The Origins of Compact Systems of Low Mass Planets

The first extrasolar planets discovered where the so-called 'Hot Jupiters', which are Jupiter-mass planets whose orbital periods are only a few days. However, these were also the easiest to find because their masses and proximity to their host star makes the radial velocity wobble of the star much larger. We now understand that only about 1 percent of stars host a hot Jupiter, whereas as many as 50 percent of stars may host less massive planets on compact orbits. Thus, the central issue in extrasolar planet formation is how such planetary systems formed.
Of course, M dwarf environments are quite different from G star environments, and so there are many questions regarding how to properly define a habitable zone and what factors, both orbital and atmospheric, influence the conditions on the surface of a rocky planet in this environment. We are beginning to also investigate these factors in collaboration with Aomawa Shields , an NSF Postdoctoral Fellow at UCLA.

The empirical success of these models raises some interesting questions about planet formation. If the planets do assemble in place, it means that the mass inventory in planetesimals is several times that inferred in the case of our own solar system, and likely cannot have simply sedimented out in the gas phase. This implies that some level of migration of solid material from large scales to small scales must still have occured, but that it likely happened at an earlier stage in the protoplanetary disk lifetime. I have recently begun to model this process as well, and have been able to demonstrate that we can reproduce the kinds of mass reservoirs required with a model in which micron-sized particles spiral in due to aerodynamic drag in the gaseous disk but are then expelled to larger radii by a protostellar outflow near the star. This circulation of solid material leads to a steady state distribution that matches well the initial conditions required for the later stage assembly. Such models also find some supporting evidence through measurements in our own solar system, because the Stardust Sample return mission to Comet Wild-2 found that much of the cometary material was processed at high temperatures, more consistent with the inner solar nebula than the distant, cold, cometary reservoir. The relevant paper can be found here .