2026ApJ...996L..23D

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 H₂-dominated water-rich to bring the mean molecular weight (μ) to intermediate values (∼4-10) when He/H₂ 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/H₂ 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.