The Heavy Element Masses of Extrasolar Giant Planets, Revealed

TL;DR

This study infers heavy element content in transiting giant exoplanets with moderate insolation, revealing correlations with stellar metallicity and planetary mass, and suggesting all such planets contain significant heavy elements.

Contribution

It provides the first evidence that extrasolar giant planets generally have enhanced heavy element content, revealing correlations with stellar metallicity and planetary mass.

Findings

All giant planets have at least 10-15 Earth masses of heavy elements.

A correlation exists between stellar metallicity and planetary heavy element mass.

Enrichment levels are similar across planets of different masses, indicating common formation processes.

Abstract

We investigate a population of transiting planets that receive relatively modest stellar insolation, indicating equilibrium temperatures $< 1000$ K, and for which the heating mechanism that inflates hot Jupiters does not appear to be significantly active. We use structural evolution models to infer the amount of heavy elements within each of these planets. There is a correlation between the stellar metallicity and the mass of heavy elements in its transiting planet(s). It appears that all giant planets posses a minimum of $\sim$ 10-15 Earth masses of heavy elements, with planets around metal-rich stars having larger heavy element masses. There is also an inverse relationship between the mass of the planet and the metal enrichment ($Z_{\textrm pl} / Z_{\textrm star}$), which appears to have little dependency on the metallicity of the star. Saturn- and Jupiter-like enrichments above solar composition are a hallmark of all the gas giants in the sample, even planets of several Jupiter masses. These relationships provides an important constraint on planet formation, and suggests large amounts of heavy elements within planetary H/He envelopes. We suggest that the observed correlation can soon also be applied to inflated planets, such that the interior heavy element abundance of these planets could be estimated, yielding better constraints no their interior energy sources. We point to future directions for planetary population synthesis models and suggest future correlations. This appears to be the first evidence that extrasolar giant planets, as a class, are enhanced in heavy elements.