Mass-radius diagram of all small planets (left) and USP planets (right) with masses and radii constrained to a precision better than 35 and 20%, respectively. The colour code represents the measured equilibrium temperatures. In both panels, we represent different composition curves for rocky planets derived in Marcus et al. (2010); Zeng et al. (2016, 2019); Turbet et al. (2020). The black arrow indicates the 3σ upper limit of K2-157 b’s mass: 2.3 M_⊕. Data: NASA Exoplanet Archive (24/03/2025). This plot was generated with mr-plotter (https://github.com/castro-gzlz/mr-plotter; Castro-González et al. 2023).