  Neutron-rich isotopes with masses near that of iron are produced in type Ia
and II supernovae. Traces of such nucleosynthesis are found in primitive
meteorites in the form of variations in the isotopic abundance of 54Cr, the
most neutron-rich stable isotope of chromium. The hosts of these isotopic
anomalies must be presolar grains that condensed in the outflows of supernovae,
offering the opportunity to study the nucleosynthesis of iron-peak nuclei in
ways that complement spectroscopic observations and can inform models of
stellar evolution. However, despite almost two decades of extensive search, the
carrier of 54Cr anomalies is still unknown, presumably because it is
fine-grained and is chemically labile. Here we identify in the primitive
meteorite Orgueil the carrier of 54Cr-anomalies as nanoparticles, most likely
spinels that show large enrichments in 54Cr relative to solar composition
(54Cr/52Cr ratio >3.6xsolar). Such large enrichments in 54Cr can only be
produced in supernovae. The mineralogy of the grains supports condensation in
the O/Ne-O/C zones of a type II supernova, although a type Ia origin cannot be
excluded. We suggest that planetary materials incorporated different amounts of
these nanoparticles, possibly due to late injection by a nearby supernova that
also delivered 26Al and 60Fe to the solar system. This idea explains why the
relative abundance of 54Cr and other neutron-rich isotopes vary between planets
and meteorites. We anticipate that future isotopic studies of the grains
identified here will shed new light on the birth of the solar system and the
conditions insupernovae.
