Why do more massive stars host larger planets?

M Lozovsky,J Venturini,R Feldmann,C Dorn,I Pascucci,R Helled

doi:10.1051/0004-6361/202140563

Abstract

Aims. It has been suggested that planetary radii increase with stellar mass for planet sizes smaller than 6 R⊕ and host masses lower than 1 M⊙. In this study, we explore whether this inferred relation of planetary size and host star mass can be explained by a higher planetary mass of planets orbiting higher-mass stars, inflation of the planetary radius due to the difference in stellar irradiation, or different planetary compositions and structures. Methods. Using exoplanetary data of planets with measured masses and radii, we investigated the relations between stellar mass and various planetary properties for G and K stars. We confirm that more massive stars host larger and more massive planets. Results. We find that the differences in the planetary masses and temperatures are insufficient to explain the measured differences in radii for planets surrounding different stellar types. We show that the larger planetary radii can be explained by a larger fraction of volatile material (H-He atmospheres) in planets surrounding more massive stars. Conclusions. We conclude that planets around more massive stars are most probably larger as a result of larger H-He atmospheres. Our findings imply that planets forming around more massive stars tend to accrete H-He atmospheres more efficiently.

Highlights

Exoplanetary exploration has led to the discovery of several thousand exoplanets in the past several decades
In this study we investigated the effect of the planetary mass, equilibrium temperature, and possible bulk composition on the planetary radius for planets surrounding G and K stars
The larger radii of planets surrounding more massive stars cannot be explained by inflation through higher irradiation of the star, nor by a higher planetary mass: the effect of both properties on the radius was found to be significantly smaller than the observed difference

Summary

Introduction

Exoplanetary exploration has led to the discovery of several thousand exoplanets in the past several decades. We have learned that planets are very diverse astronomical objects and do not follow a single mass-radius (hereafter M-R) relation, but instead show intrinsic scatter (Wolfgang et al 2015; Chen & Kipping 2016; Ning et al 2018; Neil & Rogers 2020). This scatter suggests a large diversity in planetary formation and evolution, and in interior structure and bulk composition (e.g., Weiss & Marcy 2014; Turrini et al 2015; Petigura et al 2018; Plotnykov & Valencia 2020). While determining the planetary structure and composition from the measured mass and radius is a highly degenerate problem because very different compositions and structures can yield similar masses and radii (e.g., Rogers & Seager 2010; Dorn et al 2015, 2017), some trends and limits on the planetary compositions can be found using statistical analysis (e.g., Rogers 2015; Owen & Wu 2017; Lozovsky et al 2018; Neil & Rogers 2020; Otegi et al 2020a)

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Results

Discussion

Conclusion