Mass−radius diagram for planets with masses M_p < 25 M_⊕ and radii R_p < 6 R_⊕, as retrieved from the catalogue for transiting planets TEPCat (available at http://www.astro.keele.ac.uk/jkt/tepcat/; Southworth 2011). Planets whose masses and radii are known with a precision better than 25% are plotted with grey circles. Composition models from Zeng et al. (2016) are displayed with different lines and colours. The red circle marks the position of HD 219666 b. Planets closer in mass to HD 219666 b are labelled.