Here is another planet definition:

On the low-mass planethood criterion

Authors: Bojan Pecnik, Christopher Broeg

Comments: 13 pages, 1 table, submitted to journal of Planetary and Space Science

We propose a quantitative concept for the lower planetary boundary, requiring that a planet must keep its atmosphere in vacuum. The solution-set framework of Pecnik and Wuchterl (2005) enabled a clear and quantitative criterion for the discrimination of a planet and a minor body. Using a simple isothermal core-envelope model, we apply the proposed planetary criterion to the large bodies in the Solar System

http://arxiv.org/abs/astro-ph/0608367

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There seems to be some confusion between an objective, quantitative definition, and a "scientifically-motivated" one. The seeming need for the latter is driving people to these tortured definitions involving atmospheres, hydrostatic equilibrium, barycenters, domination of neighborhoods, etc. Maybe I've been consorting with engineers for too long, but what would be so "unscientific" about simply setting a size limit so that Pluto stays a planet and anything larger would be one, and anything smaller wouldn't?

I'm reminded of the effort to define the meter as a fraction of the Earth's circumference before it was actually possible to measure the Earth to the needed level of accuracy. Much effort was spent but in the end the meter doesn't really match the Earth (and they knew this when the definition was made) -- see THE MEASURE OF ALL THINGS by Ken Alder, very interesting read.

I'm also wondering who goes to these IAU meetings. Wouldn't the real scientists stay home and do productive work?

I'm glad you asked that question, as I have obtained the following photograph of their secret deliberations that may shed some light on the subject:

Glad you explained what it was, I might have thought it was the very first UMSF bar-b-q picture otherwise...

I was going to suggest the same thing (or some nice round number for diameter that retains Pluto) but that seemed too simple to me. After all, who am I to argue with the IAU?

Note also that TPS has a couple of new pieces on this issue. Jim Bell "glogging" for Emily and Dava Sobel on Planetary Radio.

OK I've had enough, clearly we need an omipotent dictator to decide on this and conveniently we have a good one here.

Doug - tell us what a planet is and we'll go forth and make it so.

Ooo - don't let me decide - at least these ideas get some support - I'd come up with something that'd piss EVERYONE off

At least, I'd try to

Doug

A Planet is an object less than 13 times the mass of Jupiter and is large enough to have it shape determined by hydrostatic equilibrium (round or nearly round). no further qualifications.

Why such an esoteric boundary? Why not something nice and simple - average equatorial G of 1 m/s^2 or higher. Or average diameter of 2000 KM or larger?

The difference between minor planet and planet is admittedly arbitrary. Why not use something simple like radius, or gravitational pull to mark the cutoff?

And within that group, planets are classified as "Colossal" (Jupiter, Saturn), "Jumbo" (Uranus, Neptune), "Extra large", "Large" (Venus, Earth), "Medium" (Mercury, Mars, Ganymede, Callisto, Titan), "Small" (Moon, Io, Europa, Triton, Pluto, 'Xena'), and 'Peewee' (everything else).

Note that "Extra large" has no members in our Solar system... hmm... I think I see a candidate for "scientific division between planets and non-planets" there. Any extrasolar planets detected yet with masses somewhere between Uranus and Earth?

OK all jokes aside I'm with you on this one. The barycenter thing may have some convoluted logic to it but "planetness" has to be something inherent in a thing and cannot be dependant on its environment\situation. It is silliness in the extreme to make Charon a planet but not Titan, Luna or Io.

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Sorry, hydrostatic equilibrium. Post fixed.

The IAU executive commitee just needs to grow a pair and state that "Everything above XX km in diameter is a planet, unless it has participated in nuclear fusion." and just tell everyone else to screw off.