By Sean Raymond, astrophysicist.
Understanding how our Earth formed is a grand challenge of planetary science, and at the heart of the search for other life-bearing planets in the Galaxy. We have images of planet-forming disks around young stars, and have discovered thousands of exoplanets orbiting other, older stars. A variety of physical processes is thought to be responsible for the growth of micrometer-sized dust grains to planets tens of thousands of kilometers in size. Some processes are well-behaved and predictable. Yet others are stochastic in nature such that a small change may result in a drastically different outcome. Computer simulations can address the branching of outcomes, and the robustness of the formation of our planet and Solar System. The question then becomes, what characteristics make Earth truly 'special'?