Advances in our understanding of terrestrial planet formation have come from a multidisciplinary approach. Studies of the ages and compositions of primitive meteorites with compositions similar to the Sun have helped to constrain the nature of the building blocks of planets. This information helps to guide numerical models for the three stages of planet formation from dust to planetesimals (∼10 6 y), followed by planetesimals to embryos (lunar to Marssized objects; few × 10 6 y), and finally embryos to planets (10 7 -10 8 y). Defining the role of turbulence in the early nebula is a key to understanding the growth of solids larger than meter size. The initiation of runaway growth of embryos from planetesimals ultimately leads to the growth of large terrestrial planets via large impacts. Dynamical models can produce inner Solar System configurations that closely resemble our Solar System, especially when the orbital effects of large planets (Jupiter and Saturn) and damping mechanisms, such as gas drag, are included. Experimental studies of terrestrial planet interiors provide additional constraints on the conditions of differentiation and, therefore, origin. A more complete understanding of terrestrial planet formation might be possible via a combination of chemical and physical modeling, as well as obtaining samples and new geophysical data from other planets (Venus, Mars, or Mercury) and asteroids.H ow terrestrial planets grew out of the solar nebula has long been a topic of interest, and is still being pursued by a combination of studies involving samples of terrestrial and extraterrestrial materials, as well as computational models. Early nebular models by Descartes, Kant, and Laplace proposed that the planets formed from the solar nebula by various mechanisms such as vortices (analogous to spinning galaxies), or by shedding rings, but all of these general concepts failed to produce fundamental features of our Solar System (e.g., ref. 1). Subsequent models included homogeneous accretion (e.g., refs. 2, 3), in which the planets condensed from the nebula and were composed of homogeneous material that differentiated internally, and heterogeneous accretion (4), in which planets grew like layer cakes as the phases condensing out of the nebula changed with decreasing temperature. The purpose of this review is to elucidate how new samples, experiments, and modeling are contributing to a more thorough and sophisticated understanding of the many factors that helped to shape our inner Solar System ( Fig. 1; see SI Text) and specifically the Earth. We also highlight areas where more extensive work is necessary or more samples are needed to better constrain our models, and where physical and chemical constraints may benefit from combined approaches.
CompositionPrimitive chondrites were likely the building blocks for terrestrial planets, with bulk compositions similar to the Sun, but with differing oxygen fugacities (Fig. S1) and other chemical characteristics such as volatile element depletions (5; SI Text, Fig. S2). In a...