Enhanced Stability in Planetary Systems with Similar Masses

This study employs numerical simulations to explore the relationship between the dynamical instability of planetary systems and the uniformity of planetary masses within the system, quantified by the Gini index. Our findings reveal a significant correlation between system stability and mass uniformity. Specifically, planetary systems with higher mass uniformity demonstrate increased stability, particularly when they are distant from first-order mean motion resonances (MMRs). In general, for non-resonant planetary systems with a constant total mass, non-equal mass systems are less stable than equal mass systems for a given spacing in units of mutual Hill radius. This instability may arise from the equipartition of the total random energy, which can lead to higher eccentricities in smaller planets, ultimately destabilizing the system. This work suggests that the observed mass uniformity within multi-planet systems detected by \textit{Kepler} may result from a combination of survival bias and ongoing dynamical evolution processes.