What controls the bulk iron content of rocky planets?
(2025)
Authors:
Claire Guimond, Oliver Shorttle, Philipp Baumeister, Raymond Pierrehumbert
Abstract:
Iron is a powerful element shaping rocky planets. The bulk iron content of a planet exerts a first-order control on its interior structure, of fundamental importance to geodynamic processes. Across the rocky planets and dwarf planets in the solar system, bulk iron contents vary considerably, appearing to correlate with orbital distance, and possibly the Sun’s magnetic field strength (McDonough & Yoshizaki, 2021). Potentially-rocky exoplanets show an even greater spread in bulk density and hence inferred bulk iron content. Such exoplanet censuses have begun to give us access to cosmic-scale statistics. We build on McDonough & Yoshizaki (2021) to present a tentative, positive trend between rocky exoplanets’ iron contents and the energy they receive from their host star (instellation). Previous studies have searched for such a trend in iron content with other factors; in particular, with host star iron abundance, as such a link would be evidence for a planet-star compositional connection. If planet bulk iron content is also affected by disk processes, then any other trends would become more complicated to interpret. We use our results to address exoplanet bulk compositional diversity, including the formation of super-Mercuries, and discuss potential implications of high iron contents on broader planet evolution.McDonough, W. F., & Yoshizaki, T. (2021). Terrestrial planet compositions controlled by accretion disk magnetic field. Progress in Earth and Planetary Science, 8, 39.