Dynamics, mineralogy and structure of exoplanets

Ongoing work:

  • Mass-Radius-Composition Models

Modeled chi-squared goodness-of-fit for the TRAPPIST-1 planets as a function of the relative water fraction in weight percent added to the system. Mass-radius-composition curves were created using the ExoPlex calculator. From Unterborn et al., in review

Modeled chi-squared goodness-of-fit for the TRAPPIST-1 planets as a function of the relative water fraction in weight percent added to the system. Mass-radius-composition curves were created using the ExoPlex calculator. From Unterborn et al., in review

 

  • Gauging likelihood of planetary systems to host long-term, sustained plate tectonics

The presence of plate tectonics on Earth is a primary factor in its habitability, regulating atmospheric carbon as well as creating a planetary-scale water cycle. While the exact mechanism for initiating plate tectonics is unknown, determining the likelihood of a planet sustaining tectonics provides a first-order constraint on it being habitable over geologic timescales. One driver for sustaining plate tectonics on Earth is the slab-pull produced as the basalt present in the subducting plate undergoes a phase transition to the denser eclogite phase. The plate, now more dense than the surrounding mantle, continues to sink rather than stagnating in the upper mantle. For the more generalized exoplanet case, the magnitude of this chemical buoyancy force is dependent on the mineralogy of the melt- extracted, subducting crust. Currently, I am utilizing the MELTS and HeFESTo software packages to calculate the melt chemistry, mineralogy and relative buoyancy force of potential subducting plates over a compositional range indicative of ~1000 Sun-like stars. By comparing the magnitude of the negative buoyancy force across this compositional space, we can gauge the liklihood of those stellar systems producing planets able to undergo long-term plate tectonics and provide a measure of its potential habitability. 

Previous work:

  • Defining "Earth-like" Planets

C. T. Unterborn, E. E. Dismukes and W. R. Panero, 2016 ApJ 819 32. preprint

 An exoplanet’s structure and composition are first-order controls of the planet’s habitability. In my work, I explore which aspects of bulk terrestrial planet composition and interior structure affect the chief observables of an exoplanet: its mass and radius. Applying perturbations to the planet we know best, the Earth, my work finds that core radius, presence of light elements in the core and an upper-mantle consisting of low-pressure silicates have the largest effect on the final calculated mass at a given radius, none of which are included in current mass-radius models. I expanded these results to produce a self-consistent grid of compositionally as well as structurally constrained terrestrial mass-radius models for quantifying the likelihood of exoplanets being “Earth-like.”

Using the Sun's composition to reverse engineer the Earth. From Unterborn, Dismukes & Panero, 2016. 

Using the Sun's composition to reverse engineer the Earth. From Unterborn, Dismukes & Panero, 2016. 

Contours of constant mass (black, in Earth masses) and constant mantle radius fraction (MRF = (R—Core Radius)/R, blue-dashed) vs. planetary radius and [Si/Fe]. Contours were calculated adopting a three-layer planetary model with an Fe core with 6% density deficit due to 8.5 wt% Si and 1.6 wt% O, brigmanite/periclase lower mantle and pyroxene/olivine upper mantle with Mg/Si = 1.25. Our calculated model for Kepler-36b is shown as a red diamond. Earth and Venus (adopting Mg/Si = 1.25; open square, and Mg/ Si = 1.35; solid square) are shown as well. If stellar composition is a proxy for planetary composition and Kepler-36b is indeed "Earht-like," this model predicts a [Si/H] abundance for Kepler-36 of ∼−0.32.

Contours of constant mass (black, in Earth masses) and constant mantle radius fraction (MRF = (R—Core Radius)/R, blue-dashed) vs. planetary radius and [Si/Fe]. Contours were calculated adopting a three-layer planetary model with an Fe core with 6% density deficit due to 8.5 wt% Si and 1.6 wt% O, brigmanite/periclase lower mantle and pyroxene/olivine upper mantle with Mg/Si = 1.25. Our calculated model for Kepler-36b is shown as a red diamond. Earth and Venus (adopting Mg/Si = 1.25; open square, and Mg/ Si = 1.35; solid square) are shown as well. If stellar composition is a proxy for planetary composition and Kepler-36b is indeed "Earht-like," this model predicts a [Si/H] abundance for Kepler-36 of ∼−0.32.

