Helium Atmospheres May Hide in Current Exoplanet Analysis Frameworks

TL;DR

Current exoplanet atmospheric analyses may overlook helium-rich atmospheres, which can mimic water-rich or cloudy atmospheres, thus affecting interpretations of planetary composition and evolution.

Contribution

This paper highlights the potential for helium-rich atmospheres to be misinterpreted and demonstrates a reanalysis approach treating He/H$_2$ as a free parameter, revealing the importance of considering helium enhancement.

Findings

Helium-rich atmospheres can mimic water-rich spectra.

Reanalysis of HD 209458 b shows multiple helium-rich solutions.

Considering helium is crucial for accurate atmospheric characterization.

Abstract

The increasing number of detailed exoplanet observations offers an opportunity to refine our analyses and interpretations. Here, we show that atmospheres that appear volatile-rich and/or cloudy may instead be helium-rich. As transmission spectra constrain the atmospheric scale height ($H$), a He-enriched atmosphere can be misinterpreted as H$_2$-dominated water-rich to bring the mean molecular weight ($\mu$) to intermediate values ($\sim$4$-$10) when He/H$_2$ is fixed. Similarly, a cloud deck can reduce the spectral features, and thus the apparent (i.e., cloud-free equivalent) $H$. We present a proof-of-concept reanalysis of HD~209458~b's JWST transmission spectrum treating He/H$_2$ as a free parameter, resulting in sets of He-rich solutions. We argue that He enhancement must be considered to reliably constrain atmospheric composition, be sensitive to a more diverse planetary population, and ultimately yield robust trends to inform formation and evolution pathways. Looking ahead, we suggest leveraging insights from differences in pressure-broadening effects, outflow measurements, and atmospheric chemistry to disentangle reliably between He-, volatile-rich, and cloudy atmospheres -- while recognizing that associated models need targeted upgrades to reach the fidelity level required to this end.