Formation of the Solar System in two steps

Internal geophysical evolution alters the structure and volatile abundances of forming protoplanets, which govern the long-term fate of rocky worlds. A new two-step accretion scenario offers an explanation for the compositional and chronological split between the inner and outer Solar System.

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About two years ago we reported in Nature Astronomy that volatile degassing from rocky planetesimals during accretion can lead to systematic depletion of water in planetary systems, potentially leaving statistical tell-tale signs in the population of solid exoplanets (Fig. 1). However, it was unclear if this mechanism – by which accreting protoplanets are devolatilized due to outgassing from precursor planetesimals – could match the detailed meteoritic chronology of the Solar System. At the beginning of this year, we reported on our results of this attempt in Science.

The Solar System has two quite different parts: the terrestrial planets of the inner Solar System — Mercury, Venus, Earth, and Mars — are all relatively small and dry, with little water. Even though the vast surface oceans make the Earth appear to be an ‘ocean world’, water makes up only about 0.1 per cent of the whole planet mass. On the other hand, the outer Solar System appears much more volatile-rich, with much larger and wetter planetary bodies — Jupiter, Saturn, Uranus, Neptune, their icy satellites, and the dwarf planets. Comets, for example, have a lot of volatile ices, which presumably accreted onto these bodies during the circumstellar disk phase of the Solar System. This split in system architecture and chemical inventory inspired the classical idea that the planets of the inner Solar System accreted mostly dry, interior to the so-called snow line, where inward-drifting grains rich in water ice sublimate and lose their ice component. Water is usually thought to be delivered to the early Earth much later: when and how exactly remains debated.

Fig. 1: Artist's impression of the water budget dichotomy of solid exoplanets as suggested by the models from Lichtenberg et al. (2019, Nature Astronomy). Planetary systems born in dense and massive star-forming regions inherit substantial amounts of Aluminium-26, comparable to the early Solar System, which devolatilizes planetary building blocks during accretion. Planets formed in low-mass star-forming regions accrete many water-rich bodies and emerge as ocean worlds. Illustration by Thibaut Roger.

Recent complementary evidence from astronomical observations of protoplanetary disks and geochemical laboratory analyses of meteorites provided new important nuances to that storyline. Planetary materials from the inner and outer Solar System show a distinct split in the abundances of isotopes that are formed in stellar nucleosynthesis processes; this observation is often referred to as the ‘isotopic dichotomy’. Radiometric dating of early-formed extraterrestrial materials (i.e., meteorites) suggest that planet formation in the early Solar System started nearly contemporaneously with the accretion of the proto-Sun and preserved some spatial inhomogeneity throughout accretion: individual planetary bodies can be identified by their distinct nucleosynthetic fingerprints! We see potentially comparable evidence for orbital separation during planet formation in the form of dust rings and gaps in extrasolar circumstellar disks observed with the Atacama Large Millimeter/submillimeter Array (ALMA).

Building  on earlier research of our author team, we developed computational models of how the earliest accretion may have proceeded in our own home planetary system. Our numerical experiments show that the relative chronologies of early onset and protracted finish of accretion in the inner Solar System, and a later onset and more rapid accretion of the outer Solar System planets, can be explained by two distinct formation epochs of planetesimals, the building blocks of the planets. Recent observations of planet-forming disks in other planetary systems show that disk midplanes, where planets form, may feature relatively low levels of turbulence. Under such conditions, the interactions between the inward-drifting dust grains and water around the snow line can trigger an early formation burst of planetesimals in the inner Solar System and another one later and further out (Drążkowska & Dullemond 2018, A&A).

The two distinct formation episodes of the planetesimal populations, which further accreted material from the surrounding disk and via mutual collisions, resulted in different geophysical modes of internal evolution for the forming protoplanets. The different formation time intervals of these planetesimal populations mean that their internal heat engine from the radioactive decay of the short-lived isotope Aluminium-26 differed substantially. Inner Solar System planetesimals became very hot, developed internal magma oceans, quickly formed iron cores, and degassed their initial volatile content, which eventually resulted in a dry planet composition. In comparison, outer Solar System planetesimals formed later and therefore experienced substantially less internal heating and thus both limited iron core formation and limited volatile release by degassing from precursor planetesimals. The early-formed and dry inner Solar System and the later-formed and wet outer Solar System were therefore set on two different evolutionary paths very early on in their history (Fig. 2).

Fig. 2: Schematic illustration of our proposed chronology of early Solar System accretion. Nucleosynthetic isotope variability (left) across the disk due to varying composition of infall material is retained by the pile-up of inward-drifting dust grains at the snow line. The formation of two distinct planetesimal populations initiates divergent evolutionary pathways of inner and outer Solar System (right) due to the secular variation of local material composition by dust grain redistribution, internal geophysical evolution – iron core formation and volatile degassing – and dominant mode of planetary growth (Lichtenberg et al. 2021, Science).

The early split in formation epochs and sustained accretion of the outer Solar System planetary population offers a plausible explanation for the apparent dichotomy in supernovae-derived isotopes recorded in meteoritic materials (the aforementioned ‘isotopic dichotomy’). The two planetary populations were formed at different times and orbital locations. During later disk stages, material from the outer disk parts was incorporated into outer Solar System planets but did not substantially contribute to the inner terrestrial planets. Our simulations of planetary migration during the disk phase indicate that these initial orbits are consistent with the terrestrial protoplanets migrating to their present-day orbits within the anticipated lifetime of the solar protoplanetary disk.

Our simulations provide a number of predictions for distinct stages of accretion. For instance, they indicate that the precursor protoplanets of the terrestrial planets switched between accretion dominated by mutual collisions early-on to a phase dominated by accretion of smaller dust-grains, so-called pebbles. The consequences for the accretion of asteroids and the underlying cause for the observed substructure in extrasolar circumstellar disks can be further examined by astronomical observations of planet-forming disks and predicted chemical signatures in meteorites that are samples from asteroids.

Finally, the initial accretion locations of the two planetesimal populations were each at orbits outside of the drifting water snow line. This suggests that the initial inventory of radioactive isotopes in a planetary system, outgassing from planetary precursor bodies, and the geophysical evolution of the protoplanets during accretion are decisive factors to form worlds with clement climates and dry land surfaces. Upcoming surveys of extrasolar planets will start to trace the chemical signatures of the atmospheres — and hence volatile inventories — of rocky exoplanets. These will let us determine whether other systems underwent a similar history as our own Solar System, or formed and evolved very differently, and hence bring us ever-closer to understanding whether our own home world is special or rather the cosmic norm.


Header image: Artist's impression of the formation of the Solar System in two distinct planetary populations. The inner terrestrial protoplanets accrete early, inherit a substantial amount of radioactive Aluminium-26, and hence melt, form iron cores, and degas their primordial volatile abundances rapidly. The outer Solar System planets start to accrete later and further out with less radiogenic heating, and hence retain the majority of their initially accreted volatiles. Illustration by Mark A. Garlick / markgarlick.com
A video explaining the paper's findings in more details can be found at exoplanet-talks.org.

Tim Lichtenberg

SCOL Research Fellow, University of Oxford

I am a research fellow in the department of Atmospheric, Oceanic and Planetary Physics at the University of Oxford. I investigate how terrestrial worlds form and evolve to gain a more comprehensive understanding of how the origins of planets and life are interconnected.