Re-evaluating the Planetary Taxonomy: A Quantitative Framework for a Ten-Planet Solar System

The definition of a ``planet'' adopted in 2006 by the International Astronomical Union (IAU) under Resolution 5A hinges upon an extrinsic orbital dynamical requirement: that a candidate body must have ``cleared the neighborhood around its orbit.'' We demonstrate that this clearing criterion inherently penalizes celestial bodies situated at larger heliocentric distances due to the steep power-law scaling of orbital volume and period (Λ ∝ M² a-3/2). Consequently, dynamical clearing characterizes surrounding orbital space rather than intrinsic physical constitution. Drawing upon the standard syntax of stellar taxonomy—in which a dwarf star remains fundamentally a star—we present an objective, two-parameter geophysical framework. We define a major planet as any non-stellar body orbiting the Sun that satisfies: (1) internal hydrostatic equilibrium driven by self-gravity, and (2) an intrinsic mass threshold of M ≥ 1.0 × 10²² kg. This criterion circumvents arbitrary boundary debates, establishes an empirical mass gap of nearly an order of magnitude, and yields a definitive Ten-Planet Solar System comprising the eight classical planets alongside Pluto and Eris, while systematically categorizing Ceres as a protoplanet and Charon as a planetary-mass satellite.