Earth and other terrestrial planets formed from inner solar system material, study finds

Two fundamentally different processes of rocky planet formation exist, but it is unclear which built Earth and the other terrestrial planets in the solar system. They were formed either by collisions between planetary embryos of the inner solar system, or by the accretion of tiny “pebbles” drifting towards the sun of the outer solar system. New research shows that the isotopic compositions of Earth and Mars are governed by two-component mixing among inner solar system materials, including materials from the innermost disk not sampled by meteorites, whereas the contribution of materials from the outer solar system is limited to a few percent by mass.

Possible scenarios of terrestrial planet formation: In the classic Wetherill-type oligarchic growth model, terrestrial planets formed by mutual collisions between Moon-to-Mars-sized planetary embryos after gas disk dissipation and accretion only a small fraction of the CC planetesimals, which were dispersed inward during Jupiter’s growth and/or putative migration; alternatively, terrestrial planets may have formed during the lifetime of the gas disk by effectively accreting “pebbles” from the outer solar system, which drift toward the sun through the disk due to the gas drag; the two models differ in the amount of material from the outer solar system accreted by terrestrial planets, which can be quantified using nucleosynthetic isotopic anomalies. Image credit: Burkhardt et al., doi: 10.1126/sciadv.abj7601.

“Our data refute a pebble accretion origin of terrestrial planets, but are consistent with collisional growth of inner solar system embryos,” said Dr Jan Render, a researcher in the Division of Nuclear and Chemical Sciences at the Lawrence. Livermore National Laboratory.

“This low fraction of outer solar system material on Earth and Mars suggests the presence of a persistent dust and drift barrier in the disc and highlights the specific pathway of formation of rocky planets in the solar system.”

Determining which of the two processes governed the formation of the terrestrial planets of our solar system is crucial for understanding the architecture and dynamic evolution of the solar system, and for placing the formation of planets in the solar system in the context of the general processes of formation of planets, such as those observed in exoplanetary systems.

The amount of material from the outer solar system accreted by terrestrial planets can be determined using nucleosynthetic isotopic anomalies.

These arise from the heterogeneous distribution of presolar material within the solar protoplanetary disk and provide a record of the legacy of a planet’s building materials.

These isotopic anomalies help distinguish between non-carbonaceous (NC) and carbonaceous (CC) meteorites, which are generally assumed to represent planetary bodies that accreted into the inner and outer solar system, respectively.

Dr. Render and colleagues used recent observation of correlated isotopic variations among NC meteorites to show that Earth and Mars incorporated unsampled material among meteorites, determined the provenance and isotopic composition of this planetary building material lost and used this information to estimate the amount of CC material accreted by Earth and Mars.

“The isotopic composition of this type of building material, as inferred by our computer simulations, implies that it must come from the innermost region of the solar system,” said Dr Christoph Burkhardt, a researcher at the Institut für Planetology from the University of Münster.

“Since bodies from such close proximity to the Sun have almost never been dispersed in the asteroid belt, this material has been almost completely absorbed into the inner planets and therefore does not occur in meteorites.”

“It is, so to speak, a ‘lost building material’ to which we no longer have direct access today,” said Dr Thorsten Kleine, a researcher at the Institut für Planetologie at the University of Münster and the Max Planck Institute for the Solar System. Research.

“The startling discovery does not change the implications of the study for the theory of planet formation.”

“The fact that Earth and Mars apparently contain mostly inner Solar System material correlates well with the formation of planets from collisions of large bodies in the inner Solar System,” Dr Burkhardt said.

The results were published in the journal Scientists progress.


Christopher Burkhardt et al. 2021. Formation of Terrestrial Planets from Lost Materials of the Inner Solar System. Scientists progress 7 (52); doi: 10.1126/sciadv.abj7601

Arline J. Mercier