| In order to form both terrestrial planets and the cores of gas giant planets, sub-micron dust grains must coagulate and grow by over ten orders of magnitude while still embedded in gas-rich circumstellar disks. During this process there are two physical mechanisms that threaten the retention of these planetary building-blocks. First, because the rotation of the disk gas will often be non-Keplerian due to pressure support, pebbles and boulders embedded in the disk experience hydrodynamic drag which, in standard disk models, cause their orbits to decay on timescales much shorter than the growth timescale. Second, once a protoplanet grows to a significant fraction of an Earth mass, tidal interactions with the gaseous disk cause the embryo to migrate.;These two effects have proven to be significant challenges to planet formation in standard benchmark protoplanetary disk models. In this dissertation I describe how these problems can be ameliorated by physically motivated disk structures. I demonstrate that, in a disk that evolves due to magnetohydrodynamic (MHD) turbulence, driven by the magnetorotational instability (MRI), there are special locations in which solid materials can be preferentially trapped to promote planet formation. These traps are created because the efficiency of angular momentum transport is not a smooth function of radius in MRI-active disks. As the disk evolves towards a quasi-steady state, the disk surface density changes to reflect this variable disk "viscosity.";In chapters 2 and 3 we use a simple disk model to calculate the radial drift of solids in MRI-active disks. In chapter 2 we identify two points in MRI-active disks in which the surface density will change such that drifting material can be trapped, the inner edge of the dead zone and the snow-line. In chapter 3 we demonstrate that, in the protoplanetary disks surrounding rapidly-accreting intermediate-mass stars, the inner edge of the dead zone can be push out to 1 AU. This provides a mechanism to form the cores of Jovian mass planets preferentially around intermediate-mass stars.;In chapters 4 and 5 we present a new model to self-consistently calculate the surface density and temperature profiles of MRI-active disks and use this model to investigate the impact on planetary migration. Type I migration, the migration of an embedded planet due to a tidal interaction with the gaseous disk, poses both a challenge to Jovian planet formation and a source of short-period, low-mass planets. In chapter 5 we investigate consequences of new estimates of the type I migration rate in both a-disk models and in MRI-active disks. |
