One other point I would raise in regards to a possible double-planet system is that someday we might well find a double planet system somewhere in the heavens. 2 objects of similar sizes, maybe even 3 in interlocking and similar orbits - depending on the gravitational effects and the stability offered by the LaGrange points etc.
I'm not entirely familiar with how that all works - but I've read here that it is theoretically possible. So, we may well find this eventuality someday - but one would have to assume that such a system has an inherent susceptibility to being unstable - to a higher degree than a more traditional planet - moon, or planet - moons system. The gravitational effects of passing objects would surely be multiplied to some degree by the twin gravity wells in such a system - and produce more unpredictable results.
So while there's nothing to stop such a system theoretically being discovered, its also possible that these type of systems might not endure for the same length of time as a traditional system - and its in fact likely that there will be some event over the tremendous time periods we acknowledge is relevant to discussion of this sort of thing that will likely disrupt such a double-planet system.
So what I'm getting at is that we may discover a double-planet system and then over a period of thousands, tens of thousands, millions of years - length of time not being important to this particular argument, it may evolve into something else. Perhaps the 2 bodies could end up in differing orbits that no longer leaves them as double-planets - perhaps planets in their own right, or perhaps 1 could end up in orbit around another, larger world (like the Earth around Jupiter for instance - if that ever happened the Earth would become a moon).
The point I'm trying to make is that in classifying heavenly bodies you have to accept that things are not set in stone and that the Universe is a dynamic place that is subject to change on grand unimaginable scales and that given that, how we describe an object also needs to recognise this dynamic element and can't be set in stone. I think, baric, that you adhere to this understanding, but that there are some that don't. What particularly irks me are those who call for all objects that have gravitationally rounded to be called `planets.' And because of this, they call for all the large moons in the Solar System to be referred to as planets - because if they orbited the Sun in a different orbit they would be planets!
A spurious argument that ignores the understanding built up over centuries, millennia perhaps, about how we differentiate between planets and moons - and also a recipe for inviting confusion and ridicule with the general public.
One has to recognise the importance of orbit and gravitational/orbital dominance in how we've defined heavenly bodies in the past - and differentiated between planets and moons for instance. With any classification system that relies as heavily on empirical data as our understanding of what a planet is - there are going to need to be tweaks and small changes made to how that definition stacks up as we discover more information and gain a better understanding of the vast Universe out there - and that is truly something we are only just beginning to really do.
I say this because I think the orbital dominance classification system baric champions here adheres to the principles and understandings we've had in the past of how to define a planet and just tweaks things slightly at the edges given the new discoveries we've been making in the past 15 or so years - and which is why I also believe its the best way to go. But at the end of the day, that's just IMO.
