Seeing the Forest and the Trees: Unveiling Small Planet Atmospheres with a Population-Level Framework

The last decades of exoplanet exploration have revealed that the diversity of planets within the Galaxy far exceeds that within our solar system. Specifically, Kepler revealed a new population of 1-3 Rearth short period planets that seem to bridge the gap between giants and terrestrials in our own bimodal planetary system. Their bulk properties imply diminishingly thin hydrogen envelopes, producing an intermediate physical state between planets with predominantly primordial atmospheres and those with secondary atmospheres. However, we lack observations to determine whether this is truly the case, how such atmospheres are produced, and how similar or different they are to solar system planets. Atmospheric composition provides the necessary additional dimension to unveil the nature of this new class of planet, which we maintain will be JWST's greatest exoplanet legacy. We have carefully constructed the first exoplanet atmospheres survey designed to build a critical link between atmospheric characterization and planetary demographics. We will observe 11 transiting exoplanets, including four pairs of planets in the same system. By utilizing JWST's unique capabilities, we will measure the relative abundances of major molecular species expected to provide key insights into the formation and evolution pathways of exoplanets. Through our holistic approach we will analyze planets individually, within their own system architectures, and ultimately as a population. This will culminate in a program that will provide the community with a necessary kickstart to future information-rich observations of small planets, and will ultimately sculpt JWST's lasting legacy.