Abstract
Exoplanet transmission spectra provide rich information about the chemical composition, clouds and temperature structure of exoplanet atmospheres, where different spectral features dominate at different wavelengths. We can attempt to uncover the nature of atmospheres through applying retrieval codes to fit parameterised models to the transmission spectrum. With the recent launch of JWST, we have gained significant information at infrared wavelengths, however cloud and atomic opacities dominate the optical spectrum which we can observe with Hubble.We explore the question of how retrieved atmospheric parameters from exoplanet transmission spectra change with the addition of optical data. From a sample of 14 giant planets with transit spectra from 0.3 - 4.5 μm primarily from Hubble and Spitzer, we apply the free chemistry retrieval code, POSEIDON, to planetary spectra for wavelength ranges 0.3-4.5 μm, 0.6-4.5 μm and 1.1-4.5 μm. We analyse the posterior distributions of these retrievals and perform an information content analysis, finding wavelengths below 0.6 μm are necessary to constrain cloud scattering slope parameters (log a and ) and alkali species Na and K. There is limited improvement in the constraints of the remaining atmospheric parameters. With results from a population of planets, trends between the atmosphere and planetary parameters are investigated. Across the population, we find limb temperatures are retrieved lower than planetary equilibrium temperatures but have an overall good agreement with GCM models. The water abundance spans from supersolar to subsolar values, in contrast to previous studies which favour low abundances, and we do not find a significant trend with mass. JWST extends to a minimum wavelength of 0.6 μm, therefore exploration into complementing JWST observations with optical Hubble data can further our understanding of cloud structure in exoplanet atmospheres.
| Date of Award | 5 Dec 2023 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Hannah R Wakeford (Supervisor) & Zoe M Leinhardt (Supervisor) |