No Metallicity Correlation Associated with the Kepler Dichotomy

TL;DR

This study investigates whether stellar metallicity correlates with planetary system architecture in Kepler data, finding no significant metallicity difference between single and multiple transiting planet systems, thus challenging the external giant planet hypothesis for the Kepler dichotomy.

Contribution

It provides a comprehensive statistical analysis of stellar metallicities in Kepler systems, testing the hypothesis that external giant planets cause the Kepler dichotomy, and finds no supporting evidence.

Findings

No significant metallicity difference between single and multiple transiting systems.

A 55% upper bound for systems with a single non-giant planet hosting a hidden giant.

Results suggest alternative explanations for the Kepler dichotomy beyond external giant planets.

Abstract

NASA's Kepler mission has discovered thousands of planetary systems, ~20% of which are found to host multiple transiting planets. This relative paucity (compared to the high fraction of single transiting systems) is postulated to result from a distinction in the architecture between multi-transiting systems and those hosting a single transiting planet: a phenomenon usually referred to as the Kepler dichotomy. In this paper, we investigate the hypothesis that external giant planets are the main cause behind the over-abundance of single- relative to multi-transiting systems, which would be signaled by higher metallicities in the former sample. To this end, we perform a statistical analysis on the stellar metallicity distribution with respect to planet multiplicity in the Kepler data. We perform our analysis on a variety of samples taken from a population of 1166 Kepler main sequence planetary hosts, using precisely determined metallicities from the California-Kepler survey and Swift et al. (2015). Contrary to some predictions, we do not find a significant difference between the stellar metallicities of the single- and multiple-transiting planet systems. However, we do find a 55% upper bound for systems with a single non-giant planet that could also host a hidden giant planet, based on metallicity considerations. While the presence of external giant planets might be one factor behind the Kepler dichotomy, our results also favor alternative explanations. We suggest that additional radial velocity and direct imaging measurements are necessary to constrain the presence of gas giants in systems with a single transiting planet.