HST Hot-Jupiter Transmission Spectral Survey: Clear Skies for Cool Saturn WASP-39b

Abstract

We present theHubble Space Telescope(HST)Space Telescope Imaging Spectrograph(STIS) optical transmission spectroscopy of the cool Saturn-mass exoplanet WASP-39b from 0.29-1.025μm, along with complementary transit observations from SpitzerIRAC at 3.6 and 4.5μm. The low density and large atmospheric pressure scale height of WASP-39b make it particularly amenable to atmospheric characterization using this technique. We detect a Rayleigh scattering slope as well as sodium and potassium absorption features; this is the first exoplanet in which both alkali features are clearly detected with the extended wings predicted by cloud-free atmosphere models. The full transmission spectrum is well matched by a clear H2-dominated atmosphere, or one containing a weak contribution from haze, in good agreement with the preliminary reduction of these data presented in Sing et al.WASP-39b is predicted to have a pressure-temperature profile comparable to that of HD189733b and WASP-6b,making it one of the coolest transiting gas giants observed in our HSTSTIS survey. Despite this similarity, WASP-39b appears to be largely cloud-free, while the transmission spectra of HD189733b and WASP-6b both indicate the presence of high altitude clouds or hazes. These observations further emphasize the surprising diversity of cloudy and cloud-free gas giant planets in short-period orbits and the corresponding challenges associated with developing predictive cloud models for these atmospheres.