and I see why it could be omitted.

[Blue Fire]

Hmmm... further on in the announcement article, they list Pluto's moon Charon as a Planet (Pluton), and yet they clearly state (in their proposed definition of a planet) that a planet cannot be any body that is a satellite of a planet. Perhaps this is because Pluto and Charon are a kind of "double planet" in that they both orbit around a point that is not at the center of Pluto? Or is the listing of Charon just a typo or some other mistake in the announcment?

[stuart]

Originally Posted by Blue Fire

Hmmm... further on in the announcement article, they list Pluto's moon Charon as a Planet (Pluton), and yet they clearly state (in their proposed definition of a planet) that a planet cannot be any body that is a satellite of a planet. Perhaps this is because Pluto and Charon are a kind of "double planet" in that they both orbit around a point that is not at the center of Pluto? Or is the listing of Charon just a typo or some other mistake in the announcment?

That's because the center of gravity of the Pluto-Charon system is between the two objects, above Pluto's surface. Thus, they together orbit the Sun, as opposed to our Earth-Moon system where the center of gravity is within Earth, so the Moon orbits Earth which orbits the Sun.

That's also how they don't count any of the Galilean satellites nor Titan.

[Blue Fire]

Hmmm. . . After reading a bunch more from other sites and forums, I've got a sneaking suspicion that the IAU is gonna have to revise this original planet definition standard before they vote just ye or nay. Seems like there's just too much to argue about yet. Everybody else is sure debating it!

[stuart]

Originally Posted by Blue Fire

Everybody else is sure debating it!

What I don't like is the news coverage of it. Pretty much any story you read about this says that 2500 astronomers are meeting for 2 weeks to figure out the status of Pluto -- and that's it. Um, they are doing other stuff!

[Blue Fire]

Originally Posted by stuart

What I don't like is the news coverage of it. Pretty much any story you read about this says that 2500 astronomers are meeting for 2 weeks to figure out the status of Pluto -- and that's it. Um, they are doing other stuff!

Ditto!
For those interested, here's a link to the IAU's Q&A re planet definitions. It's repetitive in places, but it serves to get the proposed rules quite straight.
http://www.iau2006.org/mirror/www.ia...u0601_Q_A.html

[stuart]

An update is that yet another criterion has been added so that a planet must also be able to dominate its region of space. In that manner, Pluto/Charon, Xena, and Ceres would NOT be planets.

I'm still not really a fan of this definition, but I don't object to it as much as the former proposition. I think that this definition is a bit too bulky, that it shows its history (to include Pluto by whatever means possible) but that it was quickly patched because so many astronomers and bloggers thought that it was a cop-out.

[Blue Fire]

Originally Posted by stuart

An update is that yet another criterion has been added so that a planet must also be able to dominate its region of space. In that manner, Pluto/Charon, Xena, and Ceres would NOT be planets.
[snip]

Yeah,.. so what do they mean by dominate? Isn't this going to be too arguable also?