Detailed Stellar Chemistry and its Role in Planet Occurrence

Because the composition of stellar atmospheres preserves the composition of the nebulae in which the stars formed, stellar abundances offer a unique glimpse into the epoch of planet formation and evolution of planetary systems. Comparing the demographics of planet-hosting stars with field stars has shown strong correlations between stellar metallicity and planet occurrence, with intricate relationships depending on planet size and orbital distance. These correlations have been used to validate planet formation scenarios such as core accretion. However, it is still unclear whether the trends between planet occurrence and metallicity are truly caused by an increase in overall dust as is commonly interpreted, or if stellar metallicity is simply a proxy for more complicated chemical relationships.

Using a high reliability sample of Kepler planets observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE), we make the first ever measurement of the correlation between planet occurrence and chemical abundances for ten different elements (C, Mg, Al, Si, S, K, Ca, Mn, Fe, and Ni). We find that an increase in the abundance of any one element leads to an increase in planet occurrence of the same magnitude. As seen in previous works, the strength of the correlations depend strongly on planet size and orbital period, with the occurrence of shorter period and larger radius planets showing the strongest correlations with enhanced abundances. Though we measure correlations with planet occurrence for each element, we are unable to uniquely attribute the correlations to any one particular element due to the chemical homogeneity of the stars in the Kepler field.

These results point to larger challenges in exoplanet demographics, namely the difficulty in disentangling the effects of Galactic Chemical Evolution (GCE) with the role that specific elements play in planet formation. To fully disentangle these various effects, we advocate for targeted planet searches over distinct populations of the Milky Way, and in particular, outside the Solar neighborhood. In addition to correlations in stellar chemistry, we also note strong correlations between stellar chemistry and age, and demonstrate the degeneracies that may arise between demographic trends between these two variables.