Masters of Their Own Fate: Modeling the Chemical and Dynamic Feedback Mechanisms of Planet Formation

With over 6,000 confirmed exoplanets, numerous high angular resolution protoplanetary disk surveys, and multiple sample return missions from meteorites, the field of planet formation has a wealth of data on different stages of planet formation, yet still no clear pathway to fully connect these data.  One of the most striking discoveries of the past decade is the dynamic natures of protoplanetary disks, with evidence of dust growth, drift, and substructures appearing ubiquitous in disks.  These substructures include rings, gaps, spiral arms, and azimuthal asymmetries in both the dust and gas emission from the disks, with some such substructures directly linked to the forming planets embedded in the disk.  At the same time, isotopic measurements of meteorites and, recently, asteroids, show that parent bodies in our own solar system formed from two spatially distinct reservoirs, that persisted throughout the millions of years of prolonged planetesimal formation.

While this dichotomy has been explained by invoking substructures, it is unclear how these reservoirs might remain separated over the lifetime of the disk while still allowing for some degree of transport of calcium-aluminum-rich inclusions and chondrules.  While it is often proposed that ongoing planet formation may affect the remaining solids within a disk through dust growth, fragmentation, stirring, mixing, and pileups, it is not yet clear how the creation of these planet-induced disk substructures influence the composition of the growing planet, nor how they may affect the formation of subsequent planets.

In this thesis, I examine the feedback of planet formation on the protoplanetary disk, and how the composition of both the disk and the planets that grow there influence one another over the disk lifetime. I explore these feedback mechanisms — including planet-induced disk substructures, grain growth and fragmentation, and planetary heating — using a variety of computational techniques to constrain the effects on the composition of embedded planets in self-consistent ways. I compare the results of these computational models with meteorite compositions to place constraints on the Solar Nebula, but also more broadly with detected disks and exoplanets.

First, I present the results of a newly developed chemical model that simultaneously solves the chemical composition of the ice and gas in a protoplanetary disk while accounting for grain growth and ice sequestration.  I show that while depleting small grains from the disk by growing larger pebbles accelerates photon-driven chemistry throughout the disk, the bulk of solids in the midplane retain their inherited composition from the protostellar cloud. As such, early forming solids in the outer disk, including some meteorite parent bodies in our solar system, were impacted by chemical processing during the protoplanetary disk phase less than previously recognized. The new CANDY model (Chemistry ANd DYnamics) is freely available on github\footnote{https://github.com/ervc/newcandy} and is distributed under an open source MIT License.

Second, I examine the extent of radial mixing in a protoplanetary disk containing a growing giant planet. These results are presented within the context of the observed meteorite isotopic dichotomy, for which it has been proposed that Jupiter acted as a physical barrier between the inner and outer solar nebula, preventing mixing of carbonaceous and non-carbonaceous material. While a massive giant planet is capable of halting the inward drift of large pebbles in the disk, I show that small dust is capable of filtering past the embedded planet, and the efficiency of this passing varies as the planet grows in mass. Further, the planet can affect transport and mixing at more distant regions in the disk.  As such, the extent of radial mixing in the solar nebula evolves dramatically over time, varying with time and location, leading to a complex means of exchange across different spatial reservoirs. The particle tracking algorithm used in the model presented in this chapter is also available for download online via github\footnote{https://github.com/ervc/cpartrace}.

Finally, I examine how the stirring of small dust by a growing giant planet may act to enrich the atmospheres of the planet with volatile ices after it reached its pebble isolation mass, a point where accretion of solids is expected to stall or significantly slow. As giant planets accrete gas from the disk, they stir small dust to upper layers of the disk where temperatures are higher than the disk midplane. If these small grains are abundant as a result of particle fragmentation outside the disk gap carved by the planet, then this may be a promising mechanism to enrich these atmospheres in carbon, oxygen, and nitrogen, explaining observed enrichments in both Jupiter and directly imaged giant exoplanets.