  A correlation between giant-planet mass and atmospheric heavy elemental
abundance was first noted in the past century from observations of planets in
our own Solar System, and has served as a cornerstone of planet formation
theory. Using data from the Hubble and Spitzer Space Telescopes from 0.5 to 5
microns, we conducted a detailed atmospheric study of the transiting
Neptune-mass exoplanet HAT-P-26b. We detected prominent H2O absorption bands
with a maximum base-to-peak amplitude of 525ppm in the transmission spectrum.
Using the water abundance as a proxy for metallicity, we measured HAT-P-26b's
atmospheric heavy element content [4.8 (-4.0 +21.5) times solar]. This likely
indicates that HAT-P-26b's atmosphere is primordial and obtained its gaseous
envelope late in its disk lifetime, with little contamination from metal-rich
planetesimals.
