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Core Accretion (CA), the de-facto accepted theory of planet formation,requires formation of massive solid cores as a prerequisite for assembly of gasgiant planets. The observed metallicity correlations of exoplanets are puzzlingin the context of CA. While gas giant planets are found preferentially aroundmetal-rich host stars, planets smaller than Neptune orbit hosts with a widerange of metallicities. We propose an alternative interpretation of theseobservations in the framework of a recently developed planet formationhypothesis called Tidal Downsizing (TD). We perform population synthesiscalculations based on TD, and find that the connection between the populationsof the gas giant and the smaller solid-core dominated planets is non linear andnot even monotonic. While gas giant planets formed in the simulations in theinner few AU region follow a strong positive correlation with the host starmetallicity, the smaller planets do not. The simulated population of thesesmaller planets shows a shallow peak in their formation efficiency at aroundthe Solar metallicity. This result is driven by the fact that at lowmetallicities the solid core's growth is damped by the scarcity of metals,whereas at high metallicities the fragments within which the cores growcontract too quickly, cutting the core's growth time window short. Finally,simulated giant gas planets do not show a strong host star metallicitypreference at large separations, which may explain why one of the best knowndirectly imaged gas giant planet systems, HR 8799, is metal poor.

Tidal Downsizing model. II. Planet-metallicity correlations