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History shows that every time we think we understand our universe or even our solar system, some new discovery comes up which makes us completely rethink everything, because we were previously being a little simplistic or narrow. I think there are shades of that here - we've looked for a definition to include the classical 8 planets and leaves the others out. Pluto was always controversial.
Several problem appear here ;
(1) "Clear the neighbourhood around it's orbit". How vague is that? How far is one's neighbourhood? And how large do surrouding objects need to be before that neighbourhood is deemed "unclear"? Asteroids? Boulders? Pebbles? A few specks of dust?
I'm also assuming that things in orbit of the planet itself (eg moons, rings) are being excluded from being cleared since they are trapped by the planet itself.
Does this mean that if some event happened which brought some clutter into Earth's path of orbit that Earth would be demoted from planet status?
(2) "has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape" - I read this to mean that a round rock the size of my fist will be a planet if it's orbit is all alone.
Different materials require different pressures to bring them into a round shape. A small blob of water will very quickly form into a sphere. If it's "neighbourhood is clear", then it's a planet.
I think I'm missing some key pieces of knowledge about what these rules mean. Because it's all too easy to think up dozens of scenarios which would appear to break the definition.
I'm left a little unsatisfied by all this. While I completely bow to the superior astromonical knowledge of those concerned, on the surface this appears to be an extremely vague definition which has the hallmarks of needing to be updated in the future when something new forces us to confront the narrowness of our definition. Perhaps not in our solar system, but sometime we are going to start getting more and detail about other solar systems. I am convinced that our understanding is going to be completely blown apart as we learn more and more.
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