Dr Jake Taylor (he/him)

Abstract:

The characterisation of exoplanetary atmospheres has entered a new era with the advent of the James Webb Space Telescope (JWST), which enables the detection of molecular species in the atmospheres of both gas giants and smaller, potentially rocky planets. This thesis focuses on the development and application of atmospheric retrieval methods to interpret transmission spectra from JWST observations, with particular attention to assumptions in atmospheric models and their impact on the resulting inferences. The first part of this thesis examines how specific modelling choices can influence atmospheric retrievals. I investigate the role of planetary rotation in modifying effective gravity and demonstrate that the inclusion of centrifugal forces can alter retrieved atmospheric properties, particularly for low-density, fast-rotating planets that are common targets for transmission spectroscopy. I also explore the implementation of Centered Log-Ratio (CLR) priors for atmospheric composition, a method designed to better represent the statistical priors of gas abundances in retrievals. This is particularly relevant for smaller exoplanets, where the dominant atmospheric constituents are often unknown and conventional priors can lead to biased results. The second part of the thesis presents retrievals on JWST Early Release Science observations for the hot Saturn WASP-39 b. I apply the NEMESISPY retrieval framework to transmission spectra obtained using multiple JWST instruments using both free retrieval and equilibrium chemistry approaches. These analyses highlight the strengths and limitations of each method and illustrate how the choice of parameterisation and integration of chemical equilibrium codes can impact inferences about the atmospheric conditions on a planet. The final chapters focus on a 1.5 R_earth exoplanet L 98-59 d. Using a JWST transmission spectrum obtained with the NIRSpec G395H instrument, I perform a range of retrievals to test what can be inferred about its atmosphere. My analysis suggests the potential presence of a high mean molecular weight atmosphere dominated by the sulfur-bearing gases H2S and SO2. While these initial results are tentative, I outline an observational strategy targeting additional transits using complementary wavelengths to constrain its composition more robustly. Taken together, the work presented in this thesis examines assumptions in the technique of atmospheric retrieval and demonstrates its application to a diverse set of exoplanets. It shows how careful treatment of model assumptions and priors can improve the robustness of interpretations and enhance the scientific return from JWST observations. As the focus of exoplanet science shifts toward smaller and cooler planets, these tools and approaches will be essential for addressing fundamental questions about atmospheric diversity and the potential for habitability beyond our Solar System

Abstract:

We present an atmospheric analysis of LTT 9779 b, a rare planet situated in the hot-Neptune desert, that has been observed with Hubble Space Telescope (HST)/WFC3 with G102 and G141. The combined transmission spectrum, which covers 0.8–1.6 μm, shows a gradual increase in transit depth with wavelength. Our preferred atmospheric model shows evidence for H2O, CO2, and FeH with a significance of 3.1σ, 2.4σ, and 2.1σ, respectively. In an attempt to constrain the rate of atmospheric escape for this planet, we search for the 1.083 μm helium line in the G102 data but find no evidence of excess absorption that would indicate an escaping atmosphere using this tracer. We refine the orbital ephemerides of LTT 9779 b using our HST data and observations from TESS, searching for evidence of orbital decay or apsidal precession, which are not found. The phase-curve observation of LTT 9779 b with JWST NIRISS should provide deeper insights into the atmosphere of this planet and the expected atmospheric escape might be detected with further observations concentrated on other tracers such as Lyα

Abstract:

The near-infrared transmission spectrum of the warm sub-Neptune exoplanet GJ 1214 b has been observed to be flat and featureless, implying a high metallicity atmosphere with abundant aerosols. Recent JWST MIRI Low Resolution Spectrometer observations of a phase curve of GJ 1214 b showed that its transmission spectrum is flat out into the mid-infrared. In this paper, we use the combined near- and mid-infrared transmission spectrum of GJ 1214 b to constrain its atmospheric composition and aerosol properties. We generate a grid of photochemical haze models using an aerosol microphysics code for a number of background atmospheres spanning metallicities from 100 to 1000× solar, as well as a steam atmosphere scenario. The flatness of the combined data set largely rules out atmospheric metallicities ≤300× solar due to their large corresponding molecular feature amplitudes, preferring values ≥1000× solar and column haze production rates ≥10−10 g cm−2 s−1. The steam atmosphere scenario with similarly high haze production rates also exhibits sufficiently small molecular features to be consistent with the transmission spectrum. These compositions imply that atmospheric mean molecular weights ≥15 g mol−1 are needed to fit the data. Our results suggest that haze production is highly efficient on GJ 1214 b and could involve non-hydrocarbon, non-nitrogen haze precursors. Further characterization of GJ 1214 b’s atmosphere would likely require multiple transits and eclipses using JWST across the near- and mid-infrared, potentially complemented by ground-based high-resolution transmission spectroscopy.