The Complete Transmission Spectrum of WASP-39b with a Precise Water Constraint

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 μ m) and G141 (1.1–1.7 μ m) spectroscopic grisms. We measure the predicted high-amplitude H 2 O feature centered at 1.4 μ m and the smaller amplitude features at 0.95 and 1.2 μ m, 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 μ m. From these observed water features, combined with features in the optical and IR, we retrieve a well constrained temperature T eq = 1030 K, and atmospheric metallicity 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.