The Observational Diversity of sub-Neptunes
This dissertation focuses on characterizing the atmospheres of sub-Neptunes (planets larger than Earth but smaller than Neptune), the most common exoplanets in the galaxy. With no examples in our own Solar System, we are limited to the data quality of remote-sensing capabilities. I aim to unveil the atmospheres of these most-common exoplanets with space-based atmospheric spectroscopy, as well as models of their atmospheres including the effects of clouds and hazes. First, I present the near-infrared atmospheric transmission spectrum of the warm Neptune TOI-674 b, observed with the \textit{Hubble} Space Telescope's WFC3/G141 instrument. My observations of this planet indicate the presence of both water vapor and opaque clouds in its atmosphere, and I present the results of atmospheric retrievals to support these conclusions. Next, I present an ensemble analysis of 15 sub-Neptune atmospheric spectra from \textit{Hubble}, showing a parabolic trend relating their equilibrium temperatures with the clarity of their atmospheres. Temperate and hot sub-Neptunes appear to both have relatively strong atmospheric features and few atmospheric aerosols, while warm sub-Neptunes appear to have particularly abundant clouds and hazes that obscure our views of their constituent atmospheric species. Rather than be randomly subject to the whims of atmospheric microphysics, my work shows instead we may be able to plan our observations to target particular planets likely to show strong features. Finally, I present the \textit{JWST} NIRSpec/G395H dayside emission spectrum of the ultrahot Neptune LTT 9779 b, showing the presence of CO$_2$, H$_2$O, and CO. It appears to have high atmospheric metallicity ([Fe/H]=2.76) and C/O=0.9, consistent with earlier NIRISS transmission, but not emission observations. I also discuss how silicate clouds (inferred by the NIRISS data) may not significantly affect my eclipse spectrum, but could be visible at longer wavelengths. I conclude by discussing how LTT 9779 b's atmospheric composition relates to its formation, and call for future analyses to help harmonize the planet's inconsistent transmission and emission spectra.
Astronomy, Atmospheric Aerosols, Emission Spectroscopy, Exoplanet Atmospheres, Observational Astronomy, Space Telescopes, Transmission Spectroscopy
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