Mass-radius diagram of small exoplanets (with radii ranging from 2⊕4 R_⊙) with precise densities¹ from the PlanetS catalog² orbiting FGK stars. The red triangles correspond to exoplanets with stellar irradiation lower than 30 S_⊕. The composition lines of pure silicates (gray) from Zeng et al. (2016), Earth-like planets (light-brown), and pure iron (brown) from Dorn et al. (2015) are displayed. The purple and orange points represent TOI-815b and c, respectively. Two composition lines (Aguichine et al. 2021) that incorporate both water and terrestrial elements matching the equilibrium temperatures of the two planets are plotted as purple dashed and orange dotted lines.