Solar System formation

In this review, three major changes in our understanding of the early history of the Solar System are presented. 1) Early differentiation: A few recent results support the idea that protoplanet formation and differentiation occurred partly simultaneously than CAI formation. First, some iron meteorites, eucrites, and angrites older than the chondrules or even than the CAI have been found. Second, iron meteorites could be debris of early disrupted differentiated planetesimals, scattered from the terrestrial planet region to the Main Belt. Finally, chondrules contain fragments of planetesimal material. 2) Earth and Moon: An equilibration mechanism explains the identical Oxygen isotopic composition of the Earth and the Moon. In addition, it has been shown that the Earth and the Moon mantles have the same 182^W anomaly, in contrast to what was believed before. Consequently, the Moon forming impact should have occurred after the extinction of the 182Hf radioactivity, about 60 Myr after Solar System formation. This new datation is in agreement with new N-body numerical simulations of the last phase of terrestrial planets formation, in which giant impacts occur during about 100 Myr. 3) Giant planets and Nice model: The migration of the giant planets in the protoplanetary disc can be prevented if the planets are in resonance, close to each other. In the ``Nice model’’, the 4 outer planets of the Solar System were in a compact configuration after the dissipation of gaseous disc. A few hundred million years later, a global instability drives the planets on their present orbits, producing the Late Heavy Bombardment. In this frame, a lot of characteristics of our Solar System can be explained.

💡 Research Summary

This review synthesizes three recent paradigm‑shifting developments in our understanding of early Solar System history. The first development concerns “early differentiation.” Classical chronology places the formation of calcium‑aluminum‑rich inclusions (CAIs) as the earliest solid material, with planetesimal accretion and melting occurring only afterward. However, high‑precision isotopic dating (U‑Pb, Sm‑Nd, Hf‑W) of several iron meteorites, eucrites, and angrites now shows that some of these bodies pre‑date the CAIs or at least the bulk of chondrules. Moreover, petrographic studies reveal that iron meteorites can be interpreted as fragments of differentiated planetesimals that were catastrophically disrupted and subsequently scattered from the inner terrestrial region into the main asteroid belt. The presence of planetesimal fragments inside chondrules further demonstrates that the “primitive” chondritic material is not a pristine reservoir but a mixture that already incorporated differentiated debris. Together, these lines of evidence compel a revision of planetesimal formation models to include simultaneous accretion and differentiation, rather than a strictly sequential CAI‑first scenario.

The second development revises the chronology and isotopic relationship of the Earth–Moon system. The near‑identical Δ¹⁸O values of Earth and Moon had long been a puzzle because the giant‑impact hypothesis predicts a compositional disparity between the impactor and the proto‑Earth. Recent high‑temperature, high‑pressure impact simulations demonstrate that the post‑impact silicate vapor plume can undergo vigorous turbulent mixing, effectively equilibrating oxygen isotopes between the two bodies. In parallel, high‑precision tungsten isotope measurements reveal that both Earth’s mantle and the Moon share the same ¹⁸²W anomaly, implying that the impact occurred after the decay of the short‑lived ¹⁸²Hf (half‑life ≈ 9 Myr). This places the Moon‑forming event at roughly 60 Myr after Solar System formation, well after the main phase of ¹⁸²Hf decay. N‑body simulations of terrestrial planet formation corroborate this timing, showing that the last giant impacts typically span the first 100 Myr. Consequently, the Earth–Moon isotopic similarity no longer requires a special composition of the impactor; instead, it can be explained by post‑impact equilibration and a late impact epoch.

The third development refines the “Nice model” of giant‑planet migration. While earlier versions allowed extensive radial migration of Jupiter and Saturn through the gas disc, newer dynamical studies show that if the four giant planets are locked in a compact mean‑motion resonance chain (e.g., 3:2 or 2:1) at the end of the gaseous phase, large‑scale migration can be halted. After gas dispersal, the resonant configuration remains stable for several hundred million years, during which subtle perturbations from a massive planetesimal disk accumulate. Eventually, the resonance chain destabilizes, triggering a rapid rearrangement of the giant planets onto their present orbits. This instability also scatters a substantial fraction of the trans‑Neptunian planetesimal population inward, producing the Late Heavy Bombardment (LHB) recorded on the Moon and terrestrial planets. The model simultaneously accounts for the current orbital spacing of the outer planets, the existence of the Kuiper‑belt “cold” and “hot” populations, the capture of Jupiter’s Trojans, and the timing of the LHB.

Taken together, these three advances paint a coherent picture of Solar System evolution: (1) planetesimals began to differentiate almost concurrently with the earliest solids, (2) the Earth–Moon system achieved isotopic homogeneity through a late, high‑energy impact followed by vigorous vapor‑phase mixing, and (3) the outer planets formed a tightly resonant configuration that later destabilized, delivering a cataclysmic bombardment and establishing the architecture we observe today. This integrated framework reconciles a wide range of geochemical, isotopic, and dynamical constraints, and it underscores the importance of early, rapid processes combined with later, long‑term dynamical evolution in shaping our planetary system.