Tidal Downsizing model. II. Planet-metallicity correlations

TL;DR

This paper explores the Tidal Downsizing model for planet formation, showing it explains observed exoplanet metallicity correlations better than Core Accretion, especially for smaller planets and distant gas giants.

Contribution

It provides population synthesis results demonstrating non-linear metallicity correlations in the Tidal Downsizing framework, contrasting with traditional models.

Findings

Gas giant planets show a positive metallicity correlation.

Smaller planets have a shallow peak at Solar metallicity.

Giant planets at large separations do not prefer metal-rich stars.

Abstract

Core Accretion (CA), the de-facto accepted theory of planet formation, requires formation of massive solid cores as a prerequisite for assembly of gas giant planets. The observed metallicity correlations of exoplanets are puzzling in the context of CA. While gas giant planets are found preferentially around metal-rich host stars, planets smaller than Neptune orbit hosts with a wide range of metallicities. We propose an alternative interpretation of these observations in the framework of a recently developed planet formation hypothesis called Tidal Downsizing (TD). We perform population synthesis calculations based on TD, and find that the connection between the populations of the gas giant and the smaller solid-core dominated planets is non linear and not even monotonic. While gas giant planets formed in the simulations in the inner few AU region follow a strong positive correlation with the host star metallicity, the smaller planets do not. The simulated population of these smaller planets shows a shallow peak in their formation efficiency at around the Solar metallicity. This result is driven by the fact that at low metallicities the solid core's growth is damped by the scarcity of metals, whereas at high metallicities the fragments within which the cores grow contract too quickly, cutting the core's growth time window short. Finally, simulated giant gas planets do not show a strong host star metallicity preference at large separations, which may explain why one of the best known directly imaged gas giant planet systems, HR 8799, is metal poor.