Equilibrium temperature (T_eq) vs planet density (ρ_p) diagram for the He I detections (blue circles) and non-detections (red crosses). The black line (drawn by-eye) indicates the hinted upper boundary for the He I detections. The coloured dashed vertical lines indicate the Earth’s density (ρ_⊕; orange line at ~5.5 g cm⁻³), and a representative density where the puffy planet population starts (ρ_puffy; blue line at ~1.3 g cm⁻³; Luque & Pallé 2022). All known planets with T_eq and ρ_p determined with a precision better than 50% are marked with grey dots (data from NASA Exoplanet Archive).