Contemporary formation of early solar system planetesimals at two distinct radial locations

TL;DR

This study models early solar system disk evolution, showing planetesimals could form at both the snowline and silicate sublimation line within the first 0.5 million years, explaining compositional differences observed in meteorites.

Contribution

It demonstrates that planetesimals can form at multiple radial locations early on, driven by dust pileup and gas dynamics, expanding understanding beyond the snowline-only formation models.

Findings

Planetesimals formed at 1 au and 5 au within 0.5 million years.

Mass fraction from early material is higher at the snowline (60%) than at the silicate line (30%).

Distinct isotopic compositions are expected due to different formation histories.

Abstract

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 pileup of dust in the protoplanetary disk. Until now, this has been successfully modeled exclusively at the disk's snowline, whereas rocky planetesimals in the inner disk were obtained only 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 modeling 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 pileup at both the snowline (at approximately 5 au) and the silicate sublimation line (at approximately 1 au), provided turbulent diffusion was low. Particle concentration at approximately 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 (approximately 60 percent) from materials delivered to the disk in the first few 10,000 yr, whereas this fraction is only 30 percent 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.