Why Massive Rocky Planets Tend Toward Hydrogen Atmospheres: Atmospheric Retention Thresholds with a USP Field Theory Overlay

Planetary atmospheres act as mass selectors, preferentially retaining heavier gases on small planets while allowing hydrogen to escape. As planetary mass-to-radius ratio increases, hydrogen becomes increasingly stable and ultimately dominant if supply exists. This paper reviews standard atmospheric retention physics using thermal escape, escape velocity, the Jeans parameter, and atmospheric scale height, and shows why extremely massive yet hydrogen-poor rocky planets are non-generic outcomes. A USP Field Theory overlay reframes atmospheric retention as a resonance-lock condition in Δf corridors: shallow corridors cannot phase-lock hydrogen, while deep corridors phase-lock hydrogen first. Planetary rotation is treated as a modulating perturbation that amplifies escape or mixing without acting as the primary retention mechanism. (Equations presented descriptively: thermal velocity depends on temperature and molecular mass; escape velocity depends on planetary mass and radius; the Jeans parameter compares gravitational binding to thermal energy.)