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Abstract:

Context. The detection of Earth-like planets with the radial velocity (RV) method is extremely challenging today due to the presence of non-Doppler signatures such as stellar activity and instrumental signals that mimic and hide the signals of exoplanets. In a previous paper, we presented the YARARA pipeline, which implements corrections for telluric absorption, stellar activity, and instrumental systematics at the spectral level, and then it extracts line-by-line (LBL) RVs with a significantly better precision than standard pipelines.

Aims. In this paper, we demonstrate that further gains in RV precision can be achieved by performing principal component analysis (PCA) decomposition on the LBL RVs.

Methods. The mean-insensitive nature of PCA means that it is unaffected by true Doppler shifts, and thus can be used to isolate and correct nuisance signals other than planets.

Results. We analysed the data of 20 intensively observed HARPS targets by applying our PCA approach on the LBL RVs obtained by YARARA. The first principal components show similarities across most of the stars and correspond to newly identified instrumental systematics for which we can now correct. For several targets, this results in an unprecedented RV root-mean-square of around 90 cm s⁻¹ over the full lifetime of HARPS. We used the corrected RVs to confirm a previously published 120-day signal around 61 Vir, and to detect a super-Earth candidate (K ~ 60 ± 6 cm s⁻¹, m sin i = 6.6 ± 0.7 M_⊕) around the G6V star HD 20794, which spends part of its 600-day orbit within the habitable zone of the host star.

Conclusions. This study highlights the potential of LBL PCA to identify and correct hitherto unknown, long-term instrumental effects and thereby extend the sensitivity of existing and future instruments towards the Earth analogue regime.

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