Contemporary formation of early Solar System planetesimals at two distinct radial locations

The formation of planetesimals is expected to occur via particle-gas instabilities that concentrate dust into self-gravitating clumps. Triggering these instabilities requires the prior pile-up of dust in the protoplanetary disk. This has been successfully modelled exclusively at the disk's snowline, whereas rocky planetesimals in the inner disk were only obtained by assuming either unrealistically large particle sizes or an enhanced global disk metallicity. However, planetesimal formation solely at the snowline is difficult to reconcile with the early and contemporaneous formation of iron meteorite parent bodies with distinct oxidation states and isotopic compositions, indicating formation at different radial locations in the disk. Here, by modelling the evolution of a disk with ongoing accretion of material from the collapsing molecular cloud, we show that planetesimal formation may have been triggered within the first 0.5 million years by dust pile-up at both the snowline (at ~5 au) and the silicate sublimation line (at ~1 au), provided turbulent diffusion was low. Particle concentration at ~1 au is due to the early outward radial motion of gas and is assisted by the sublimation and recondensation of silicates. Our results indicate that, although the planetesimals at the two locations formed about contemporaneously, those at the snowline accreted a large fraction of their mass (~60%) from materials delivered to the disk in the first few tens of thousands of years, whereas this fraction is only 30% for the planetesimals formed at the silicate line. Thus, provided that the isotopic composition of the delivered material changed with time, these two planetesimal populations should have distinct isotopic compositions, consistent with observations.