Three regimes of extrasolar planets inferred from host star metallicities

TL;DR

This study classifies exoplanets into three groups based on host star metallicities, revealing how metallicity influences planetary formation and composition, with implications for understanding planet diversity and habitability.

Contribution

It provides the first large-scale statistical analysis linking host star metallicities to three distinct exoplanet populations, enhancing understanding of planet formation regimes.

Findings

Exoplanets are categorized into three populations by metallicity.

Host star metallicity correlates with planetary size and composition.

Transitions between planet types align with dynamical mass estimates.

Abstract

Approximately half of the extrasolar planets (exoplanets) with radii less than four Earth radii are in orbits with short periods. Despite their sheer abundance, the compositions of such planets are largely unknown. The available evidence suggests that they range in composition from small, high-density rocky planets to low-density planets consisting of rocky cores surrounded by thick hydrogen and helium gas envelopes. Understanding the transition from the gaseous planets to Earth-like rocky worlds is important to estimate the number of potentially habitable planets in our Galaxy and provide constraints on planet formation theories. Here we report the abundances of heavy elements (that is, the metallicities) of more than 400 stars hosting 600 exoplanet candidates, and find that the exoplanets can be categorized into three populations defined by statistically distinct (~ 4.5{\sigma}) metallicity regions. We interpret these regions as reflecting the formation regimes of terrestrial-like planets (radii less than 1.7 Earth radii), gas dwarf planets with rocky cores and hydrogen-helium envelopes (radii between 1.7 and 3.9 Earth radii) and ice or gas giant planets (radii greater than 3.9 Earth radii). These transitions correspond well with those inferred from dynamical mass estimates, implying that host star metallicity, which is a proxy for the initial solid inventory of the protoplanetary disk, is a key ingredient regulating the structure of planetary systems.