Probing Mini-Neptune Atmospheres: the case of K2-18b

The Kepler and TESS missions found a multitude of planetary systems with Super-Earths and/or Mini-Neptunes in compact orbits around their host stars. Planets in this range of sizes and orbits appear as a common outcome of planetary formation, although they are not present in our Solar System. Remote-sensing spectroscopy of their atmospheres is key to understanding their bulk composition and possible formation pathways, such as the relative amounts of rock and volatiles, the persistence of a primordial H-He envelope, and ideally their elemental ratios. K2-18b is one of the smallest exoplanets with robust atmospheric detections (CO2 and CH4), leading to hypotheses that it is a hycean, likely inhabited, planet (Madhusudhan et al. 2023), or a gas-rich mini-Neptune (Wogan et al. 2024). The previous molecular detections, albeit with different possible interpretations, make it a particularly relevant case of study. We re-reduced and analysed the JWST NIRISS and NIRSpec data taken during two transits in 2023, confirming the detection of CO2 and CH4 in the atmosphere of K2-18 b. Moreover we quantitatively test the impact of different treatments of stellar limb-darkening, corrections for an occulted spot (and unocculted ones), instrumental ramps, spectroscopic binning and outlier rejection in the atmospheric transmission spectrum and the effect these have on possible molecular detections, highlighting the case of CO.