Did the terrestrial planets of the Solar System form by pebble accretion?OPEN ACCESS
Alessandro Morbidelli, Thorsten Kleine, Francis Nimmo
In press as a Frontier paper in Earth and Planetary Science Letters
“The dominant accretion process leading to the formation of the terrestrial planets of the Solar System is a subject of intense scientific debate. Two radically different scenarios have been proposed. The classic scenario starts from a disk of planetesimals which, by mutual collisions, produce a set of Moon to Mars-mass planetary embryos. After the removal of gas from the disk, the embryos experience mutual giant impacts which, together with the accretion of additional planetesimals, lead to the formation of the terrestrial planets on a timescale of tens of millions of years. In the alternative, pebble accretion scenario, the terrestrial planets grow by accreting sunward-drifting mm-cm sized particles from the outer disk. The planets all form within the lifetime of the disk, with the sole exception of Earth, which undergoes a single post-disk giant impact with Theia (a fifth protoplanet formed by pebble accretion itself) to form the Moon. To distinguish between these two scenarios, we revisit all available constraints: compositional (in terms of nucleosynthetic isotope anomalies and chemical composition), dynamical and chronological. We find that the pebble accretion scenario is unable to match these constraints in a self-consistent manner, unlike the classic scenario.”
Update (27 November 2024): Comment on “Did the terrestrial planets of the Solar System form by pebble accretion?”
Anders Johansen, Peter Olson, Zachary Sharp
“Morbidelli, Kleine & Nimmo (2024) (MKN) recently published a critical analysis on whether the terrestrial planets in the Solar System formed by rapid pebble accretion or by the classical route of multiple giant impacts between planetary embryos after the dissipation of the protoplanetary disc. They arrive at the conclusion that the terrestrial planets did not form by pebble accretion. Although we welcome debate on this topic, we want to emphasize here several points that we disagree on. We will not address in detail every claim made in MKN, but rather stick to four main points. Our conclusion is that pebble accretion remains a viable mechanism to drive significant growth of protoplanets in the protoplanetary disc, with as much as 70% of Earth formed by pebble accretion. This rapid growth phase must nevertheless have been followed by an extended period of collisional growth after the end of the protoplanetary disc phase, likely culminating with the moon-forming giant impact. We emphasize here an important recent result from Sharp & Olson (2023), namely that significant growth by pebble accretion can be reconciled with the Hf-W decay system even for a canonical moon-forming giant impact with a Mars-mass protoplanet – a near-equal mass impactor, as proposed in Johansen et al. (2023), is not necessary. Given that terrestrial planet formation naturally involves both pebble accretion and a combination of small and large impactors, this challenges the very notion of making an either/or distinction between the classical collision model and the pebble accretion model.”
