Dependence of mutual separation of adjacent planets on their average sizes (top) and average masses (bottom). Observed exoplanetary populations (CKSM by W18) show a negative correlation between average sizes and mutual separation of adjacent planets, which is well reproduced here (top left). However, this negative correlation arises from the dependence of mutual Hill radii on the 1∕3 power of planetary masses. This is shown by the dashed line in the plots on the right. The vertical line marks the Hill stability criterion from Chambers et al. (1996). Points on the right side of this line are Hill stable ().