Spectral Analysis and Atmospheric Retrieval of Terrestrial Exoplanets and Mini-Neptunes Using the TauREx 3 Framework

We report the results of the atmospheric characterization and Bayesian retrieval analysis for two distinct exoplanetary classes: the mini-Neptune K2-18b and the Earth-sized terrestrial exoplanet LHS 475b. For K2-18b, our retrieval models successfully reproduce the spectral signatures of methane (CH₄) and carbon dioxide (CO₂) and effectively rule out the existence of high-altitude opaque clouds at the 10 mbar pressure level. For LHS 475b, the study focuses on evaluating the presence of a secondary atmosphere and establishing upper limits on the abundances of major volatile species.
The investigation utilizes high-precision transmission spectra retrieved from the NASA Exoplanet Archive (JWST observations). We integrate datasets from the NIRISS (0.8–2.8 μm) and NIRSpec (2.7–5.2 μm) instruments to ensure broad-band infrared coverage essential for identifying molecular absorption features. Forward modeling based on radiative transfer equations and Bayesian retrieval pipelines were implemented within the TauREx 3 framework. To maintain consistency with the observational resolving power, all theoretical spectra were convolved with a Gaussian filter (σ = 30).
Additionally, by applying comparative retrieval analysis, we quantify the impact of photochemical hazes and cloud deck muting on spectral interpretation. This approach allows for the mitigation of parameter degeneracies and the reconstruction of precise Temperature-Pressure (T-P) profiles. Our findings highlight the critical role of multi-instrument retrieval in verifying atmospheric conditions on sub-Neptunes and terrestrial planets.