Exploring the Relationship Between Planet Mass and Atmospheric Metallicity

Observations of transiting planets provide an invaluable window into the processes of planet formation and evolution. By measuring the masses and radii of these planets, we can calculate their average densities and constrain their bulk compositions, which presumably vary depending on their formation locations and accretionary histories. Results from large surveys such as Kepler indicate that the average densities of planets tend to increase with decreasing mass, suggesting that low-mass planets have comparatively large rocky or icy cores. In this proposal we will test a fundamental finding of planetary science over the past few decades: the correlation between core mass fraction and atmospheric metallicity. In our own solar system, Neptune and Uranus have both a larger percentage of their masses tied up in solid cores and more metal-rich atmospheres as compared to Jupiter and Saturn. However, with a sample size of just four planets it is difficult to know whether or not this relation holds for all exoplanets, or just for systems that meet particular conditions. Our program focuses on secondary eclipse observations of cool planets, where the relative abundances of methane and CO provide a sensitive tracer of atmospheric metallicity. We select a sample of planets with masses ranging from sub-Neptune to super-Jupiter sizes and temperatures cooler than 1100 K; these systems offer the first opportunity to confront this fundamental question prior to the launch of JWST.