Time-Lapse of Exoplanets: Watching Sub-Neptunes Evolve with JWST

Sub-Neptunes are the most common type of exoplanets in the Galaxy, yet our own solar system does not have one. These worlds sit between Earth and Neptune in size, and their diversity makes them prime targets for understanding planetary habitability with upcoming missions such as the Habitable Worlds Observatory (HWO).

Two competing formation pathways have been proposed. In the gas-dwarf scenario, sub-Neptunes form in-situ, accumulating puffy H/He atmospheres that subsequently evolve through intense mass loss, cooling, and contraction. Alternatively, they could form farther out as volatile-rich worlds that migrate inward. Distinguishing between these scenarios requires answering several fundamental questions:
 

• What is the atmospheric composition of sub-Neptunes?
• How do young and mature sub-Neptune atmospheres compare with each other? 
• How diverse are sub-Neptunes immediately after formation?
• What physical processes govern early evolution  and on what timescales?
• 
   

For the first time, JWST allows us to unravel the atmospheric composition of these mysterious sub-Neptunes with unprecedented precision. In this talk, I will present new JWST results for both young (<100 Myr) and mature (~Gyr) sub-Neptunes, compare their atmospheric compositions across age, temperature, and stellar irradiation, and discuss emerging patterns that hint at their origins. I will connect these insights to formation pathways and early evolutionary mechanisms, and conclude with the key open questions that will define the next decade of observations and modeling as we work towards understanding the most common planets in our Galaxy.