Dynamics and chemistry of planet formation

Ongoing work:

  • Feeding Zones as first-order gauge of protoplanetary disk mixing and dynamics

The condensation sequence method described above only considers an “average” exoplanet in a planetary system. In nature, however, planets are built from various radial areas within the protoplanetary disk. These “feeding zones” will have variable chemistry based on the stability of minerals within that zone as determined by the condensation sequence. I am currently working with Drs. Steven Desch and Alan Jackson of ASU to combine their recent formulations of spatial feeding zones from N-body planet formation models with ArCCoS’s predicted stable minerals within these zones. This method allows us to calculate how the composition of a planet changes as a function of the zone in a disk in which it formed. We are currently working to benchmark this model to the Earth and predict possible bulk compositional differences between the Earth and Venus given this model. Once benchmarked, this method can then be applied to specific exoplanetary systems and predict such observables as an exoplanet’s mass, thus providing an observational test and laying the groundwork for future interior dynamic studies. 

Previous work:

  • Mineral stability and condensation sequence in the protoplanetary disk

Unterborn, C. T. & Panero W. R. ApJ in review. preprint

During star formation the protoplanetary disk is initially hot and entirely gaseous. As the disk cools, solids begin to condense directly from the gas phase. The relative stability of these minerals provides insight into the composition and mineralogy of the building blocks of terrestrial planets. To calculate these stabilities, I developed ArCCoS, an equilibrium condensation sequence calculator. Unlike the current generation of condensation sequence calculators, ArCCoS is open-source (https://github.com/CaymanUnterborn/ArCCoS) and able to be updated with new thermodynamic data as it becomes available. Currently I am using ArCCoS to examine the effects of bulk stellar composition on the initial oxidation state of a planet as measured by the relative amount of O to the major cations: Mg, Fe and Si. This method removes significant degeneracy in the interpretation of the structures of exoplanets as well as providing observational predictions for an exoplanet’s mass as a check of the validity of this model. 

Mg and Si phase diagrams for condensation sequence calculations adopting the Solar composition of Asplund et al. (2005) as input. Both figures are on the same scale for comparison. Higher temperature condensed solids are shown as dashes. 

Mg and Si phase diagrams for condensation sequence calculations adopting the Solar composition of Asplund et al. (2005) as input. Both figures are on the same scale for comparison. Higher temperature condensed solids are shown as dashes. 

The percentage of oxygen condensed in refractory phases as a function of Mg/Si for independent changes in both Mg (crosses) and Si (squares). The Sun (circle; Asplund et al. 2005) is included for reference. Results of the stoichiometric determination of the core mass fraction (assuming only Fe-Ni alloy) and mantle mineralogy are appended for the Sun and each end-member. 

The percentage of oxygen condensed in refractory phases as a function of Mg/Si for independent changes in both Mg (crosses) and Si (squares). The Sun (circle; Asplund et al. 2005) is included for reference. Results of the stoichiometric determination of the core mass fraction (assuming only Fe-Ni alloy) and mantle mineralogy are appended for the Sun and each end-member.