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

Abstract

The formation of planetesimals is expected to occur via particle-gas instabilities that concentrate dust into self-gravitating clumps1–3. Triggering these instabilities requires the prior pile-up of dust in the protoplanetary disk4,5. This has been successfully modelled exclusively at the disk’s snowline6–9, whereas rocky planetesimals in the inner disk were only obtained by assuming either unrealistically large particle sizes10,11 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 states13,14 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 cloud16–18, 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 silicates20,21. 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. An evolutionary model of the solar protoplanetary disk that includes the decrease of its viscosity with time and the accretion of gas from the interstellar medium shows that planetesimals formed simultaneously in two locations: at the water snowline (~5 au) and at the silicate sublimation line (~1 au), explaining the observed isotopic dichotomy of iron meteorites.