I figure having been on a bit of an offensive over the definition of planet in a few places, I owe those I've been debating with a chance to shoot back at my position. So to the best of my ability, I'm going to spell it out here.
The foundation of my position on what makes a planet, and my opposition to the current trend in trying to define one, is that a body is a body, no matter where it is. A star is a star, whether it forms inside a galaxy, between a galaxy, orbits a galaxy, or is shot from a galaxy. No matter where it is, a star is a star is a star, the physical properties of the body define its label.
With brown dwarves we're pretty much in the same situation. Whether the brown dwarf is a body within a star system, or an independent body, if its a non-fusing deuterium burner, its a brown dwarf. Size doesn't matter, where and how it was born doesn't matter.
Defining what is a planet, for reasons I see poorly bound in centuries of tradition, drags in this belief that where a body is formed and currently located is as critical to the definition as what it actually is physically. It completely contradicts almost every other definition on the books.
From my perspective, the definition of a planet should be the result of non-arbitrary, defined attributes that are unique to planets. Where they are, how they formed, what caused their formation, and where they happen to be at the time of discovery is irrelevent. With rare exception, and object is what it is, or its something else. (I acknowledge the fact that comets are in fact KBOs/Oort objects that have changed status without substantial change in physical make up. Exceptions do exist, but they should be strenuously avoided where possible, and corrected when feasible).
Ok, enough rambling, here's my list.
1) Spheroid shape.
2) Differentiated internal structure. A defined and distinct core, and a crust.
3) Must either be an independent body or of at least equal status to a co-orbital body. Meaning, its either on its own, or its not the satellite of another major body. By establishing mass equilibrium as a clearly defined, non-arbitrary break point, you can look at a binary set of solid objects like Earth-Moon and Pluto-Charon and clearly say "Earth is the planet" and "Pluto is the planet". The actual sizes of the bodies isn't relevent, its how they relate to their companions in their particular circumstances.
Now #3 to me is actually negotiable. I don't see any particular crime in saying we have some satellite planets in the solar system. The Moon, Io, Europa, Ganymeade, Callisto, Titan, Triton and probably some of the larger moons of Uranus, are potentially deserving of planet status because of their very clearly planet like composition. Keep in mind, as I see it, location and orbit aren't relevent to the definition because it creates unnecessary overlap. A body is, or it is not.
Some time ago on Space.com, I read about some objects that were observed in some manner in the Orion nebula, that were pretty clearly NOT brown dwarves, but were independently formed bodies of a physically planet-like nature (the size of Saturn approximately, as I recall). The controversy at the time was, are they a new type of planet born free of a stellar disk, or are they brown dwarves by right of being formed independently. To me, this was a silly arguement. If they weren't burning deuterium, calling them brown dwarves muddies the waters that definition needlessly. If they are substellar bodies that aren't burning deuterium, they aren't brown dwarves, its that simple.
Recently, I was introduced to the arguement against planetary status for Pluto known as "Multiplicity". This, quite honestly, is a joke. That's like saying that the 40 plus billion objects in the Milky Way aren't stars because there's just too many of them in the same place so using the term "star" for all of them is too indistinct to be meaningful. I didn't know whether to laugh or cry. With this argument, some eight planet defenders are straying into the same realm of bogus logic as geocentrists, making up rules more in line with protecting the traditional status quo, than truly and meaningfully creating a definition that can be equally applied to all situations where objects of a given type are found.
As I see the situation, we're now face to face with one of the last spectres of primitive planetary astronomy. The fight to defeat these spectres of ignorance started centuries ago with the acceptance of Earth as one of nine originally known planets, not something unique and special in the universe.
Then with the discovery of pulsar planets in 1992, and the discovery of 51 Pegasus B in 1995, we've learned definitively that the nine planets in this system were no longer the only ones out there. The planets we've discovered elsewhere have also shaken the very foundation of what we believed a "normal" planetary system would be in form and structure (from post supernova pulsar planets to "normal" main sequence stars with planets with migrating gas giants practically hugging the central star to red giants with protoplanetary disks to multistar systems with stably orbitting planets).
Now back here at home, we're seeing the existance of candidate planets with orbits that are not the comfortably near circular, ecliptically aligned type we've known since antiquity. Now that new discoveries are forcing us to put some meat on the skeletal definition that for so long was "understood" faces us with the possibility that our handhold on tradition may have been a mistake. Its interesting to me that some astronomers seem to be so fascinated with the wildly variable nature of other star systems, yet to challenge the regularity of this system is somehow offensive. Where is it written that the star system orbitting Sol MUST be this rigidly defined, clockwork mechanism into which the elevation of bodies to planetary status that defy that mechanistic tranquilty are forbidden? Why must we write special rules for defining the term planet that absolutley respect the "traditional" structure of this star system as a baseline? What's the crime in shaking up the family a little? Because its definitely a crime NOT to shake it up through the use of artificial rules a geocentrist could take a shine to. Its the same mindset, the difference being that instead of just the Earth being special, its the Earth and these seven other massive bodies that exist here in the Solar system with it that are special, and its just as wrong.
There she is, folks, the official standing of Doodler on what makes a planet. The bull's eye's drawn for you, have at it.
