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Excerted from the International Astronomical Union meeting newsletter:
Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation `planets'. The word `planet' originally described `wanderers' that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.
The IAU therefore resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way: (1) A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. (2) A dwarf planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite. (3) All other objects orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".
footnotes:
planet: The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn,
Uranus, and Neptune.
shape: An IAU process will be established to assign borderline objects into
either dwarf planet and other categories.
objects(3): These currently include most of the Solar System asteroids,
most Trans-Neptunian Objects (TNOs), comets, and other small bodies.
The IAU further resolves: Pluto is a dwarf planet by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.
Before I present my thoughts let me state a couple of things for the record. First, I am an IAU member but could not attend for reasons of schedule conflicts and lack of funding to pay for travel and time to be in Prague. I have tried to be a part of the official discussion but my offers to help have been ignored. Thus, my opinions have not been incorporated into the process in any way that I can see. Second, my opinions on the planet definition question have nothing to do with the "status of Pluto" despite my career-long pursuit of understanding Pluto. As long as this issue has been in the public eye I have steadfastly refused to argue the planethood of Pluto. I have grudgingly taken on the role of discussing and debating the concept of planet. I don't really consider the issue that interesting but it comes up in every public talk I give, every interview I give for print, radio, and TV, and dominates the email I see year after year. In thinking about this issue I have had to revise my opinion more than a few times as a consequence of continued thought, discussions, and debate. My stance on this issue is a result of a decade long process. The IAU process does not appear to have spent nearly enough time while also involving the scientific community. Instead, the resolution that passed appears to be a consequence of a strong and vocal minority that succeeded in confusing enough of those in attendance to pass the resolution. Furthermore, the concept that a vote validates a scientific truth or concept is misguided. The facts or truths we discover in science come from thinking about the problem and developing a consensus through the scientific method that guides our research. Perhaps the lack of consensus really tells us that we don't know enough yet to define what a planet is.
Regardless, a new resolution from the IAU is before us. Part of this definition is similar to my own definition. 1b) matches my own proposal though they leave out any discussion of the upper limit to the size of a planet. Beyond this point we do not agree.
The first thing that bothers me about this resolution is that it explicitly refuses to address the issue of planets outside our solar system. Surely we can write a definition that could work for objects elsewhere in the universe. The lack of generality in this resolution is perhaps my biggest problem. It seems to me that the concept of planethood shouldn't depend so much on where it is. Similarly, I think that planethood shouldn't depend that much on how it got to be where it is, either.
The concept discussed in 1c) is an interesting scientific question but is too complicated and too hard to prove to have merit in a definition such as this. For instance: can it be proven that the Earth was, in fact, responsible for clearing out it's orbit zone? I know some think this is likely but what if Jupiter or the Sun caused more clearing near the Earth that was done by the Earth. Does that make it not a planet? Another example: What if we someday find an object as big as Neptune in the distant outer reaches of the solar system? We don't think there was ever a lot of material there and such an object would have to have been ejected to such a distant locale. An object like that couldn't have cleared its own zone since there was nothing to clear. Another way to state my objection is to note that the same object, if put in a different location around the Sun, would not be categorized in the same way. This, to me, is the very epitome of a bad definition.
Marc W. Buie, Southwest Research Institute, September 2006