Mass-period (upper panel) and mass-radius (lower panel) diagram for USP planets (P_orb < 1 day, R_p < 2 R_⊕) with measured masses. The solid green circle marks the position of K2-141 b. USP planets in the literature are marked with blue squares. The composition models from Zeng et al. (2016) are displayed with different lines and colors. The Earth-like density curve is also shown with a dot-dashed black line.