SubNeptunes - Probing Exoplanetary Atmospheres in the Sub-Neptune Regime

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Sub-Neptune planets represent the new frontier in exoplanet science. Exoplanet demographics reveal that planets between Earth and Neptune in size (1-4 Earth radii) overwhelmingly dominate the known exoplanet population. These planets, collectively referred to as "sub-Neptunes", encompass the entire range of planet types from rocky super-Earths to volatile-rich mini-Neptunes. These planets dominate the exoplanet population but have no analog in the solar system. A defining feature of this population is the "Radius Valley", a bimodal distribution in radii with two peaks separated by a valley around 1.8 Earth radii. As competing theories of planet formation, interiors and atmospheres strive to understand this exotic regime, it is becoming clear that atmospheric observations are essential to place definitive constraints on their properties, physical processes and origins. We propose to conduct a comprehensive atmospheric survey of ten nearby sub-Neptunes across the Radius Valley. The project will address three fundamental questions in exoplanetary science: How diverse are the atmospheric compositions of sub-Neptunes? How do disequilibrium processes operate in sub-Neptune atmospheres? How do sub-Neptunes form and evolve? These questions will be answered using a combination of pioneering approved observations with state-of-the-art facilities in space (HST and JWST) and on ground, and key advancements in theoretical modelling and atmospheric retrieval techniques. This will enable new detections of key molecular species such as H2O, CH4, NH3, and HCN in diverse planets across the radius valley and key constraints on physical and chemical processes in the atmospheres and on their formation conditions. In addition to answering the above transformational questions, we will conduct an in-depth atmospheric characterisation of a habitable-zone sub-Neptune using approved JWST observations.