PLANET TRAPS AND PLANETARY CORES: ORIGINS OF THE PLANET–METALLICITY CORRELATION

Massive exoplanets are observed preferentially around high metallicity ([Fe/H]) stars while low-mass exoplanets do not show such an effect. This so-called planet–metallicity correlation generally favors the idea that most observed gas giants at r < 10 AU are formed via a core accretion process. We investigate the origin of this phenomenon using a semi-analytical model, wherein the standard core accretion takes place at planet traps in protostellar disks where rapid type I migrators are halted. We focus on the three major exoplanetary populations—hot Jupiters, exo-Jupiters located at r ≃ 1 AU, and the low-mass planets. We show using a statistical approach that the planet–metallicity correlations are well reproduced in these models. We find that there are specific transition metallicities with values [Fe/H] = −0.2 to −0.4, below which the low-mass population dominates, and above which the Jovian populations take over. The exo-Jupiters significantly exceed the hot Jupiter population at all observed metallicities. The low-mass planets formed via the core accretion are insensitive to metallicity, which may account for a large fraction of the observed super-Earths and hot-Neptunes. Finally, a controlling factor in building massive planets is the critical mass of planetary cores (Mc, crit) that regulates the onset of rapid gas accretion. Assuming the current data is roughly complete at [Fe/H] > −0.6, our models predict that the most likely value of the “mean” critical core mass of Jovian planets is 〈Mc, crit〉 ≃ 5 M⊕ rather than 10 M⊕. This implies that grain opacities in accreting envelopes should be reduced in order to lower Mc, crit.