Disk Instability vs. Core Accretion: Observable Discriminants

I will discuss ways to distinguish between disk instability and core accretion, the two competing paradigms for giant planet formation. Disk instability happens when a massive disk fragments into planet-sized self-gravitating clumps. Scattered light from these disks will illuminate high altitude density variations that result from stirring of the disk by the forming planet. These variations will evolve quickly, within several years, but do not correlate with the position of the planet itself. Alternatively, core accretion happens when solid particles collide and coagulate into larger and larger bodies until a body large enough to accrete a gaseous envelope forms -- around 10-20 Earth masses. This process is thought to be more quiescent than gravitational instability, so the disk should appear smooth. Although a 10-20 Earth mass core is insufficiently massive to fully clear an annular gap in the disk, it does perturb the disk material immediately in its vicinity, creating shadows and brightenings at the protoplanet's location. The planet may also begin to clear a partial gap. Shadowing and illumination on this partial gap can alter the thermal structure at the upper layers of the disk on a sufficiently large scale to be observable. Observing the signatures of either disk instability or core accretion requires milliarcsecond resolution and high contrast imaging. Advances in coronography, adaptive optics, and interferometry are bringing us ever closer to begin able to make these detections. Observational confirmation of either process taking place in a young circumstellar disk will help resolve the long-standing debate over how giant planets form.