A study by the TESS-Keck Survey using 17 USP planets found that USP planets predominantly have an Earth-like compositions with iron core mass of about 32% and have masses below runaway accretion.
A research conducted by the TESS-Keck Survey on 17 planets in the USP system revealed that these planets are mostly Earth-like in composition, with a core made up of iron and a mass below the threshold for runaway accretion.