A Dichotomy of the Mass-Metallicity Relation of Exoplanetary Atmospheres Demarcated by their Birthplace

Abstract: Atmospheric observations by JWST raise a growing evidence that atmospheric metallicity exhibits an anti-correlation with masses of giant exoplanets. While such a trend was anticipated by planetesimal-based planet formation models, it remains unclear what kind of atmospheric metallicity trends emerge from pebble-based planet formation. Moreover, while recent studies of solar system Jupiter suggest that uppermost observable atmosphere may not represent the bulk envelope composition, it remains uncertain how the envelope inhomogeneity influences the atmospheric metallicity trend. In this study, we develop disk evolution and planet formation models to investigate the possible atmospheric metallicity trends of giant exoplanets formed via pebble accretion and how they depend on the metallicity inhomogeneity within the envelope. We find that pebble-based planet formation produces two distinct mass-metallicity relations depending on planetary birthplace. Planets formed beyond the H2O snowline exhibit a mass-metallicity anti-correlation similar to that predicted by planetesimal-based models if their atmospheres are fully convective. This anti-correlation disappears if the convective mixing is inefficient. In contrast, planets formed inside the H2O snowline show a shallower mass-metallicity anti-correlation, regardless of the efficiency of atmospheric mixing. Many gas giants observed by JWST observations lie around the mass-metallicity relation predicted for formation at close-in orbits, although some planets with sub-stellar atmospheric metallicity appear to require unmixed envelopes and formation beyond the H2O snowline. We also examine the relationship between bulk and atmospheric metallicity and find a clear correlation that closely follows atmospheric metallicity that is comparable to bulk metallicity.