Planet-metallicity correlation for Kepler’s giant planets

Abstract

This study is part of a newly growing science in exoplanet characteristics. We present an initial attempt to investigate the relationship between the eclipse depth of giant exoplanets and the metallicity of their host stars to show the best eclipse depth-metallicity trend. This will help us to better understand the conditions that lead to giant planets formation, which helps to postulate about the formation of our Solar System and more generally the planet formation theories. Previous studies have shown that stars with giant planets tend to have higher metallicities than stars without giant planets. This was explained by the core accretion theory. According to this theory, one must expect to see an increase in median eclipse depth with metallicity. Furthermore, since metal-rich stars have smaller radii than metal-poor stars of the same mass and age, a uniform population of planets should show a rise in median eclipse depth with [M/H]. Investigation of the relation between exoplanets and their host stars has greatly evolved due to the Kepler Mission. Since this mission is providing a large sample of candidate planets, Sarah E. Dodson Robinson 2012 studied the relation between the eclipse depth of 213 Kepler gas giant candidates (from Q0 till Q5) and the metallicity of their parent stars. The author found a negative eclipse depth-metallicity trend with−2.3σ significance level. This could be explained by the disk instability model: the higher the metallicity, the higher the disk opacity which leads to less cooling therefore, the smaller the probability to form giant planets. Is there really a negative correlation between the eclipse depths of Kepler gas giant planets and the metallicity of their parent stars? In an attempt to answer this question and to figure out a more accurate trend we start by removing the biased planets (false positive and planets that are out of range of the sample selection criteria) from the sample of Sarah E. Dodson Robinson 2012. Based on the Kendall’s τ correlation coefficient we found that the significance level decreased to−0.7σ. This suggests that this negative trend may not be so significant. We then identified a larger sample of candidate and confirmed planets (From Q0 till Q12), provided by the Kepler mission as on June 2013. We quantify a positive eclipse depth-metallicity trend with 0.4σsta-tistical significance. With the publication of an extended data on March 2014 (FromQ0 till Q16) we found, again, a positive eclipse depth-metallicity trend with a bigger significance level of 0.9σ.From this work, we can conclude that the negative eclipse depth-metallicity trend is not that obvious and it tends to be more likely a positive trend. Nevertheless, we propose a scenario that supports the formation of planets by the core accretion process at high metallicity and by the disk instability at small metallicity.