NYT: Debate Lingers Over Definition for a Planet (protest petition signed)

[DeepModem Mom]

Debate Lingers Over Definition for a Planet
By KENNETH CHANG
Published: September 1, 2006

More than 300 scientists have signed a petition protesting the definition of “planet” decided by the International Astronomical Union last week. That definition demoted Pluto, leaving the solar system with eight planets.

The petition states: “We, as planetary scientists and astronomers, do not agree with the I.A.U.’s definition of a planet, nor will we use it. A better definition is needed.”

Mark V. Sykes, director of the Planetary Science Institute in Tucson, Ariz., and S. Alan Stern, of the Space Science and Engineering Division of the Southwest Research Institute in Boulder, Colo., organized the petition. Many planetary scientists, they said, were dissatisfied with the new definition, some because of how it was decided, some because they did not find it sound.

The signers of the petition included NASA scientists, astronomers at major observatories, university professors and graduate students.

The astronomical union allowed only scientists attending a conference last week in Prague to vote....

http://www.nytimes.com/2006/09/01/science/space/01planet.html

[kurth]

1. Maybe they should have shown up?

News reports seem to indicate that many scientists figured the proposal wouldn't go anywhere, so they never bothered to attend.

[IndianaGreen]

2. The vote was taken on the last day of the conference, only 424 got to

vote! The new definition is idiotic for it defines a planet as a celestial body that has swept clean its neighborhood. By that definition, neither the Earth nor Jupiter would be defined as planets, the Earth having 10,000 trojan asteroids in its orbital path, while Jupiter has 50,000 trojans.

[Bill McBlueState]

3. that's not what they mean by "swept clean"

The astronomers who did vote are smart people; they wouldn't accidentally vote to approve a proposal that takes Earth and Jupiter off the list.

The orbit clearing parameter is proportional to (M^2)/P, where M is the mass of the prospective planet and P is its period. The eight remaining planets all have orbit clearing parameters that are orders of magnitude higher than those of the dwarf planets. For example, if Earth's orbital clearing parameter is set to 1, then Mars's (the lowest of the eight remaining planets) is 6.1x10^-3. After that, the next highest is UB313, all the way down at 3.5e-8. (Here's where I'm getting these: http://lanl.arxiv.org/pdf/astro-ph/0608359 )

This concept of "orbital clearing" neatly divides the solar system into eight important objects and a greater number of somewhat less important objects.

I encourage people to have a look at the table of orbital clearing parameters, on page 17 of the above link. (It's the column labeled lambda/lambdaE, where lambda is the upside-down "V".) It's interesting that if any of the eight remaining planets is only marginally a planet, it's Mars because of the influence of the huge, relatively nearby Jupiter.

(edited to be more specific)

[Gman]

5. And oops...

posted above before I read your post.

[muriel_volestrangler]

6. Thanks for that link

It explains the thought behind the classification much better than the phrase that all the news reports have used.

I find it interesting that the numerical definition he uses for the "planetary discriminant" could give the Earth a problem - with the Moon having about 1/80th of the Earth's mass - if he hadn't put "non-resonant" into the definition of "sharing an orbital zone". The current theory of a Mars-sized body hitting a proto-Earth to produce the Moon would also mean you can't call the Earth a planet until that had happened - because by definition it hadn't cleared its orbit until then.

[DeepModem Mom]

7. Thanks for your post, Bill! nt

[TechBear_Seattle]

9. Nine important objects, actually

Yeah, yeah, I know that the sun isn't a planet. But I would certainly classify it as an "important object." :hi:

I particularly like a description I heard years ago about our solar system: it consists of the Sun, Jupiter, and a bit of leftover debris.

[Gman]

4. Very good!

I'm impressed with your knowledge of this topic and why the decision was apparently arbitrary! It sounds like they based the decision on mass and gravity which doesn't necessarily imply size and is why this seems arbitrary.

I've only paid minimal attention to the topic as it progressed and hadn't learned the new definition. At one time when I was a kid, I was very much into astronomy. I do have a telescope and have enjoyed it (despite the city lights). But its become one of those things that begin to go by the wayside while I go on to others.

[gratuitous]

8. Thousands dead, billions of dollars squandered

But it's nice to see people all worked up to sign a petition to "save" Pluto's status as a planet. I'm so glad our priorities are so clearly in order. I wonder if there's intelligent life on Pluto? Because to quote a great philosopher, there's bugger-all down here.