The Influences of Hydrogen-Silicate-Iron Miscibility on the Demographics of Sub-Neptunes and Super-Earths

TL;DR

This study models how hydrogen-silicate-iron miscibility influences the formation and characteristics of sub-Neptunes and super-Earths, aligning with observed planetary distributions and features.

Contribution

It introduces a unified model linking planetary composition, phase equilibria, and observed demographics through hydrogen-silicate-iron miscibility effects.

Findings

Models reproduce the observed occurrence density and radius gap.

Hydrogen content determines whether planets have discrete cores or fully miscible interiors.

Miscibility explains the diversity of sub-Neptune and super-Earth architectures.

Abstract

Models based on variable miscibility among hydrogen, molten silicate, and molten iron, coupled with atmospheric escape, can reproduce the observed occurrence density structure of sub-Neptunes and super-Earths in mass-radius space. The models are also consistent with the radius gap and the observed radius-period relationship exhibited by these planets. The degree of overlap between predicted and observed planetary occurrences suggests that hydrogen-silicate-iron miscibility may serve as a unifying concept for the formation and evolution of these planet classes. The well-defined equilibrium conditions at the boundary between supercritical magma oceans and the overlying hydrogen-rich envelopes are important features of the models. Planets formed with less than ~1 % hydrogen by mass develop discrete, terrestrial-like metallic cores, while those accreting greater hydrogen concentrations are predicted to have fully miscible interiors and no discrete metal cores. Hydrogen-silicate-iron miscibility provides an overarching explanation for the full range of sub-Neptune and super-Earth architectures based on the accreted hydrogen mass fraction and the phase equilibria governing silicate, iron metal, and H$_2$ miscibility.