Title: Water, High-Altitude Condensates, and Possible Methane Depletion in the Atmosphere of the Warm Super-Neptune WASP-107b,
Abstract: The super-Neptune exoplanet WASP-107b is an exciting target for atmosphere
characterization. It has an unusually large atmospheric scale height and a
small, bright host star, raising the possibility of precise constraints on its
current nature and formation history. We report the first atmospheric study of
WASP-107b, a Hubble Space Telescope measurement of its near-infrared
transmission spectrum. We determined the planet's composition with two
techniques: atmospheric retrieval based on the transmission spectrum and
interior structure modeling based on the observed mass and radius. The interior
structure models set a $3\,\sigma$ upper limit on the atmospheric metallicity
of $30\times$ solar. The transmission spectrum shows strong evidence for water
absorption ($6.5\,\sigma$ confidence), and the retrieved water abundance is
consistent with expectations for a solar abundance pattern. The inferred
carbon-to-oxygen ratio is subsolar at $2.7\,\sigma$ confidence, which we
attribute to possible methane depletion in the atmosphere. The spectral
features are smaller than predicted for a cloud-free composition, crossing less
than one scale height. A thick condensate layer at high altitudes (0.1 - 3
mbar) is needed to match the observations. We find that physically motivated
cloud models with moderate sedimentation efficiency ($f_\mathrm{sed} = 0.3$) or
hazes with a particle size of 0.3 $\mu$m reproduce the observed spectral
feature amplitude. Taken together, these findings serve as an illustration of
the diversity and complexity of exoplanet atmospheres. The community can look
forward to more such results with the high precision and wide spectral coverage
afforded by future observing facilities.