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

Abstract

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∼10.2), large ($1.37 R_J$), sub-Saturn mass ($0.19 M_J$) exoplanet WASP-127b across the near-UV to near-infrared wavelength range (0.3–5 μm), using the Hubble and Spitzer Space Telescopes. Our results show a feature-rich transmission spectrum, with absorption from Na, $H_2O$, and $CO_2$, 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 super-solar, with abundances relative to solar values of $9^{+15}_{-6}$, $16^{+7}_{-5}$⁠, and $26^{+12}_{-9}$ respectively. Despite giving conflicting C/O ratios, both retrievals gave super-solar $CO_2$ volume mixing ratios, which adds to the likelihood that WASP-127b’s bulk metallicity is super-solar, since $CO_2$ abundance is highly sensitive to atmospheric metallicity. We detect water at a significance of 13.7 σ. 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 JWST will be able to constrain WASP-127b’s C/O ratio, and may reveal the formation history of this metal-enriched, highly observable exoplanet