HD 209458b in new light: evidence of nitrogen chemistry, patchy clouds and sub-solar water

Interpretations of exoplanetary transmission spectra have been undermined by apparent obscuration due to clouds/hazes. Debate rages on whether weak H2O features seen in exoplanet spectra are due to clouds or inherently depleted oxygen. Assertions of solar H2O abundances have relied on making a priori model assumptions, for example, chemical/radiative equilibrium. In this work, we attempt to address this problem with a new retrieval paradigm for transmission spectra. We introduce POSEIDON, a two-dimensional atmospheric retrieval algorithm including generalized inhomogeneous clouds. We demonstrate that this prescription allows one to break vital degeneracies between clouds and prominent molecular abundances. We apply POSEIDON to the best transmission spectrum presently available, for the hot Jupiter HD 209458b, uncovering new insights into its atmosphere at the day–night terminator. We extensively explore the parameter space with an unprecedented 108 models, spanning the continuum from fully cloudy to cloud-free atmospheres, in a fully Bayesian retrieval framework. We report the first detection of nitrogen chemistry (NH3 and/or HCN) in an exoplanet atmosphere at 3.7–7.7σ confidence, non-uniform cloud coverage at 4.5–5.4σ, high-altitude hazes at >3σ and sub-solar H2O at ≳3–5σ, depending on the assumed cloud distribution. We detect NH3 at 3.3σ, and 4.9σ for fully cloudy and cloud-free scenarios, respectively. For the model with the highest Bayesian evidence, we constrain H2O at 5–15 ppm (0.01–0.03) × solar and NH3 at 0.01–2.7 ppm, strongly suggesting disequilibrium chemistry and cautioning against equilibrium assumptions. Our results herald a new promise for retrieving cloudy atmospheres using high-precision Hubble Space Telescope and James Webb Space Telescope spectra.