Are Helium Atmospheres Hiding as Volatile-rich in Current Exoplanet Analysis Frameworks?

Abstract: The increasing number of detailed exoplanet observations offers an opportunity to refine our analyses and interpretations, and uncover previously-hidden trends. One such trend is unexpectedly high water abundances ($\sim$1$-$10\%) for several hot Jupiters. Here, we show that such atmospheres may instead be helium-rich, and the perceived high water abundances simply biases in analyses compensating for ``ad hocly'' fixed helium-to-hydrogen ratio (He/H$_2$ = 0.157). As transmission spectra constrain the atmospheric scale height ($H$), a helium-enriched atmosphere can be misinterpreted as H$_2$-dominated water-rich to bring the mean molecular weight ($\mu$) to intermediate values ($\sim$4$-$15) when He/H$_2$ is fixed. 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 solutions with helium enrichment and non-extreme water abundance. The He-enhancement found requires an order of magnitude less enrichment/evolution than does a water abundance of 10\%, suggesting that the He-rich hypothesis is a viable alternative. 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 metallicity trends to inform formation/evolution pathways. Looking ahead, we suggest leveraging insights from differences in pressure-broadening effects, outflow measurements, and atmospheric chemistry to help disentangle reliably between He- and volatile-rich atmospheres -- while recognizing that existing opacity/outflow/chemistry models may need targeted upgrades to reach the required fidelity level.