Formation of the Solar System in two steps

The earliest history of the Solar System is inscribed in meteorites and the present-day structure of the inner terrestrial and outer gaseous and ice-rich planet population. We developed a new theory that explains our own home planetary system as the result of formation in two distinct episodes. This sets the inner and outer planets on divergent evolutionary paths already during the accretion of the proto-Sun and reinterprets the origins of the Earth’s earliest atmosphere and oceans.

Tim Lichtenberg
Jan 21 · 5 min read

This blog article summarises the research published in Lichtenberg et al., Science (2021), openly accessible at arXiv:2101.08571.

The Solar System has two quite different parts: the inner Solar System features small planets — Mercury, Venus, Earth, and Mars — that are all relatively 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 by mass of the whole planet. The outer Solar System on the other hand appears much more water- and volatile-rich, with much bigger and wetter planets — Jupiter, Saturn, Uranus, Neptune, their satellites, and the dwarf planets. Comets, for example, have a lot of volatile ices, which presumably accreted onto these bodies early in the history of the Solar System. This split in system architecture and chemical inventory long inspired the classical idea that the inner Solar System planets accreted mostly dry, interior to the so-called snow line, where water ice evaporates during the protoplanetary disk phase. Water is then thought to be delivered to the early Earth much later: when and how exactly is debated (see for example dynamical arguments on this topic here).

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, which is often referred to as the ‘isotopic dichotomy’. Together with radiometric dating of early-formed extraterrestrial materials, laboratory analyses of meteorites suggest that planet formation in the early Solar System started nearly contemporaneously with the accretion of the proto-Sun and took place in isolated orbital regions. In other planetary systems — ones that are currently forming — this can look like this.

Based on earlier work of our author team, we developed computational models of how the earliest accretion may have proceeded in our own 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 system 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.

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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. Credit: Mark A. Garlick /

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 therefore limited iron core formation, and volatile release. 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.

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.

From our simulations follow a number of predictions for distinct stages of accretion. For instance, our simulations 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 meteorite families 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 meteorite parent bodies.

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 given planet-forming systems is a first-order requirement to form worlds with clement climates and dry land surfaces. If there are no radioactive isotopes, then this may lead to the formation of very volatile-rich planets, so called ‘ocean worlds’, potentially like scaled-up versions of the Jovian moon Ganymede in our Solar System. Upcoming surveys of extrasolar planets will start to trace the chemical signatures of the atmosphere — and hence volatile inventory — of rocky exoplanets analogous to Earth. These will help to decide whether other systems may have undergone a similar history as our own Solar System, or formed very differently, and hence bring us ever-closer to understanding whether our own home world is special or rather the cosmic norm.

This research was supported by funding from the Simons Collaboration on the Origins of Life, the Swiss National Science Foundation, and the European Research Council.

A video summary of the research can be found here:

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