paper

Helium Atmospheres May Hide in Current Exoplanet Analysis Frameworks

arXiv:2506.22537 · doi:10.3847/2041-8213/ae2f5c

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 (), a He-enriched atmosphere can be misinterpreted as H-dominated water-rich to bring the mean molecular weight () to intermediate values (410) when He/H is fixed. Similarly, a cloud deck can reduce the spectral features, and thus the apparent (i.e., cloud-free equivalent) . We present a proof-of-concept reanalysis of HD~209458~b's JWST transmission spectrum treating He/H 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.

Submitted for review. Comments welcome

References in corpus (30)