Helium Atmospheres May Hide in Current Exoplanet Analysis Frameworks

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 ( μ ) to intermediate values (∼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.