Planet Mass and Metallicity: The Exoplanets and Solar System Connection

Theoretical studies of giant planet formation suggest that substantial quantities of metals—elements heavier than hydrogen and helium—can be delivered by solid accretion during the envelope-assembly phase. This process of metal enhancement of the envelope is believed to diminish as a function of planet mass, leading to predictions for a mass-metallicity relationship. Supporting evidence for this picture is provided by the abundance of CH4 in solar system giant planets, where CH4 abundance, unlike H2O, is unaffected by condensate cloud formation. However, all of the solar system giants exhibit some evidence for stratification of metals outside of their cores. In this context, two fundamental questions are whether metallicity of giant planets inferred from observations of the outer envelope layers represents the bulk metallicity of these planets, and if not, how are metals distributed within giant planets. Comparing the mass-metallicity relationship for solar system giant planets, inferred from the observed CH4 abundance, with various tracers of metallicity in the exoplanet population, has yielded a range of results. There is evidence of a solar-system-like mass-metallicity trend using bulk density estimates of exoplanet metallicity. However, transit-spectroscopy-based tracers of exoplanet metallicity, which probe only the outer layers of the envelope, are less clear about a mass-metallicity trend and raise the question of whether radial composition gradients exist in some giant exoplanets. The large number of known exoplanets enables statistical characterization of planet properties. We develop a formalism for comparing both the metallicity inferred for the outer envelope and the metallicity inferred using the bulk density and show this combination may offer insights into the broader question of metal stratification within planetary envelopes. Our analysis suggests that future exoplanet observations with JWST and Ariel will be able to shed light on the conditions governing radial composition gradients in exoplanets and, perhaps, provide information about the factors controlling stratification and convection in our solar system gas giants.