Reconciling Giant Impacts with the Radius Valley: The Role of Silicate-Hydrogen Miscibility for Sub-Neptunes

Abstract

Giant impacts are believed to play a major role in the assembly, long-term orbital stability, and resonant structure of exoplanetary systems. We revisit giant impact driven atmospheric escape while chemically coupling the interior and atmosphere by allowing for silicate-hydrogen miscibility. Incorporating hydrogen-silicate miscibility fundamentally alters atmospheric escape compared to chemically-inert models. While previous work on chemically-inert planets has found sub-Neptunes to undergo total atmospheric loss following collisions with impactors only a tenth of their mass, we find that including silicate-hydrogen miscibility can significantly reduce mass loss. This results from an increased envelope mean molecular weight, interior hydrogen sequestration, and modified compositional gradients in miscible planets. Miscible planets sequester hydrogen in their interiors post-impact, reducing the hydrogen available in the envelope for mass loss. As planets cool, hydrogen exsolves into their envelopes. Because more massive planets cool slower, they sequester hydrogen for longer, undergoing less mass loss. Thus, atmospheric retention depends more strongly on planet mass than impact energy, provided the impact does not gravitationally unbind the planet. Planets with masses $\gtrsim$4.50 $M_{\oplus}$ and hydrogen mass fractions > 5 % typically retain atmospheres of several percent after a giant impact, while planets with masses $\lesssim$3.30 $M_{\oplus}$ are almost entirely stripped of their envelopes. This allows sub-Neptunes with hydrogen-dominated envelopes to survive giant impacts. Contrastingly, lower-mass planets, which ultimately populate the super-Earth regime, are frequently stripped of their envelopes. Our findings suggest that super-Earths and sub-Neptunes could originate from a single population with rocky interiors even if post-disk-dispersal giant impacts were common.