The metal-rich atmosphere of the exo-Neptune HAT-P-26b

Transmission spectroscopy is enabling precise measurements of atmospheric H₂O abundances for numerous giant exoplanets. For hot Jupiters, relating H₂O abundances to metallicities provides a powerful probe of their formation conditions. However, metallicity measurements for Neptune-mass exoplanets are only now becoming viable. Exo-Neptunes are expected to possess super-solar metallicities from accretion of H₂O-rich and solid-rich planetesimals. However, initial investigations into the exo-Neptune HAT-P-26b suggested a significantly lower metallicity than predicted by the core-accretion theory of planetary formation and Solar system expectations from Uranus and Neptune. Here, we report an extensive atmospheric retrieval analysis of HAT-P-26b, combining all available observations, to reveal its composition, temperature structure, and cloud properties. Our analysis reveals an atmosphere containing 1.5^+2.1_-0.9 % H₂O, an O/H of 18.1^+25.9_-11.3 × solar, and C/O < 0.33 (to 2σ). This updated metallicity, the most precise exo-Neptune metallicity reported to date, suggests a formation history with significant planetesimal accretion, albeit below that of Uranus and Neptune. We additionally report evidence for metal hydrides at 4.1σ confidence. Potential candidates are identified as TiH (3.6σ), CrH (2.1σ), or ScH (1.8σ). Maintaining gas-phase metal hydrides at the derived temperature (563⁺⁵⁸_-54 K) necessitates strong disequilibrium processes or external replenishment. Finally, we simulate the James Webb Space Telescope Guaranteed Time Observations for HAT-P-26b. Assuming a composition consistent with current observations, we predict JWST can detect H₂O (at 29σ), CH₄ (6.2σ), CO₂ (13σ), and CO (3.7σ), thereby improving metallicity and C/O precision to 0.2 dex and 0.35 dex, respectively. Furthermore, NIRISS observations could detect several metal hydrides at >5σ confidence.