THE HEAVY-ELEMENT COMPOSITION OF DISK INSTABILITY PLANETS CAN RANGE FROM SUB-TO SUPER-NEBULAR

Transit surveys combined with Doppler data have revealed a class of gas giant planets that are massive and highly enriched in heavy-elements (e.g., HD 149026b, GJ436b, and HAT-P-20b). It is tempting to consider these planets as validation of core accretion plus gas capture because it is often assumed that disk instability planets should be of nebular composition. We show in this paper, to the contrary, that gas giants that form by disk instability can have a variety of heavy-element compositions, ranging from sub-to super-nebular values. High levels of enrichment can be achieved through one or multiple mechanisms, including enrichment at birth, planetesimal capture, and differentiation plus tidal stripping. As a result, the metallicity of an individual gas giant cannot be used to discriminate between gas giant formation modes.