2016ApJ...831...64T

Exoplanet discoveries of recent years have provided a great deal of new data for studying the bulk compositions of giant planets. Here we identify 47 transiting giant planets (20 M_⊕ < M < 20 M_J) whose stellar insolations are low enough (F_* < 2 x 10⁸ erg s^–1 cm^–2, or roughly T_eff < 1000) that they are not affected by the hot-Jupiter radius inflation mechanism(s). We compute a set of new thermal and structural evolution models and use these models in comparison with properties of the 47 transiting planets (mass, radius, age) to determine their heavy element masses. A clear correlation emerges between the planetary heavy element mass M_z and the total planet mass, approximately of the form M_z∝\sqrtM. This finding is consistent with the core-accretion model of planet formation. We also study how stellar metallicity [Fe/H] affects planetary metal-enrichment and find a weaker correlation than has previously been reported from studies with smaller sample sizes. We confirm a strong relationship between the planetary metal-enrichment relative to the parent star Z_planet/Z_star and the planetary mass, but see no relation in Z_planet/Z_star with planet orbital properties or stellar mass. The large heavy element masses of many planets (>50 M_⊕) suggest significant amounts of heavy elements in H/He envelopes, rather than cores, such that metal-enriched giant planet atmospheres should be the rule. We also discuss a model of core-accretion planet formation in a one-dimensional disk and show that it agrees well with our derived relation between mass and Z_planet/Z_star.