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 σ upper limit on the atmospheric metallicity of 30× solar. The transmission spectrum shows strong evidence for water absorption (6.5 σ 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 σ 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 sed = 0.3) or hazes with a particle size of 0.3 µm 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.

Helium in the eroding atmosphere of an exoplanet

“…We found the broadband data were well explained by a grey absorbing cloud deck across the full 8,780-11,370 Å wavelength range, in addition to H 2 O absorption. We obtained a volume mixing ratio for H O of 5×10 -3 -4×10 -2 , consistent with previous estimations 18 .…”

“…The broadband transmission spectrum is consistent with a previous transmission spectrum for WASP-107b obtained using the WFC3 G141 grism, which covers the 11,000-16,000 Å wavelength range 18 . The latter exhibits a muted water absorption band centred at 14,000 Å, with an otherwise flat spectrum implying an opaque cloud deck.…”

“…Following standard methods 18 we interpolated each spectrum onto the wavelength range of the first to account for shifts in the dispersion axis over time.…”

“…Table 2 | All results from transit light curve fits. Modified results from a previous study 18 are included. RMS is the root mean squared of the model residuals in parts per million (PPM); the second-to-last column is the RMS divided by the expected photon noise; the last column is the correction factor we applied to account for stellar variability.…”

A sub-Neptune exoplanet with a low-metallicity methane-depleted atmosphere and Mie-scattering clouds

“…None of our twelve HST and Spitzer light curves show any direct evidence for star spot crossing events. The strongest constraints present the three HST/WFC3 white light curves which, with a Gaussian residual scatter of approximately 40 ppm per single data point, would have been highly sensitive to identifying the crossing of star spots along the planet chord 18 . In addition, we also find that all individual transit depth measurements taken over multiple epochs are consistent within their statistical uncertainties and with the joint fit ( Supplementary Figure 4).…”