| dc.description.abstract | The interior conditions of planets are highly uncertain, because two types of intrinsic degeneracies – compositional degeneracy and structural degeneracy – prevent precise characterization. In this thesis, I develop a planetary interior code package, CORGI, incorporating state-of-the-art physical properties of planet-forming materials. Using CORGI, I eliminate unmixed interior scenarios for Uranus, rule out the fossil-compressed formation hypothesis for high-density exoplanets, and establish a link between formation history and atmospheric composition for hypothetical Earth-like white dwarf (WD) exoplanets, reducing interior degeneracy for these planets. However, I also identify a novel carbon-rich interior composition for sub-Neptunes, introducing an additional degeneracy to this already ambiguous category.
It is heatedly debated that whether Uranus is a distinct-layer “ice giant” with greater than 70 wt% ice or a “rock giant” with compositional gradients and roughly equal amounts of ice and rock. Gravity field measurements from spacecraft, which directly probe interior mass distribution, are expected to resolve this debate. However, I show that the degeneracy will persist even with future Uranus Orbiter and Probe (UOP) mission, but the level of degeneracy can be reduced. My models indicate that only highly mixed interiors – either those with smooth density gradients or those with substantial light elements in the mantle and heavy elements in the atmosphere – are consistent with previous Voyager 2 measurements. Additionally, I demonstrate that the UOP can distinguish between high- and low-atmospheric metallicity scenarios and constrain the J6 harmonic, and potentially J8, if placed in close-in polar orbits, informing the mission and orbit design of UOP.
For exoplanets with no solar system counterparts, interior models are essential for understanding their composition, structure, formation, and evolution. I apply CORGI to a category of high-density planets that are consistent with greater than 50% core mass fraction, substantially higher than that of the Earth (33%). By combining planetary interior modeling with photoevaporation modeling, I investigate into one of the hypotheses – the fossil-compressed hypothesis – for the origin of high-density planets. My models revealed that most high-density planets are highly improbable to be fossil-compressed cores, because most or even all of the iron-silicate core is molten during the evolution, while the fossil-compressed hypothesis requires a solid core. Kolmogorov–Smirnov test statistics show that this result is robust for planets with both hydrogen-dominated and steam envelopes.
Planetary interior models sometimes reveal new degeneracies rather than resolving them. By combining interior, atmospheric chemistry, and transmission spectra models, I identify a new possible interior composition for sub-Neptunes: carbon-rich composition. I posit that sub-Neptunes formed between the “soot line” – a condensation line for refractory organic carbon – and the water snow line would have high bulk C/O ratios and a substantial carbon layer. Interior models revealed that such carbon-rich compositions are consistent with the masses and radii of sub-Neptunes, given appropriate atmospheric metallicity. Atmospheric chemistry and transmission spectra models found that the spectral features predicted for carbon-rich sub-Neptunes are compatible with observations by the Hubble Space Telescope and the James Webb Space Telescope.
Finally, I explore the connection between post-main-sequence evolutions and the atmosphere and interior conditions of hypothetical Earth-like planets orbiting WDs. I showed that first-generation WD planets that have experienced significant atmospheric loss and second-generation WD planets that are formed in WD debris disks under a more clement radiation environment can be distinguished by the presence of a hydrogen-dominated atmosphere. Additionally, the interior conditions of second-generation WD planets can be inferred from WD pollution observations. | |
