Pluto should without a doubt be "grandfathered" in as a planet, and then welcome any newcomer in. After all any "newcomer" has been in our solarsystem longer than humans.

I agree with Frank and will add that anything small than Pluto be called a planetoid.

Anything orbiting an asteroid be classified as orbiting debris of that body. A moon would still be a moon.

This would simplify matters and settle the debate to most everyone's satisfaction.

Perhaps the thing to do is come up with a new set of terms (terrestrials, jovians, asteroids, micros, satellites, gwerkels?) that would be applied to bodies around other stars as well as in our own solar system. The "classical" planets of our solar system that we all know should continue to be called "planets" as a common or colloquial term. However, call the eight large ones by the new term for technical usage and for comparison with similar bodies around other stars as we continue to discover them.

Yes, I also agree with Frank. As well as the fact that Pluto is still classified as a planet in the old text books. But still classified as a planet in my mind. Pluto should be "grandfathered" in as a planet.

On the Naming of Rocks - a colloquial proposal:

I have thought long and hard on this matter with especial reference to the exploitation of these 'lesser bodies'.

Size
0/ Less than 1 metre its Debris/Hazard to Navigation and exempt from any U.N. Asteroid Treaty!

1/ CHUNK max diameter:

2/ HUNK max diameter:

3/ LUMP max diameter:

4/ PLANETISSIMAL 1km+. Irregular. All Planetissimals should be named and memorised by space geeks in Schools!

Note the difference in spelling and pronunciation between Planet*i*ssimal and Planet*e*simal.
Although for most purposes these bodies are identical except that the latter only exist in proto-planetary disks.

5/ PLANETOID a Planetissimal with sufficient mass for differentiation, a roughly spherical form and probably a trace atmosphere. All Planetoids should have schools. (One day!)

6/ WORLDLET any one of the above with an artificial internal biosphere and resident workforce/homesteaders/teacher.

Thus Ceres and Pluto are Planetoids with the potential of being Worldlets.
But both are 'historical' Planets.

Composition
Iceteroid - extractable quantities of water ice and other volatiles
Feteroid - ditto iron, nickel and platinum!
Asteroid - rock (made of star stuff!)

Orbit
Around a Sun (any sun) itsa Planet
Around a Planet itsa Moon
Around a Moon itsa Moonlet
Around (shares) an orbit itsa Co-orbital (aka Trojan)
Around any other lesser body itsa Companion cos Planetoidlet is just silly!
SORTED.

Whoops something went wrong there!

Edit:
1/ CHUNK max diameter: less than 10 metres i.e. big enough for a bootprint, a flag and a claim marker before being towed to the nearest RockProcessor.

2/ HUNK max diameter: less than 100 metres

3/ LUMP max diameter: less than 1km (processed in situ)
Sorry.

The whole problem is that nature has proven that it does not oblige humans by sticking to nice, neat, easily-divisible categories. In stars, we have brown dwarfs right up to the limit where they become red dwarfs. In planets, we get /every possible/ size from

The problem is that we /need/ to find some method of classification. My suggestions are as follows:

Any type of nuclear heat-generation in a body where most of the mass lies above any solid surface = star; Gravitational-only heat generation in such a body = J-class gas giant

Spherical body with solid surface and distinct atmosphere = planet; Spherical body with solid surface and either no atmosphere or extremely teneous atmosphere = planetoid; Any significant body (>100km diameter, longest axis) in orbit around a non-stellar body = moon; Irregular solid body = asteroid.

On the whole, I think that the argument against Pluto is sound - nothing about it really makes it sound like a major system body (planet), especially that eccentric orbit. However, I think it is worth leaving it in just to avoid the rigmarole of having to rewrite all the textbooks.

The Meghar Scale of planetary mass classification works for me. It works for most of the known extrasolar planets as well.

Why not just go metric and get it over with? Everything in the universe is a planet. Once you know how big it is, you pick the right prefix and that's the end of it, everything from attoplanets to teraplanets and beyond. (Yes, this is supposed to be geekhumour.)

Pluto is by definition a planet.

We were taught in school (and dictionary or encyclopedia) that any entity revolving around a star (in this case the Sun) was by definition a planet. Who gave anyone the authority to change the definition of planet only because of it's size? That would open up all types of controversy of definitions changed on ANYTHING because of size.