Formation of Terrestrial Planets

TL;DR

This review summarizes recent advances in understanding terrestrial planet formation, including planetesimal growth, planetary embryo development, and models explaining the Solar System's unique architecture compared to exoplanet systems.

Contribution

It synthesizes current models of planetesimal and planetary embryo formation, compares different scenarios for Solar System development, and discusses formation pathways for super-Earths and exoplanets.

Findings

Streaming instability favors dust coagulation into planetesimals.

Models like Grand-Tack and low-mass asteroid belt match constraints but have uncertainties.

Super-Earths likely form via migration, not in-situ formation.

Abstract

The past decade has seen major progress in our understanding of terrestrial planet formation. Yet key questions remain. In this review we first address the growth of 100 km-scale planetesimals as a consequence of dust coagulation and concentration, with current models favoring the streaming instability. Planetesimals grow into Mars-sized (or larger) planetary embryos by a combination of pebble- and planetesimal accretion. Models for the final assembly of the inner Solar System must match constraints related to the terrestrial planets and asteroids including their orbital and compositional distributions and inferred growth timescales. Two current models -- the Grand-Tack and low-mass (or empty) primordial asteroid belt scenarios -- can each match the empirical constraints but both have key uncertainties that require further study. We present formation models for close-in super-Earths -- the closest current analogs to our own terrestrial planets despite their very different formation histories -- and for terrestrial exoplanets in gas giant systems. We explain why super-Earth systems cannot form in-situ but rather may be the result of inward gas-driven migration followed by the disruption of compact resonant chains. The Solar System is unlikely to have harbored an early system of super-Earths; rather, Jupiter's early formation may have blocked the ice giants' inward migration. Finally, we present a chain of events that may explain why our Solar System looks different than more than 99\% of exoplanet systems.