Isn’t it funny how things keep ending upon Pluto!

You’re running into the problem of the sorites: a bunch of things that add up, little by little, so that a very small difference pushes something from one (somewhat arbitrary) category to another. The canonical example is the category of “bald men.” I have a pretty full head of hair, at lest compared to most men my age; I’m not bald. If I were to lose one hair, I still wouldn’t be bal; lose two, and, well, that’s only a different oce one hair. Flash forward until I’ve alost all but one of my remaining hairs, and sure enough I’ll be bald. But you’ll be hard-pressed to find the time in between at which removing one hair moved me from “not bald” to “bald.”

I talked with Dan about this myself, a couple of years ago, with the idea that there were inflections or discontinuities in the distributions of sizes that might give us a hint. The problem there is that, at least at that time and based on an over-lunch conversation, there really aren’t; the distribution of object size versus number of objects seems pretty continuous.

Of course, if that’s true, it’s unlikely that any definition will be able to crisply determine whether an object is, or isn’t, a planet. No matter what, it’s likely that we’ll find there are objects that seem “planet-like” but which are just outside our definition.

You can create indexes showing big gaps in just about anything; you just have to taylor carefully what goes into them. [...] And the mass ratio is everything.

That isn’t what the paper says. It establishes a natural gap. (Yes, using mass ratio as I said, to establish clearing/mass dominance. And a time scale, again naturally characteristic.)

The neighborhood being dinamically decided is yet another absolute nonsense. Orbits shift, and they shift to the point that we just can’t know for sure at present where the planets orbited in the past.

That wasn’t the process I (and, IIRC, the paper; yes, it was a while since I read it) described. My very first comment mentioned the planetary formation process from the planetary disk.

As for your observation that some bodies may change status, it is exactly what I mentioned earlier in the analogy with biology. There it makes sense, and if we regard the planetary formation process as basis for defining planetary bodies it may make sense here too.

[But if we try to make the analogy exact, IAU is actually trying to establish an analogue to a 'species', in which case they will have to look for a closely related population. That, I assume would include all inner system originated bodies; planets, most asteroids and most moons.]

And the checking time is the age of the Solar System, which is why the index details the current state of affairs, not what happened in the past, nor any possible future state.

As the current state is contingent on the historical path, I don’t see how one can argue that it isn’t a result of a dynamic process. [Btw, again this is very analogous to biology, because species are historically contingent on the dynamic process of evolution. But I'm not arguing that is necessarily a good thing.]

Thank you, SteveoR, for your very comprehensive list of why the IAU vote was a travesty and why Pluto, as well as Ceres, Eris, MakeMake, and other dwarf planets should be considered a subclass of planets. With the IAU having made such a mess, I hardly trust them as the “experts” in this field. In fact, most IAU members are not planetary scientists, and most planetary scientists are not members of the IAU. That leads to the inevitable conclusion that planetary scientists should form their own organization to deal with defining the objects they study.

I was at the Great Planet Debate, and Stern makes a compelling case for hydrostatic equilibrium as the one criteria that unites both “major” or “classical” planets and dwarf planets and separates them from asteroids or small solar system bodies. Objects at the mass where they can attain hydrostatic equilibrium are fundamentally different than inert asteroids–they are differentiated geologically, the same way the terrestrial planets are. Yes, they are dynamically different, in that being smaller, they don’t dominate their neighborhoods. That’s why they are classified as dwarf planets, but it is not reason to state that they are not planets at all.

Phil, Pluto DOES fit the bill for being a planet. You argue that science shouldn’t be driven by public opinion but don’t mention that neither should it be driven by fiat or decree of a small insular body whose members have their own personal agendas. I believe that when New Horizons gets to Pluto, we will have sufficient data to illustrate Pluto’s planetary features that there will be no need to rely on a decree because “someone says so.” The same is true for Ceres regarding Dawn. In the meantime, the public is fully capable of understanding that the issue is a subject of ongoing debate with two differing schools of thought and that there is no one “right” answer, at least until 2015.