Bridging the Solar System-Exoplanets Divide: The Sub-Neptune Opportunity

Exoplanets are the next frontier for planetary science. We are witnessing an incredible transition in the study of exoplanets, where they are shifting from a purely astronomical domain to a wider domain in which planetary scientists can also contribute. We saw a similar transition near the beginning of the Space Age when the planets and moons of our solar system became worlds with complex histories that opened our eyes to broader manifestations of physics and chemistry. Each object is an experiment in planetary evolution. As of today, over 6000 confirmed exoplanets have been found. In this talk, I will focus on volatile-rich planets between the sizes of Earth and Neptune. They are commonly called sub-Neptunes. Planet surveys have revealed that sub-Neptunes are the most common class of planets in the Galaxy. Yet, we know very little about them because there are no (known) sub-Neptunes in the solar system; they are terra incognita. The range of possibilities in what they might be like is astonishing and enormous—all the way from massive and potentially habitable ocean worlds to big rocks cloaked in thick, hot atmospheres. Exotic “soot worlds” are another possibility. Launched in late 2021, the James Webb Space Telescope is accelerating the exoplanet revolution by providing compositional data on many exoplanet atmospheres, including some sub-Neptune atmospheres. Molecules like CO2 and CH4 have been detected on these distant worlds. What do they tell us? I will describe recent collaborative efforts to understand the nature of sub-Neptunes. In particular, I will show how we can gain new insights by bringing geochemical approaches developed for studying solar system objects to the exoplanet arena.