Three end-member structures for each of the TRAPPIST-1 planets, assuming constant densities for ice I and liquid water (white) with ρ_I = 1000 kg m⁻³, high-pressure ice polymorphs (ρ_hpp = 1300 kg m⁻³, light gray), rock (ρ_r = 5000 kg m⁻³, dark gray) or low-density rock as indicated in Table 2 (ρ_r = 3300 kg m⁻³, dark gray, see Sect. 2.2), and iron (ρ_Fe = 12 000 kg m⁻³, black), corresponding to the maximum density permitted by orbital solutions (left), the mean density (middle), and lowest density (right). The sizes of the planets and thicknesses of ice polymorph, rock, and iron layers are quantitatively accurate; but in some cases, the thickness of the outer water/ice I layer has been expanded slightly to show detail.