Compositions of rocky exoplanets The interior compositions of small rocky exoplanets cannot be observed directly but are expected to relate to the composition of the host star. Adibekyan et al . analyzed a sample of rocky exoplanets, inferring the planets’ iron fractions by combining their masses and radii with an interior structure model. The iron fractions of the host stars were calculated from stellar elemental abundances. The two iron fractions, that of the planets and that of the stars, correlate with each other, but the slope is steeper than 1, indicating that planet formation processes modify the compositions of rocky planets. —KTS