Isotopic enrichment of planetary systems from asymptotic giant branch stars

Short-lived radioisotopes, in particular 26Al and 60Fe, are thought to contribute to the internal heating of the Earth, but are significantly more abundant in the solar system compared to the interstellar medium. The presence of their decay products in the oldest solar system objects argues for their inclusion in the Sun's protoplanetary disk almost immediately after the star formation event that formed the Sun. Various scenarios have been proposed for their delivery to the solar system, usually involving one or more core-collapse supernovae of massive stars. An alternative scenario involves the young Sun encountering an evolved asymptotic giant branch (AGB) star. AGBs were previously discounted as a viable enrichment scenario for the solar system due to the presumed low probability of an encounter between an old, evolved star and a young pre-main-sequence star. We report the discovery in Gaia data of an interloping AGB star in the star-forming region NGC2264, demonstrating that old, evolved stars can encounter young forming planetary systems. We use simulations to calculate the yields of 26Al and 60Fe from AGBs and their contribution to the long-term geophysical heating of a planet, and find that these are comfortably within the range previously calculated for the solar system.