Abundance measurements of H2O and carbon-bearing species in the atmosphere of WASP-127b confirm its supersolar metallicity

The chemical abundances of exoplanet atmospheres may provide valuable information about the bulk compositions, formation pathways, and evolutionary histories of planets. Exoplanets with large, relatively cloud-free atmospheres, and which orbit bright stars provide the best opportunities for accurate abundance measurements. For this reason, we measured the transmission spectrum of the bright (V similar to 10.2), large (1.37 R-J), sub-Saturn mass (0.19M(J)) exoplanet WASP-127b across the near-UV to near-infrared wavelength range (0.3-5 mu m), using the Hubble and Spitzer Space Telescopes. Our results show a feature-rich transmission spectrum, with absorption from Na, H2O, and CO2, and wavelength-dependent scattering from small-particle condensates. We ran two types of atmospheric retrieval models: one enforcing chemical equilibrium, and the other which fit the abundances freely. Our retrieved abundances at chemical equilibrium for Na, O, and C are all supersolar, with abundances relative to solar values of 9(-6)(+15), 16(-5)(+7), and 26(-9)(+12), respectively. Despite giving conflicting C/O ratios, both retrievals gave supersolar CO2 volume mixing ratios, which adds to the likelihood that WASP-127b's bulk metallicity is supersolar, since CO2 abundance is highly sensitive to atmospheric metallicity. We detect water at a significance of 13.7 sigma. Our detection of Na is in agreement with previous ground-based detections, though we find a much lower abundance, and we also do not find evidence for Li or K despite increased sensitivity. In the future, spectroscopy with James Webb Space Telescope will be able to constrain WASP-127b's C/O ratio, and may reveal the formation history of this metal-enriched, highly observable exoplanet.

Automated summary

The study measured the transmission spectrum of exoplanet WASP-127b and found rich features of absorption and wavelength-dependent scattering. The abundances of Na, O, and C were found to be supersolar, as well as the volume mixing ratios of CO2. Water was detected with a significance of 13.7 sigma, while Na was detected at a lower abundance compared to previous ground-based detections, and no evidence of Li or K was found.