One of the most puzzling properties of the solar system is the high abundance at its birth of Al-26, a short-lived radionuclide with a mean life of 1 Myr. Now decayed, it has left its imprint in primitive meteoritic solids. The origin of Al-26 in the early solar system has been debated for decades and strongly constrains the astrophysical context of the Sun and planets formation. We show that, according to the present understanding of star-formation mechanisms, it is very unlikely that a nearby supernova has delivered Al-26 into the nascent solar system. A more promising model is the one whereby the Sun formed in a wind-enriched, Al-26-rich dense shell surrounding a massive star (M > 32 M-circle dot). We calculate that the probability of any given star in the Galaxy being born in such a setting, corresponding to a well-known mode of star formation, is of the order of 1. It means that our solar system, though not the rule, is relatively common and that many exo-planetary systems in the Galaxy might exhibit comparable enrichments in Al-26. Such enrichments played an important role in the early evolution of planets because Al-26 is the main heat source for planetary embryos.
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