Why do more massive stars host larger planets?

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.

Ariel stellar characterisation

“…The higher efficiency allows them to accrete a substantial gaseous envelope before the gas disc dissipates. The results shown in Figures 13 and 14 confirm the finding of Lozovsky et al (2021), extending them at higher stellar effective temperatures. For F stars, Pascucci et al (2018) expected that the presence of other effects, such as atmospheric loss by photoevaporation, might affect the properties of their planetary systems.…”

“…Pascucci et al (2018), focusing on GKM stars, showed that the radius of the planets depends on the mass of the stellar host. Very recently, Lozovsky et al (2021) confirmed the tendency, and explored the reasons for which more massive stars host larger planets. While the hypothesis bases on thermal inflation and on the effect of planet mass on the radius are less favourable, Lozovsky et al (2021) concluded that planets around more massive stars tend to be richer in gas and are, therefore, larger.…”

“…Very recently, Lozovsky et al (2021) confirmed the tendency, and explored the reasons for which more massive stars host larger planets. While the hypothesis bases on thermal inflation and on the effect of planet mass on the radius are less favourable, Lozovsky et al (2021) concluded that planets around more massive stars tend to be richer in gas and are, therefore, larger. Specifically, they suggested that the difference in planetary radii among planets that orbit stars with different masses might be caused by different H-He mass fractions: planets around hotter and more massive stars being richer in such light elements than planets around cooler and less massive stars.…”

Constraining stellar rotation and planetary atmospheric evolution of a dozen systems hosting sub-Neptunes and super-Earths

“…initial atmospheric mass fraction vs. either planetary or stellar mass and semi-major axis) that would enable planetary accretion models, and thus planet formation to be empirically constrained (e.g. Lozovsky et al 2021).…”

The Effect of Accretion Rate and Composition on the Structure of Ice-rich Super-Earths

“…The connection between planetary origin and internal structure is nontrivial, and different formation mechanisms and birth environments can lead to a large range of compositions and internal structures (e.g., Helled et al 2014;Lozovsky et al 2017;Valletta & Helled 2020 and references within). Even the host stellar type might influence the planetary structure and composition (Silva & Valio 2016;Adibekyan et al 2021;Lozovsky et al 2021). While there are numerous observations of protoplanetary disks (e.g., Andrews 2020), and various aspects of their evolution have been modeled theoretically, the properties of such protoplanetary disks are still not well constrained.…”