Two Stages of Planet Formation Could Explain Early Solar System Architecture

This image of the planet-forming disk around the young star V883 Orionis was obtained by ALMA in long-baseline mode. This star is currently in outburst, which has pushed the water snow line further from the star and allowed it to be detected for the first time. Credit: ALMA (ESO/NAOJ/NRAO).

Geochemical analyses of meteorites and other planetary materials provide important clues about the origin and evolution of the solar system. For example, nucleosynthetic isotope anomalies, which reflect primordial reservoirs in the protoplanetary disk, show that solar system materials are sharply divided into two distinct groups, which are commonly interpreted to correspond to the inner versus the outer solar system. One hypothesis that has been suggested to explain the existence of such well-separated groups is that early formation of the giant planet Jupiter (< 1 million years after the beginning of the solar system) created a gap in the protoplanetary disk and prohibited the transfer of the materials through the gap, therefore keeping these reservoirs separate.

A recent study led by Tim Lichtenberg from the University of Oxford used numerical modeling of the evolution of the protoplanetary disk to show that this compositional dichotomy might instead be explained by two stages of planet formation related to the movement of the so-called snow line. The snow line is the boundary region in the protoplanetary disk beyond which water vapor could stably condense to water ice and existed because temperature decreased with increasing distance from the protosun. According to the team’s modeling, during the earliest stages of disk evolution, the snow line moved outward, away from the Sun, as a dense disk formed and heated up viscously. Then, during the later stages of disk evolution, the snow line moved inwards because gas density decreased, and the temperature of the disk was instead controlled by stellar irradiation. Importantly, the formation of planetesimals in these simulations occurs preferentially around the snow line. As a result, this movement of the snow line created two distinct episodes of planet formation that sampled different regions of the disk. If those regions originally had different isotopic compositions, this comprehensive model could explain the isotopic dichotomy and compositional variation observed in solar system objects as a natural result of protoplanetary disk evolution and subsequent heterogenous accretion of materials. It therefore constitutes an alternative to the previously proposed model dependent on the early formation of Jupiter. READ MORE