Mass-radius relations for the upper (orange line) and lower boundary (blue line) given by Eqs. (14) and (15), respectively. We present different curves at fixed density (left). While the upper boundary is consistent with a very low mean planetary density (≈0.1 g/cm³), the lower boundary comprises different values spanning from ≈1 g/cm³ to ≈2g/cm³. We also present different mass-radius curves for different planetary compositions (right).