Mass-radius diagram of exoplanets with M_p ≤ 10 M_⊕ and R_p ≤ 4R_⊕ known with a precision better than 30%. The grey points are all the known exoplanets as of June 19, 2023, taken from the NASA Exoplanet Archive (2023). The black dashed line is the iso-density curve for the value we derived for TOI-561 b. The mass of TOI-561 b is taken from Brinkman et al. (2023) and the radius from the present work. Also shown are the theoretical models for various chemical compositions computed by Zeng et al. (2019). The mass and radius of 55 Cnc e, K2-141 b, and CoRoT-7 b are taken from Bourrier et al. (2018), Malavolta et al. (2018), and John et al. (2022).