Mass–radius diagram for small planets (R_P < 4 R_⊕), as retrieved from the catalogue for transiting planets TEPCat (Southworth et al. 2011). Planets whose masses and radii are known with a precision better than 22% are plotted with grey circles. The solid red circle marks the position of HD 93963 A c. Planets in the literature are marked with grey circles. Composition models from Zeng et al. (2016) are displayed with different lines and colours.