Interior difference between a pair of adjacent planets. In this illustrative example, planet P₁ lies closest to the blue curve (distance L₀) whereas planet P₂ is closer to the red curve (distance L₁). The blue curve, corresponding to 30% Fe and 70% MgSiO₃, is chosen as the reference curve for the pair. This means if P₂ had the same bulk composition as P₁, P₂’s bulk density should be given by the blue curve corresponding to its mass (red circle). I_p1 and I_p2 (shown in the thin red line) are measured as the vertical gap distance between planet P₁ and planet P₂ to the reference curve at their respective masses. This figure shows LTT 1445A b and c planets, and they are separated by an interior distance I = 0.053.