The bulk metallicity of giant planets around M stars

TL;DR

This study compares the bulk metallicity of giant exoplanets orbiting M-dwarf stars to those around FGK stars, revealing lower metallicities around M dwarfs and implications for planet formation theories.

Contribution

It provides the first comparative analysis of giant planet metallicities around M-dwarfs versus FGK stars using interior models and mass-radius data.

Findings

Giant planets around M dwarfs have lower bulk metallicity than those around FGK stars.

Mass-metallicity relations have similar slopes but different offsets for the two star types.

Limited data (20 planets) results in large uncertainties, highlighting the need for more detections.

Abstract

The bulk-metallicity determination of giant exoplanets is essential to constrain their formation and evolution pathways and to compare them to the solar system. Previous studies inferred an inverse relation between the mass and bulk metallicity. However, the data almost exclusively contained planets that orbit FGK stars. The recent discoveries of giant exoplanets around M-dwarf stars present an opportunity to probe whether they follow a mass-metallicity trend different from that of their FGK counterparts. Using evolution models we characterised the interiors of giant exoplanets with reliable mass-radius measurements that orbit FGK and M-dwarf stars. We then inferred the mass-metallicity trends for both populations. We found that the bulk metallicity of giant planets around M stars is overall lower compared to those around FGK stars. This yielded mass-metallicity relations for the two populations with similar slopes but significantly different offsets. The lack of metal-rich giant planets around M dwarfs could explain the difference in the inferred offset and be a result of different formation conditions. However, there were only 20 successful bulk-metallicity retrievals for the giant planets around M dwarfs, which resulted in rather large uncertainties. Therefore, it is of great importance to continue detecting these planets with both transit and radial velocities. Additionally, the characterisation of the atmospheres of giant planets around M-stars can further help to constrain their interiors and to investigate the atmosphere-interior connection. This will significantly contribute towards understanding the possible formation pathways of giant planets.