Institute for Advanced Study / Princeton University Joint Astrophysics Colloquium

Why do many stars host close-in chains of super-Earths? Why are eccentric gas giants found in some inner planetary systems? What determines which of these outcomes will occur around a particular star? I will present a possible framework for answering these questions that appeals to the “flow isolation mass,” a limiting mass for pebble accretion. Flow isolation occurs when small particles, coupled to the gas, are pulled around a growing planet along gas streamlines. Its consequences are highly dependent on the particle sizes present in the planet’s natal disk. In inner planetary systems, if fragmentation limits “pebble” sizes to Stokes numbers approaching one, then flow isolation yields limiting masses similar in scale to the thermal mass, comparable to the distinct “pebble isolation mass.” At larger orbital separations or if Stokes numbers are smaller, these processes diverge. I will present work showing that flow isolation can yield systems of super-Earths comparable to those observed. I will then discuss how a giant impact phase for giant planets results if multiple gas giants are instead produced in inner planetary systems, yielding a population of gas giants that well matches the data and in particular explaining why higher-mass giants are more likely to have high eccentricities. Finally, I will connect these two ideas to demonstrate that for a reasonable distribution of disk parameters, we can reproduce the relative frequency of super-Earth and gas giant systems.