Internal physics is a good idea; however, I still feel our knowledge of object interiors is too feeble to base classification upon it. We don't really know what's inside the known big KBOs, anything said is pure speculation really. Also, what do you mean by internal physics: grouping objects into different categories of planets, or just calling all objects with some kind of internal differentiation planets? If you do the latter, what would you call a large object like Callisto?

Another problem is that internal physics are extremely hard to analyze or even speculate for extrasolar planets. I really think any definition that is developed should be universal and able to be used for other stellar systems.

This is why I prefer size; it's much easier to gauge for bodies both in our solar system and outside. Neither the lower or higher limits are arbitrary. It's easy, and can be applied everywhere.

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