Mass-density distribution of all confirmed planets from the NASA Exoplanet Archive with mass and radius determinations of better than 20%. The red dots represent TOI-5398 b and c, while the remaining planets mentioned in Table 7 are represented by dots of different colour. We also plot theoretical mass–radius curves for planets of various pure compositions from Zeng et al. (2019, online at https://lweb.cfa.harvard.edu/~lzeng/planetmodels.html): solid red indicates a pure-iron core, brown an Earth-like rocky core (32.5% Fe and 67.5% MgSi03), light blue a 100% water world at 1000 K, and blue a 100% cold-hydrogen world.