The Edgeworth-Kuiper Belt of planetesimals, of which Pluto is the largest known member, furnishes a grand ensemble of test particles whose trajectories reflect the chronology of events in our planetary system. The intellectual merit of Dr Eugene Chiang's research project is that it will detail three different chapters in this history: (1) The formation of planetesimals. Self-gravitational collapse of a dense layer of particles at the midplane of the protoplanetary disk promises to form superkilometer-sized objects from dust grains. Numerical hydrodynamical simulations of turbulence triggered by the Kelvin-Helmholtz instability in this dense layer will be performed to assess whether the turbulent stress exerted by gas on dust can transport dust to regions of ever increasing metallicity. Such regions are ripe for planetesimal formation by gravitational instability. (2) The accretion of planets from planetesimals. New insights into the formation environment of Neptune are promised from the recent discovery of Neptune Trojans. Theoretical calculations, ranging from detailed order-of-magnitude estimates to numerical explorations with a new collisional Nbody code, will determine whether observed Neptune Trojans were inserted directly by collisions or grew in situ from smaller bodies. These two formation channels paint dramatically different portraits of the planetesimal disk at the time of Neptune's birth. The first places the bulk of the disk mass into objects dozens of kilometers across, while the second does the same for centimeter-sized objects. (3) The evolution of orbits. Can the Kuiper Belt's enormous dispersion of orbital inclinations be understood by appealing to ancient close encounters with Neptune? This hypothesis is valid only if the perihelia of scattered objects can subsequently be raised beyond Neptune's reach. The ability of secular resonances to raise perihelia will be tested, for the first time, in the presence of a self-gravitating disk of planetesimals. If Neptune's migration were responsible for capturing Kuiper Belt objects into low-order resonances, what demands does this process make on the mass spectrum of planetesimals which exchange angular momentum with the planet? The degree of stochasticity in planetary migration will be measured using analytic calculations and numerical experiments designed to test analytic scalings. How are high-order resonances filled by planetary migration? The potential of n:1 resonances to diagnose Neptune's ancient migration rate will be assessed, in anticipation of future all-sky astrometric surveys of the Kuiper Belt.
The research has several broader impacts. Scientifically, issues of planetary migration and resonance capture impact extrasolar planetary systems, particularly debris disk systems that exhibit non-axisymmetries attributed to gravitational sculpting by planets, and resonant pairs of planets such as GJ876a+b. Furthermore, understanding the requirements of planetesimal formation determines our expectation for the sizes of planetary systems, sizes that may be measured with future adaptive optics imaging of young extrasolar systems. From the perspective of human resources, Dr. Chiang will train two graduate students and a cadre of undergraduate research assistants, will teach new graduate and undergraduate classes in the physics of planetary systems, with emphasis on recently discovered Neptune-mass giants, and will continue to deliver lectures at research institutions and at public venues. ***
