  Short-lived radioisotopes, in particular 26-Al and 60-Fe, 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 disc 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 26-Al and 60-Fe 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.
