Secondary atmospheres on HD 219134 b and c

TL;DR

This study uses Bayesian inference and atmospheric escape analysis to determine that HD 219134 b and c likely do not have hydrogen-rich primordial atmospheres, suggesting they may have secondary atmospheres or volatile layers.

Contribution

It introduces a method combining interior structure modeling with atmospheric escape considerations to constrain the nature of exoplanet atmospheres without spectroscopic data.

Findings

Maximum possible atmospheres are smaller than primordial thresholds.

Planets likely have enriched, secondary atmospheres.

HD 219134 c could be rocky, while b has a larger gas layer.

Abstract

We analyze the interiors of HD~219134~b and c, which are among the coolest super Earths detected thus far. Without using spectroscopic measurements, we aim at constraining if the possible atmospheres are hydrogen-rich or hydrogen-poor. In a first step, we employ a full probabilistic Bayesian inference analysis in order to rigorously quantify the degeneracy of interior parameters given the data of mass, radius, refractory element abundances, semi-major axes, and stellar irradiation. We obtain constraints on structure and composition for core, mantle, ice layer, and atmosphere. In a second step, we aim to draw conclusions on the nature of possible atmospheres by considering atmospheric escape. Specifically, we compare the actual possible atmospheres to a threshold thickness above which a primordial (H$_2$-dominated) atmosphere can be retained against evaporation over the planet's lifetime. The best constrained parameters are the individual layer thicknesses. The maximum radius fraction of possible atmospheres are 0.18 and 0.13 $R$ (radius), for planets b and c, respectively. These values are significantly smaller than the threshold thicknesses of primordial atmospheres: 0.28 and 0.19 $R$, respectively. Thus, the possible atmospheres of planets b and c are unlikely to be H$_2$-dominated. However, whether possible volatile layers are made of gas or liquid/solid water cannot be uniquely determined. Our main conclusions are: (1) the possible atmospheres for planets b and c are enriched and thus possibly secondary in nature, and (2) both planets may contain a gas layer, whereas the layer of HD 219134 b must be larger. HD 219134 c can be rocky.