Terrestrial planet formation from a ring

The peculiar mass-orbital distribution of the terrestrial planets, with the massive Earth and Venus at the center and the much smaller Mercury and Mars at the sides, has been reproduced assuming that the solid material was initially concentrated in a narrow annulus near 1 au (Hansen, 2009; Nesvorny et al., 2021). However, these simulations assumed that planetesimals and planetary embryos were still confined in the ring when the gas disappeared from the disk. Here, for the first time, we simulate self-consistently the formation of planetary embryos from a ring of planetesimals during the gas-dominated era of the disk. We use the code GENGA (Grimm and Stadel, 2014), which allows simulating the mutual interactions and collisions among the planetesimals. We use the technique of tracer super-particles, adapted to the case of a self-gravitating disk, to simulate the growth of embryos form a ring of 100km planetesimals (the predominant size of objects formed via the streaming instability – Klahr and Schreiber, 2020) with a total mass of about 2 Earth masses. We account for gas drag and migration forces acting on planetesimals and growing embryos. We find that, while they form, the planetary embryos spread radially under the effect of mutual scattering, dynamical friction and eccentricity damping from the disk of gas. At the end of the gas-disk lifetime we obtain a large number of embryos whose orbits are so radially separated from each other that no further collisions occur, even during the giant planet instability. Thus, these simulations fail forming Earth and Venus but lead to spread-out systems of multiple small planets. Planetesimal formation in a ring is therefore not a sufficient condition for the successful formation of the terrestrial solar-system planets. We show that the formation of Earth and Venus analogues can be obtained if the gas surface density distribution is sufficiently peaked near 1 au, causing convergent migration of the embryos. Convergent migration has been already invoked to explain terrestrial planet formation starting from a radially extended disk of planetesimals or embryos (Ogihara et al., 2018; Broz et al. ). Here we show that it is also needed in the planetesimal-ring hypothesis.

This work has received support from the ERC advanced grant HolyEarth N. 101019380 and DFG TRR grant N. 170,

Hansen, B.M.S. 2009, ApJ, 703, 1131. Nesvorny, D., Roig, F.V., & Deienno, R. 2021, AJ, 161, 50. Grimm, S.L. & Stadel, J.G. 2014, ApJ, 796, 23. Klahr, H. & Schreiber, A. 2020, ApJ, 901, 54. Ogihara, M., Kokubo, E., Suzuki, T.K., et al. 2018, A&A, 612, L5. Broz, M., Chrenko, O., Nesvorny, D., et al. 2021, Nature Astronomy, 5, 898.