The solar system began to form 4568 million years ago, from a disk of dust and gas around the young Sun. Within a few million years, Jupiter and Saturn had formed and the terrestrial planets had reached significant fractions of their present size. The processes by which the planets formed are of particular interest at a time when the search for Earth-like planets around other stars occupies considerable attention. Earth-like exoplanets somewhat larger than our own are already detectable, and criteria are being developed to identify stars around which smaller Earths should be present.1
As rocky planets grow by accreting surrounding material, they suffer increasingly violent collisions. Figure 1 imagines a collision between a growing Earth-like planet and a smaller planetary embryo. One such encounter at the end of Earth’s accretion is thought to have created the Moon.
In the context of our own solar system, a combination of...
