Younger Stars Host Denser Rocky Planets

Interior compositions are key for our understanding of Earth-like exoplanets. For example, the composition of the core can influence the presence of a magnetic dynamo or the strength of gravity on a planet. However, such measurements are extremely challenging for small exoplanets, and existing data suggest a wide diversity in planet compositions. Hitherto, only certain chemical elements in the photospheres of the host stars have been considered as a tracer to explain the diversity of exoplanet compositions. Here we present a homogeneous analysis of stars hosting rocky exoplanets, revealing a correlation between rocky exoplanet compositions and the ages of the planetary systems, with denser rocky planets being found around younger stars. This suggests that the compositional diversity of rocky exoplanets can be directly linked to the ages of their host stars. We interpret this to be a result of chemical evolution of stars in the Milky Way, which modifies the material out of which stars and planets form. This therefore implies that rocky planets which form today may have different formation conditions, and thus different properties than planets which formed several billion years ago, such as the Earth.