The Fuzzy Cores of Jupiter and Saturn

New interior models of Jupiter and Saturn suggest that both planets have "fuzzy cores." These cores should be viewed as central regions that are enriched with heavy elements but are not distinct from the rest of the deep interior. These cores may contain large amounts of hydrogen and helium though small pure heavy-element cores may also exist. New measurements along with advanced planetary modeling have revolutionized the way we think about the interiors of giant planets and provide important constraints for planet formation and evolution theories. These developments are also relevant for the characterization of giant exoplanets. Plain Language Summary For decades giant planets were assumed to have distinct heavy-element cores in their deep interiors overlain by an envelope of hydrogen and helium. Measurements from the Juno and the Cassini missions strongly indicate that the internal structure of Jupiter and Saturn is much more complex, including inhomogeneities in composition and "fuzzy" cores. In Jupiter, this dilution might extend to more than one hundred Earth masses, while in Saturn it is less, though both planets have a diluted region that extends to many tens of percent of total radius. In both cases, a small central core composed of pure heavy elements may exist. These structures pose challenges for our understanding of giant planet formation and evolution.