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

By R. MacDonald, N. Madhusudhan. 2017

203 citations recorded by the source corpus.

Interpretations of exoplanetary transmission spectra have been undermined by apparent obscuration due to clouds/hazes. Debate rages on whether weak H$_2$O features seen in exoplanet spectra are due to clouds or inherently depleted oxygen. Assertions of solar H$_2$O abundances have relied on making a priori model assumptions, e.g. 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 generalised 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 10$^8$ 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 (NH$_3$ and/or HCN) in an exoplanet atmosphere at 3.7-7.7$\sigma$ confidence, non-uniform cloud coverage at 4.5-5.4$\sigma$, high-altitude hazes at $>$3$\sigma$, and sub-solar H$_2$O at $\gtrsim$3-5$\sigma$, depending on the assumed cloud distribution. We detect NH$_3$ at 3.3$\sigma$ and 4.9$\sigma$ for fully cloudy and cloud-free scenarios, respectively. For the model with the highest Bayesian evidence, we constrain H$_2$O at 5-15 ppm (0.01-0.03$\times$ solar) and NH$_3$ at 0.01-2.7 ppm, strongly suggesting disequilibrium chemistry and cautioning against equilibrium assumptions. Our results herald new promise for retrieving cloudy atmospheres using high-precision HST and JWST spectra.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/a6ac9a1ba05b7a15ff9f822e5665f5633f1e2dea

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Citations in the corpus

High Tide or Riptide on the Cosmic Shoreline? A Water-rich Atmosphere or Stellar Contamination for the Warm Super-Earth GJ 486b from JWST Observations (2023, methodology): “The POSEIDON retrievals for all three data reductions yield a spot coverage fraction of ∼10% but with relatively weak and inconsistent constraints on the spot temperature.”
H2O abundances and cloud properties in ten hot giant exoplanets (2018, methodology): “State-ofthe-art Bayesian atmospheric retrieval techniques make it possible to estimate such atmospheric properties from transmission spectra (Madhusudhan et al. 2014a; Line et al. 2013; Kreidberg et al. 2014a; MacDonald & Madhusudhan 2017a).”
ExoMol molecular line lists – XXXV. A rotation-vibration line list for hot ammonia (2019, background): “Although attempts to detect ammonia in the atmosphere of an exoplanet have so far proved inconclusive (Beaulieu et al. 2011), it is thought to be an important component of the chemistry of hot Jupiter exoplanets (MacDonald & Madhusudhan 2017).”
Retrieving scattering clouds and disequilibrium chemistry in the atmosphere of HR 8799e (2020, background): “(2011a, 2014a), Madhusudhan & Seager (2011), MacDonald & Madhusudhan (2017, 2019), Burningham et al. (2017), Gandhi & Madhusudhan (2018), Pinhas et al. (2018).”
A Population Study of Gaseous Exoplanets (2017, methodology): “As discussed above, in this analysis we do not take into account effects due to patchy or non-uniform cloud covers (e.g. MacDonald & Madhusudhan 2017).”
Exoplanetary Atmospheres: Key Insights, Challenges, and Prospects (2019, background): “Current retrieval codes are able to retrieve the chemical abundances from such spectra in addition to constraining the cloud/haze properties (16, 157).”