Planet Definition Proposal
The IAU definition of 'planet' needs improvement because it is neither quantitative nor general. I proposed a simple criterion that can be used to quantify, generalize, and simplify the definition. This criterion makes it possible to immediately classify 99% of all known exoplanets. All 8 planets and all classifiable exoplanets satisfy the criterion.
Planet test applied to solar system bodies
The figure below shows the criterion applied to the solar system (objects above the bold line are planets according to this criterion):
Planet test applied to the putative Planet Nine
Would Planet Nine pass the planet test? The answer is yes.