Abstract: Sub-Neptunes, planets ranging in size between Earth and Neptune, are the most abundant type of exoplanets discovered to date. However, their nature remains enigmatic, as no such planets exist in our Solar System. Sub-Neptunes are hypothesized to be either bloated terrestrial planets (“super-Earths”), miniature giant planets (“mini-Neptunes”), or perhaps something in between, such as “water worlds.” Of particular interest are temperate sub-Neptunes, which hold significant potential for habitability.

In this talk, I will present recent work from our group that combines photochemical modeling and modern geochemical tools to investigate the nature of temperate sub-Neptunes. I will highlight examples of our models applied to new JWST atmospheric characterization data for systems such as TOI-270 d, K2-18 b, and LHS 1140 b. Finally, I will discuss our ongoing laboratory efforts to facilitate our understanding of the puzzling nature of these intriguing worlds.