Boundaries between ice/rock, ice/rock/iron, and rock/iron compositions as a function of planetary mass and radius for the four innermost TRAPPIST-1 planets. Star indicates the location of the best-fit mass and radius from Wang et al. (2017). For TRAPPIST-1b (top left), gray regions show pairs of mass and radius for which the planet has a four-layer structure, similar to the “mean density” case illustrated in the top row in Fig. 3. For TRAPPIST-1c (top right), the vast majority of pairs of mass and radius values indicate a rock/iron composition. For planets d and e (bottom left and right), an ice/rock composition is most likely; structures completely devoid of H₂ O are indicated only if each planet has a mass close to the highest permitted value and a radius close to the minimum permitted value.