2023MNRAS.522.5062B

Precise measurements of chemical abundances in planetary atmospheres are necessary to constrain the formation histories of exoplanets. A recent study of WASP-127 b, a close-in puffy sub-Saturn orbiting its solar-type host star in 4.2 d, using Hubble Space Telescope (HST) and Spitzer revealed a feature-rich transmission spectrum with strong excess absorption at 4.5 μm. However, the limited spectral resolution and coverage of these instruments could not distinguish between CO and/or CO₂ absorption causing this signal, with both low and high C/O ratio scenarios being possible. Here we present near-infrared (0.9-2.5 μm) transit observations of WASP-127 b using the high-resolution SPIRou spectrograph, with the goal to disentangle CO from CO₂ through the 2.3 μm CO band. With SPIRou, we detect H₂O at a t-test significance of 5.3 σ and observe a tentative (3σ) signal consistent with OH absorption. From a joint SPIRou + HST + Spitzer retrieval analysis, we rule out a CO-rich scenario by placing an upper limit on the CO abundance of log₁₀[CO] ←4.0, and estimate a log₁₀[CO₂] of -3.7$^{+0.8}_{-0.6}$ , which is the level needed to match the excess absorption seen at 4.5 μm. We also set abundance constraints on other major C-, O-, and N-bearing molecules, with our results favouring low C/O (0.10$^{+0.10}_{-0.06}$ ), disequilibrium chemistry scenarios. We further discuss the implications of our results in the context of planet formation. Additional observations at high and low resolution will be needed to confirm these results and better our understanding of this unusual world.