Mass-radius diagram for exoplanets for which the mass and radius have been both measured with a relative error better than 30%. The location of the K2-3 planets is emphasized. For K2-3 d we also plot the mass derived from the GP analysis (shaded point in violet), and the value corrected using the result of the simulations described in Sect. 5.1. The curve for bulk density ρ = 3 g cm^-3 is shown in grey passing through the three positions occupied by the K2-3 planets. The planetary data are taken from the NASA exoplanet archive and updated to August 30, 2017.