Dix Planetary Science Seminar

Abstract: Much recent work on planet formation has focused on planetary growth through accretion of particles whose aerodynamic properties make them marginally coupled to the local nebula. Growth of planets through accretion of these "pebbles," often termed "pebble accretion," has several notable features when contrasted with "traditional" growth that relies on purely gravitational interactions. In this talk, I will give an overview of pebble accretion and discuss why the theory has garnered so much interest, and then go on to describe several novel and important features of growth by pebble accretion that have emerged from my work. Firstly, I demonstrate that growth by pebble accretion qualitatively changes for core masses above a minimum mass scale, above which far more pebble sizes are able to be accreted, and particles can be captured on scales comparable to the planet's hill radius. This change in behavior implies that the early stages of planetary growth must be fueled by processes other than pebble accretion, which can bring planets up to the minimum masses needed for pebble accretion to take hold. I then discuss the semi-major axes where gas giant formation can occur if these early stages are dominated by accretion of planetesimals. Finally, I show that consideration of the smallest sizes of particles that can be captured by pebble accretion leads naturally to an upper planetary mass limit known as the "flow isolation mass." This mass scale naturally results in planetary growth ending at super-Earth masses in the inner disk, where, in the absence of flow isolation, rapid pebble accretion rates imply that planets either stall at sub-Earth masses or run away to become gas giants.