Zillionth time: The proposal is poor, and so is any other because there exists no good definition. "Planet" is fundamentally cultural term, wanted we that or not.

Let's hope they vote for it so that we finally can stop that eternal dispute.

In the end, this may turn out positive for us planetary probe fanatics. If an object receives a planetary status, it gets more PR and is more likely visited by a probe (at least in the main asteroid belt). Ceres was badly outlooked because it was considered just another "rock" not a Real Planet™.

From Rob:

Hope you'll forgive me for this, Alan, but I can't resist playing devil's advocate: wouldn't that "technically" make Nix and Hydra numberable KBOs, rather than satellites of Pluto?


My comment:

The deflection of the Pluto/Charon barycenter by Nix and Hydra being less than the radius of either Pluto or Charon should secure these two bodies as satellites.

Exactly, so why bother? For PR purposes? Is that really what the IAU is there for? I'm sad and surprised to think that worlds like Ganymede or Titan might be considered less worthy of exploration because they each orbit a planet as well as the Sun. Why should that matter? It must be so because even the Cassini team seems to think so - the preamble to each Titan flyby page starts with a statement to that effect. I'm totally mystified by this.

According to Daniel Fischer from the HASTRO list, I quote:

There has just been the first open debate here at the IAU General
Assembly on the proposed resolution in which only planetary
scientists (planets of all sizes) took part: The proposal lost,
about 60:40, to an alternative put forward by a group of other
planetologists (which would have made "being by far the largest
object in the local population" plus roundness the criteria for
being a planet and thusly excluding Pluto). The term "plutons"
was rejected in a 2nd vote by an overwhelming majority, for
the linguistical confusion it may trigger (and has already done
so in places). None of these votes is binding in any way:
It's up to decide for the IAU Executive now to decide on
further action ...

Daniel


Daniel F. just posted another note with a link to the details, plus this:

More on today's events: http://www.astro.uni-bonn.de/~dfischer/mirror/300.html

I'm now being told that the vote was actually more like 75:25; the counting
was a bit chaotic ...

Daniel

This post has been edited by ljk4-1: Aug 18 2006, 04:07 PM

Groan. Okay, let complete chaos ensue... cry havoc, and let slip the dogs of war.

I don't find the alternative proposal any clearer than the primary proposal. How does one measure a "by far"? What constitutes or does not constitute a "local population"? In what system of analysis do all TNOs constitute a single "local population"?

1 Ceres is three and a half times more massive than 4 Vesta. Is 3.5 too small for "by far"?

Sedna isn't even a KBO by the most generous definitions of the Kuiper Belt. Does that make it "by far the largest object in [its] local population"?

2003 UB313 also has an orbit that takes it well beyond the outer edge of the Kuiper belt. Strictly within the KB itself, Pluto is still the largest object discovered. But "by far"? Who knows what that means?

If the KB constitutes a single "local population", then perhaps the much smaller region inside Jupiter's orbit can also be considered a single "local population". The largest objects in this region (Earth, Venus) are close in size -- and both are much smaller than Jupiter -- so neither of them can be considered a planet!

[...]

What is a planet? by Steven Soter

http://arxiv.org/abs/astro-ph/0608359

The question of what a planet is has one important ramification: it affects what the name of 2003 UB313 can be. I think we're all tired of saying 2003 UB313, so it'd be nice if the IAU could resolve the question to that level.

I agree. However, there is an easy way for the IAU to resolve the question without making a decision on "what is a planet" -- let them abolish the rule providing for a different system of nomenclature for planets.

all sun-orbiting objects get numbers.

Welcome to 145675 Earth.

Hehe, good one! Don't forget to visit 145676 Luna (Earth's Moon) while you're at it! Later!

J P

I've just read this proposal. It completely ignores the actual physical attributes in favor of pure orbital population dominance. This position is pretty diametrically opposed to Alan Stern's. It does contain some questionable assumptions, like the 100:1 miniumum ratio for object-mass:mass-of-everything-else for planet qualification, and the fact that he says every single KBO can collide with Pluto.

Also, it uses accretion terminology in the definition. He seems to feel that intermediate cases when it comes to population dominance are impossible due to accretion mechanics.

I was amazed by Soter's comment myself:



My initial comment is that of Spock on Khan:



Here's another surprising one from Soter:


So he transitions from "more massive than the total mass of all other bodies" to comparing the mass of one object to the next most massive body, and likewise transitions from "similar orbit" to "cross[ing] its orbit". I think there may also be another body in the neighborhood of Earth with an orbit that frequently crosses Earth's that's just a bit bigger than 1036 Ganymed.

[...]

I think he overlooked that possibility when he excluded objects in resonance with each other from counting when it comes to crossing orbits (when discussing one of the extrasolar planet systems). He also exludes resonance with Neptune when discussing objects crossing Neptunes orbit, but ignores that this implies some KBO's have orbits synced with each other.

Personally, I think it might be interesting if some sort of hackish compromise between this and the IAU proposal could be hammered out that might be not too disgusting to either the 'type of object' or the 'population dominance' crowds, by making a more explicit distinction between major and dwarf planets.