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

• Published in: Nature astronomy Vol. 6; no. 1; pp. 72 - 79
• Main Authors: Morbidelli, A., Baillié, K., Batygin, K., Charnoz, S., Guillot, T., Rubie, D. C., Kleine, T.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.01.2022; Nature Publishing Group
• Subjects: 639/33/445/3928; 639/33/445/849; Accretion; Astronomy; Astrophysics and Cosmology; Dust; Eddy diffusion; Letter; Meteors & meteorites; Physics; Physics and Astronomy; Sciences of the Universe; Snowline; Sublimation
• ISSN: 2397-3366; 2397-3366
• DOI: 10.1038/s41550-021-01517-7

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 metallicity 12 . 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 compositions 15 , 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 gas 19 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 time 22 , these two planetesimal populations should have distinct isotopic compositions, consistent with observations 15 . 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.