Observational Perspective on Linking Sub-Neptune Atmospheric Measurements to Planet Formation

Low-mass planets (sub-Neptunes and super-Earths) are the most common types of exoplanets with periods less than 100 days. Their radii suggest a bimodal distribution with most planets larger than 1.6–2 Earth radii maintaining a H/He envelope; a conclusion derived from their less than rocky densities. However, composition models suggest the overall structure of these planets may be more complex than previously thought. While a rocky core overlaid with a H/He atmosphere can explain their bulk densities, a volatile or water-rich world with a steam atmosphere is also possible. Mass-loss from atmospheric escape and core-atmosphere interactions further complicate our understanding of their overall composition. Atmospheric characterization of these worlds should provide essential insights into their structure and dynamics while also seeking to test current formation and evolutionary theories. However, observational attempts using transmission spectroscopy of sub-Neptunes have proved challenging due to their smaller scale heights, their expected metal-rich atmospheres, and the common presence of aerosols. In this talk I will (a) summarize our current observational knowledge about sub-Neptunes and how these results compare to theory, (b) discuss what we expect to discover as new state-of-the-art facilities and telescopes come online, (c) and connect these results with theories surrounding the formation and evolutionary pathways of sub-Neptunes. To accomplish this, I will highlight a few key systems that have the potential to dramatically enhance our understanding of the formation and evolution of these worlds.