2018AJ....155...29W

WASP-39b is a hot Saturn-mass exoplanet with a predicted clear atmosphere based on observations in the optical and infrared. Here, we complete the transmission spectrum of the atmosphere with observations in the near-infrared (NIR) over three water absorption features with the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) G102 (0.8-1.1 µm) and G141 (1.1-1.7 µm) spectroscopic grisms. We measure the predicted high-amplitude H₂O feature centered at 1.4 µm and the smaller amplitude features at 0.95 and 1.2 µm, with a maximum water absorption amplitude of 2.4 planetary scale heights. We incorporate these new NIR measurements into previously published observational measurements to complete the transmission spectrum from 0.3 to 5 µm. From these observed water features, combined with features in the optical and IR, we retrieve a well constrained temperature T_eq = 1030_–20⁺³⁰ K, and atmospheric metallicity 151_–46⁺⁴⁸×solar, which is relatively high with respect to the currently established mass-metallicity trends. This new measurement in the Saturn-mass range hints at further diversity in the planet formation process relative to our solar system giants.