Terrestrial planet formation from a ring of planetesimals (Invited)

The formation of terrestrial planets from a ring of planetesimals was initially proposed by Hansen (2009) as a solution to the planet’s mass distribution pattern, namely with two small planets at the edges of the  semi major axis distribution and two major ones at the center, rather than a system of planets with uniform masses.

An early attempt to explain why planetesimals should have been concentrated into a ring invoked a specific inward-then-outward migration pattern of Jupiter, dubbed the Grand Tack (Walsh et al., 2011). However, modern models of planetesimal formation predict that the first planetesimals naturally form in rings (Drazkowska et al., 2016; Morbidelli et al., 2022; Izidoro et al., 2022; Marchall and Morbidelli, 2024). These works support the fundaments of the ring formation model.

Whereas early simulations of terrestrial planet formations assumed the existence of a narrow ring of planetesimals and planetary embryos after gas removal (Hansen et al., 2009; Nesvorny et al., 2021), more recent simulations start from a ring of sole planetesimals (super-particles) and simulate the growth of planetary embryos during the gas-disk stage (Woo et al. 2023, 2024). This phase leads to the spreading of the ring and the formation of numerous mini-planets, unless the radial profile of the gas surface density distribution peaks itself near 1 au (Broz et al., 2021).  In these conditions, it is not necessary that the ring of planetesimals is located at 1 au.

Although the terrestrial planets formed from almost exclusively non-carbonaceous material from the inner Solar system, the chemistry of the Earth’s mantle, in particular the moderately siderophile element depletion pattern and FeO and SiO2 abundances, reveal that our planet should have started its accretion by incorporating reduced material, akin (in terms of oxygen fugacity) to Aubrites and enstatite chondrites, and incorporated later more oxidized material, akin to the known non-carbonaceous iron meteorite parent bodies or of ordinary chondrites (Rubie et al., 2011). This cannot be explained if the terrestrial planets formed from a narrow ring of planetesimals (Dale et al., 2025 and at this conference).

New, still unpublished models will be presented in this review, where the terrestrial planets form from two rings. The first ring, located near 0.5 au, is due to the silicate sublimation line and comprises reduced planetesimals, where the second one, located near 1.5 au, is made of more oxidized planetesimals. The growth of planetary embryos from each of these rings, and their merging near 1 au, gives the best reconstruction of the orbital and mass distribution of the terrestrial planets. The existence of the second ring is justified in Goldberg et al., this conference).

In conclusions, the model of terrestrial planet formation from a ring of planetesimals is definitely more complicated than originally envisioned by Hansen (2009) but is now converging towards a more complex model that can explain both the physical, chemical and isotopic properties of the planets. Moreover, the new model is consistent with the structure of the asteroid belt and with our understanding of the evolution of the giant planets.

Acknowledgment: This work is supported by the ERC grant N. 101019380 HolyEarth

References.

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Dale, K.I., Morbidelli, A., Nathan, G., Woo, J., Nesvorný, D. and Rubie, D.C. Oxidation Constraints on Terrestrial Planet Formation from a Ring. Icarus, submitted.