Atmospheric characterization of K2-18b

This thesis investigates the atmosphere of K2-18b, a sub-Neptune exoplanet located in the habitable zone of an M-dwarf star. The study aims to characterize the chemical composition and the temperature-pressure (T-P) profile of the atmosphere to understand whether K2-18b has a primary or secondary atmosphere. The research starts with an overview of small exoplanets, focusing on super-Earths and sub-Neptunes, two categories that include K2-18b. This thesis examines the gap in the radius distribution between these types of planets, exploring theories like photoevaporation and core-powered mass loss that might explain the transition between primary and secondary atmospheres. It continues by illustrating the various results relating to the atmosphere of K2-18b in the literature, concluding that the planet is a sub-Neptune with an H2-rich atmosphere. To verify the kinds of atmosphere of K2-18b, a complete analysis of the planet’s atmospheric composition and temperature-pressure profile is conducted using data from the James Webb Space Telescope (JWST), through transit spectroscopy and transmission spectrum, already reduced. Various atmospheric models are simulated and compared, in particular molecular constant abundance and equilibrium chemistry models with isothermal and 4-point T-P profiles. For all the retrievals is dominant the presence of the CH4 and CO2 molecules. The best model, in this thesis, considers a constant molecular abundance profile and isothermal temperature pressure profile. The available transmission spectrum does not give enough information to choose a more complex model with a high statistical significance. The results suggest that K2-18b’s atmosphere is likely in a transitional state, between a hydrogen-rich primary atmosphere and a secondary atmosphere containing heavier molecules. The study concludes by considering the need for additional observations with higher precision and a larger wavelength range to confirm whether K2-18b has a hybrid or fully secondary atmosphere.