Constraining the bulk composition. Boxes indicate the 95% confidence level on the retrieval of both the radius and the mass for two synchronous planets (R = 1.5 R_⊕, Msini = 4 M_⊕, and R = 2 R_⊕, Msini = 8 M_⊕), at 0.02 AU of a 0.5 and a 0.8 M_⊙ star, observed with the JWST for 3 different inclinations (2 orbits for computation). The solid, dashed and dotted lines correspond respectively to a pure iron, pure rock, and pure-ice planet according to Fortney et al. (2007).