  The current IAU definition of "planet" is problematic because it is vague and
excludes exoplanets. Here, we describe aspects of quantitative planetary
taxonomy and examine the results of unsupervised clustering of Solar System
bodies to guide the development of possible classification frameworks. Two
unsurprising conclusions emerged from the clustering analysis: (1) satellites
are distinct from planets and (2) dynamical dominance is a natural organizing
principle for planetary taxonomy. To generalize an existing dynamical dominance
criterion, we adopt a universal clearing timescale applicable to all central
bodies (brown dwarfs, stars, and stellar remnants). Then, we propose two
quantitative, unified frameworks to define both planets and exoplanets. The
first framework is aligned with both the IAU definition of planet in the Solar
System and the IAU working definition of an exoplanet. The second framework is
a simpler mass-based framework that avoids some of the difficulties ingrained
in current IAU recommendations.
