On the correlation between metallicity and the presence of giant planets

TL;DR

This paper challenges the traditional view that stellar metallicity directly influences giant planet formation, proposing instead that galactic dynamics and star migration from the inner disk explain observed correlations.

Contribution

It introduces a galactic-scale explanation for the planet-metallicity correlation, emphasizing star migration and the role of the inner galactic disk over local metallicity effects.

Findings

Giant stars with planets do not show a metallicity skew like dwarfs.

Giant planets are more common around thick disk stars at intermediate metallicities.

Star migration from the inner galactic disk can account for the observed planet-metallicity correlation.

Abstract

The correlation between stellar metallicity and the presence of giant planets is well established. It has been tentatively explained by the possible increase of planet formation probability in stellar disks with enhanced amount of metals. However, there are two caveats to this explanation. First, giant stars with planets do not show a metallicity distribution skewed towards metal-rich objects, as found for dwarfs. Second, the correlation with metallicity is not valid at intermediate metallicities, for which it can be shown that giant planets are preferentially found orbiting thick disk stars. None of these two peculiarities is explained by the proposed scenarios of giant planet formation. We contend that they are galactic in nature, and probably not linked to the formation process of giant planets. It is suggested that the same dynamical effect, namely the migration of stars in the galactic disk, is at the origin of both features, with the important consequence that most metal-rich stars hosting giant planets originate from the inner disk, a property that has been largely neglected until now. We illustrate that a planet-metallicity correlation similar to the observed one is easily obtained if stars from the inner disk have a higher percentage of giant planets than stars born at the solar radius, with no specific dependence on metallicity. We propose that the density of molecular hydrogen in the inner galactic disk (the molecular ring) could play a role in setting the high percentage of giant planets that originate from this region.