Terrestrial planet formation during giant planet formation and giant planet migration

I. The first five million years

¹ Konkoly Observatory, HUN-REN CSFK, MTA Centre of Excellence ; Konkoly Thege Miklos St. 15–17, 1121 Budapest, Hungary
² Centre for Planetary Habitability (PHAB), University of Oslo, Sem Saelands Vei 2A, 0371 Oslo, Norway

^★ Corresponding author; rbrasser@konkoly.hu

Received: 27 September 2024
Accepted: 13 January 2025

Abstract

Context. Terrestrial planet formation (TPF) is a difficult problem that has vexed researchers for decades. Numerical models are only partially successful at reproducing the orbital architecture of the inner planets, but have generally not considered the effect of the growth of the giant planets. Cosmochemical experiments suggest that the nucleosynthetic isotopic composition of bodies from beyond Jupiter is different from that of the inner Solar System. This difference could have implications for the composition of the terrestrial planets.

Aims. I aim to compute how much material from the formation region of the gas giants ends up being implanted in the inner Solar System due to gas drag from the protoplanetary disc, how this implantation alters the feedstocks of the terrestrial planets, and whether this implantation scenario is consistent with predictions from cosmochemistry.

Methods. I dynamically model TPF as the gas giants Jupiter and Saturn are growing using the Graphics Processing Unit (GPU) software Gravitational ENcounters with GPU Acceleration (GENGA). The evolution of the masses, radii, and orbital elements of the gas giants are precomputed and read and interpolated within GENGA. The terrestrial planets are formed by planetesimal accretion from tens of thousands of self-gravitating planetesimals spread between 0.5 au and 8.5 au. The total mass of the inner planetesimal disc and outer disc are typically 2 and 3 Earth masses (M_⊕) respectively, and the composition of the planetesimals changes from non-carbonaceous-like to carbonaceous-like at a prescribed distance, ranging from 2 au to 5 au.

Results. Here, I report on the first 5 million years of evolution. At this time approximately 20% of the mass of planetesimals in the Jupiter-Saturn region is implanted in the inner Solar System, which could be more than the cosmochemical models predict; this amount can be reduced by reducing the total mass of the outer planetesimal disc, and the results suggest a mass of 1 M_⊕ could suffice. The mass-weighted fraction of outer Solar System material implanted in the inner Solar System shows a flat or bimodal distribution beyond 0.7 au, with an occasional peak near 0.9 au. The planetesimals that remain in the inner Solar System have a mixed composition, which could have implications for late accretion.

Conclusions. The growing gas giants scattered the planetesimals in their vicinity into the inner Solar System, which changed the isotopic composition of the terrestrial planets. The inner planetesimal disc may not have extended much farther than 2 au, otherwise embryos do not grow fast enough to produce Mars analogues. This could mean that the region of the current asteroid belt never contained much mass to begin with. The implantation scenario could also explain the existence of active asteroids in the main belt.