Understanding Exoplanet Atmospheres

Citation

Zhang, Zhaoxi Michael (2022) Understanding Exoplanet Atmospheres. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/kpx0-an68. https://resolver.caltech.edu/CaltechTHESIS:02152022-223424046

Abstract

The study of exoplanet atmospheres is a blossoming field. Over the past two decades, dozens of hot gas giant atmospheres have been observed using a variety of techniques with both space and ground telescopes, revealing the presence of water, the ubiquity of clouds, the presence of equatorial jets, the existence of photoevaporation, and much more. For the far more abundant planets smaller than ~3 R_⊕, which potentially have a wide variety of exotic atmospheric compositions, observations are more challenging. We are just beginning to characterize their atmospheres.

This thesis consists of 6 papers on the topic of exoplanet atmospheres. In paper 1, we use Spitzer observations to make rudimentary 1D maps of two hot giant planets, allowing us to infer atmospheric circulation properties and compare them to models and to similar observations of other giant planets. In paper 2, we present PLATON, a fast, open source, easy to use, and easy to understand Python package that calculates transmission spectra for exoplanets and retrieves atmospheric characteristics based on observed spectra. PLATON supports the most common atmospheric parameters, in addition to less commonly included features such as a Mie scattering cloud model and unocculted starspot corrections. In paper 3, we add significant improvements to PLATON, including updated molecular opacities and emission spectra capability. In addition, we perform the most comprehensive retrieval on published HST and Spitzer transmission and emission spectra of the archetypal hot Jupiter HD 189733b, finding that they are well-matched by a moderately metal-enhanced atmosphere with a solar C/O ratio where the terminator is dominated by extended nm-sized hazes.

Papers 4-6 cover mass loss from sub-Neptunes. In paper 4, we present a tight upper limit on the amount of escaping helium from the the archetypal super Earth 55 Cnc e, suggesting that it has no primordial (H/He) atmosphere. In paper 5, we obtain the first detection of an escaping atmosphere from a young mini Neptune by measuring Lyα absorption from HD 63433c. We do not detect absorption from the inner planet, suggesting that the inner planet may have lost its primordial atmosphere while the outer one has not. In paper 6, we detect escaping helium from a young mini Neptune for the first time. The inferred mass loss rate is high enough to strip a significant portion of the atmosphere within the planet's lifetime; combined with the previous paper, these observations support the canonical explanation of mini Neptunes as rocky planets with a substantial primordial H/He atmosphere and validate models predicting that mini Neptunes can transform into super Earths.

Thesis Files