Transmission spectroscopy is enabling precise measurements of atmospheric H _ { 2 } O abundances for numerous giant exoplanets .
For hot Jupiters , relating H _ { 2 } O 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 H _ { 2 } O-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 } \% H _ { 2 } O , an O/H of 18.1 ^ { +25.9 } _ { -11.3 } \times solar , and C/O < 0.33 ( to 2 \sigma ) .
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 4.1 \sigma confidence .
Potential candidates are identified as TiH ( 3.6 \sigma ) , CrH ( 2.1 \sigma ) , or ScH ( 1.8 \sigma ) .
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 H _ { 2 } O ( at 29 \sigma ) , CH _ { 4 } ( 6.2 \sigma ) , CO _ { 2 } ( 13 \sigma ) , and CO ( 3.7 \sigma ) , 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 > 5 \sigma confidence .