Institute for Advanced Study Informal Astrophysics Seminar

The fast-paced discovery of extrasolar planets has heightened interest in the mechanisms by which planets form. The early stages of planet formation -- growth from dust grains to "planetesimals" larger than a kilometer -- is a long-standing problem which sets the stage for subsequent growth phases. Simple growth by collisional sticking stalls as collisions become more violent with increasing particle size. The gravitational collapse of many small solids into a larger planetesimal is a promising mechanism. However stirring of particles by turbulent gas opposes this collapse, and led to the abandonment of the hypothesis. I will describe recent dynamical work which is resurrecting the collapse mechanism for planetesimal formation. A key point is that turbulence not only stirs particles but creates transient clumps, which can act as seeds for gravitational collapse. The streaming instability provides a robust mechanism for drag forces to promote particle clumping (Youdin & Goodman, 2005; Youdin & Johansen, Johansen & Youdin 2007). Simulations by Johansen et al. (2007, Nature) confirm the viability of the collapse hypothesis when initial particle sizes are above tens of centimeters. I will describe ongoing analytic work on gravitational collapse which can be extrapolated across parameter space, including toward smaller particle sizes. A surprising and robust conclusion is that planetesimals can form with initial sizes significantly larger than the canonical value of a few km. I will describe implications both for the detailed properties of our Solar System, and for the formation of planets and exoplanets in general.