Left: mass-radius diagram for planets around stars with estimated ages below 1 Gyr. The colour-code represents the estimated median stellar age and the K2-233 planets are marked as violet square symbols. Iso-density lines corresponding to different compositions from Zeng et al. (2019) are shown as solid traces while the dashed lines correspond to iso-densities of 0.3, 1.33, 5.3, 10 g⋅cm⁻³ (from top to bottom). Right: planetary systems with ages younger than 1 Gyr against the semi-major axis of the planets. Weshow systems with one (grey symbols), two (red), three (pink), and four (green) known planets. Symbol sizes correspond to the planet size and non-transiting planets are represented as open circles. The young systems with three or more planets known (including K2-233) are also labeled.