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Abstract

The formation of planetesimals is expected to occur via particle-gas instabilities that concentrate dust into self-gravitating clumps^1-3. Triggering these instabilities requires the prior pile-up of dust in the protoplanetary disk^4,5. This has been successfully modelled exclusively at the disk's snowline^6-9, whereas rocky planetesimals in the inner disk were only obtained by assuming either unrealistically large particle sizes^10,11 or an enhanced global disk metallicity¹². 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^13,14 and isotopic compositions¹⁵, 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^16-18, 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 gas¹⁹ and is assisted by the sublimation and recondensation of silicates^20,21. 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 time²², these two planetesimal populations should have distinct isotopic compositions, consistent with observations¹⁵.