Planets Made of Star Stuff: The Diversity of Earth-Sized Exoplanet Compositions In Relation To Their Host Stars
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2024
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Planets and their host stars are born from the same primordial cloud of gas and dust. We observe similar ratios of iron to rock-building elements (such as magnesium and silicon) on Earth as we do in Venus, Mars, and the Solar atmosphere—suggesting that rocky planets tend to have similar compositions to their host stars. We observe very different ratios of iron to rock on Mercury, however, demonstrating that planets occasionally undergo processing that alters its primordial composition and differentiates it from its host star. We have discovered roughly 1000 exoplanets similar in size to the Earth, indicating thatterrestrial planets are common in our galaxy. My dissertation addresses two main questions in exoplanetary science: (1) what is the diversity compositions for Earth-sized exoplanets, and (2) what is the relationship between the compositions of these planets and that of their host star?
The mass and radius measurements of rocky exoplanets show such a diversity in density that suggest some are composed primarily of iron, while others hardly any. To test the diversity of Earth-sized exoplanet compositions, we collected high-precision Radial Velocity measurements on 10 super-Earth sized planets using Keck/HIRES, Keck/KPF, and Gemini/MAROON-X. We report updated mass measurements for seven of these planets, and confirm the existence of a new super-Earth TOI-1011 b. We then estimated the Core Mass Fraction (CMF) for each planet using updated radius measurements from TESS photometry for two planets (TOI-561 b and TOI-1011 b) and literature values for the rest. We findthat the CMF for all planets (except for TOI-561 b) became more similar to that of the Earth (CMF=0.33) after re characterization. We find that many planets classified as “super-Mercuries" have a lower CMF than previously thought after updating their masses using high-precision RVs (Kepler-100 b and HD 93963 A b), Gaussian Processes regression to account for stellar activity (Kepler-102 d), or homogeneously updating stellar parameters (Kepler-406 b, K2-106 b, K2-229 b, and K2-38 b). We also find that some planets (TOI-561 b and WASP-47 e) are more consistent with having a gaseous envelope of high mean molecular weight species than having a pure rocky composition, demonstrating that not all
planets with radii R<1.5R⊕ can be assumed to be rocky.
We then place these planet compositions in the context of their host stars. We find that 75% of planets have a CMF consistent with that of their host star to within 1σ, and that the best-fit slope of a linear fit to this relationship is consistent with planets and their host stars having a 1-1 correlation (indicating planets have the same composition of their host star). This is in contrast to previously determined slopes that suggest a much steeper relationship between host star and planet composition. At the present, however, the small sample of rocky planets and the relatively large uncertainties on CMF also place this slope within 2σ of 0 (indicating no strong correlation between star and planet) and future planet discoveries and characterization are needed to demonstrate a statistically significant correlation. Our analysis of rocky planet compositions, in the context of their host star abundances, has the power inform the planet formation and evolution pathways that are needed to produce all of the planets we observe—from iron-rich super-Mercuries to super-Earths with volatile rich envelopes. We have only begun to scratch the surface of detecting and characterizing planets similar in size to the Earth, however. Precise mass measurementsfrom instruments such as KPF, and atmospheric characterization from instruments such as JWST provide a means for us to continue answering these fundamental questions about the nature and origin of Earth-like planets throughout our galaxy.
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Astronomy, Earth, Exoplanets, Planet Compositions, Planets, Rocky Planets, Stellar Abundances
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246 pages
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