  • The distribution of radiogenic heat budgets in the Galaxy

C. T. Unterborn, J. A. Johnson and W. R. Panero, 2015 ApJ 806, 1.

The abundances of the radioactive elements Th, U and 40K are key components of a planet’s energy budget, making up 30%–50% of the Earth’s. As refractory elements, stellar Th and U abundances should be mirrored between host-star and exoplanet, thus providing an estimate of the exoplanet’s radiogenic heat budget. Through spectroscopic observations of Solar twins and analogues, I measured variation in Th between ~60 and 250% of the Solar/Earth abundance. Utilizing a parameterized convection model, I found that the first- order this variation can significantly affects the convective state and thermal history of a planet.  

Stellar Th/Si as a function of average Si abundance. Stars with observed planets are shown as closed squares and without as unfilled squares. The Sun is represented by a triangle. The Sun is depleted relative to the rest of this sample, suggesting some planets may have a greater heat budget than the Earth. 

Stellar Th/Si as a function of average Si abundance. Stars with observed planets are shown as closed squares and without as unfilled squares. The Sun is represented by a triangle. The Sun is depleted relative to the rest of this sample, suggesting some planets may have a greater heat budget than the Earth. 

  • Mineralogy and dynamics of carbon planets

C. T. Unterborn, Kabbes, J. E., Pigott, J. S., Reaman, D. M. and Panero, W. R. 2014 ApJ 793, 2. preprint

The viscoelastic structure of a planet’s mantle and subsequent dynamic state is a function of its relative temperature, composition and mineralogy of the convecting fluid. My previous work showed that terrestrial planets containing significant fractions of C relative to Mg, Si, Fe and O would stabilize diamond rather than carbonates as the dominant form of mantle carbon. Using parameterized convection models, we found that an increase in C, and thus diamond, concentration slows or stops mantle convection relative to a silicate-dominated planet, due to diamond’s 3 order of magnitude increase in both viscosity and thermal conductivity. These planets will then have drastically different thermal histories than the Earth, and likely lack tectonics as well as deep carbon and water cycles, making them inhabitable to life as we know it. 

Ternary diagram for the C–(Mg+2Si+Fe)–O system. The Earth is shown as a blue cross and HD 19994 as a red diamond. The exact position of the diamond/no diamond line on the core-free side of the ternary depends on the specific Fe/(Mg+2Si+Fe) ratio, in which planets with more Fe relative to the other cations are able to stabilize more FeCO3, and thus reduce the amount of diamond present in the mantle. 

Ternary diagram for the C–(Mg+2Si+Fe)–O system. The Earth is shown as a blue cross and HD 19994 as a red diamond. The exact position of the diamond/no diamond line on the core-free side of the ternary depends on the specific Fe/(Mg+2Si+Fe) ratio, in which planets with more Fe relative to the other cations are able to stabilize more FeCO3, and thus reduce the amount of diamond present in the mantle. 

  • Petrology and mineralogy of non-Earth compositions

Our current suite of thermodynamics software and databases used for determining bulk mineralogy and melt chemistry were designed to address phase equilibria for compositions at or near the Earth values. Thus, these models often fail when calculating phase equilibria for non-Earth compositions due to a lack of data. Working with Drs. Christy Till of Arizona State University (ASU) and Mark Ghiorso of OFM Research, I am quantifying which major-element compositions are not represented in the MELTS software package. These compositions will then be used as starting compositions for phase equilibria experiments in ASU’s EPIC lab to expand the underlying MELTS database. I am similarly working with Dr. Dan Shim of ASU to develop diamond anvil cell experiments for determining the phase equilibria of mantles with compositions of Mg/Si < 1. I am concurrently calculating the predicted phase equilibria using PerPlex for these compositions for comparison with this experimental data.