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Five carbon- and nitrogen-bearing species in a hot giant planet’s atmosphere

Abstract

The atmospheres of gaseous giant exoplanets orbiting close to their parent stars (hot Jupiters) have been probed for nearly two decades1,2. They allow us to investigate the chemical and physical properties of planetary atmospheres under extreme irradiation conditions3. Previous observations of hot Jupiters as they transit in front of their host stars have revealed the frequent presence of water vapour4 and carbon monoxide5 in their atmospheres; this has been studied in terms of scaled solar composition6 under the usual assumption of chemical equilibrium. Both molecules as well as hydrogen cyanide were found in the atmosphere of HD 209458b5,7,8, a well studied hot Jupiter (with equilibrium temperature around 1,500 kelvin), whereas ammonia was tentatively detected there9 and subsequently refuted10. Here we report observations of HD 209458b that indicate the presence of water (H2O), carbon monoxide (CO), hydrogen cyanide (HCN), methane (CH4), ammonia (NH3) and acetylene (C2H2), with statistical significance of 5.3 to 9.9 standard deviations per molecule. Atmospheric models in radiative and chemical equilibrium that account for the detected species indicate a carbon-rich chemistry with a carbon-to-oxygen ratio close to or greater than 1, higher than the solar value (0.55). According to existing models relating the atmospheric chemistry to planet formation and migration scenarios3,11,12, this would suggest that HD 209458b formed far from its present location and subsequently migrated inwards11,13. Other hot Jupiters may also show a richer chemistry than has been previously found, which would bring into question the frequently made assumption that they have solar-like and oxygen-rich compositions.

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Fig. 1: Detection significance for H2O, CH4, NH3, C2H2, HCN, CO and CO2.
Fig. 2: Constraints on the chemical composition of the atmosphere of HD 209458b.
Fig. 3: Single-molecule constraints on the chemical composition of the atmosphere of HD 209458b.

Data availability

The raw data that support the findings of this study are publicly available at the Telescopio Nazionale Galileo archive http://archives.ia2.inaf.it/tng/ hosted at the IA2 Data Center https://ia2.inaf.it/.

Code availability

The Gofio pipeline used to perform the GIANO-B data reduction is publicly available at https://atreides.tng.iac.es/monica.rainer/gofio. The procedures that perform the wavelength calibration, the telluric removal, the search for molecules via cross-correlation and the likelihood analysis employ public IDL libraries (explicitly indicated in the Methods) and are detailed in the text and/or in the cited papers. Even though they are available from the corresponding author upon reasonable request, we encourage other groups to develop similar tools independently and carry out their own analyses for an unbiased check of the results presented in this work. The corresponding author offers to provide any help needed. The high-resolution transmission models underpinning this article will be made available upon reasonable request to S.G. The molecular cross-sections for the various species are available on the Open Science Framework: https://osf.io/mgnw5/?view_only=5d58b814328e4600862ccfae4720acc3. The radiative-equilibrium profiles computed in this article are available on Zenodo at https://zenodo.org/record/4494367.

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Acknowledgements

We thank J. Bean for comments that allowed us to improve the manuscript. P.G. gratefully acknowledges support from the Italian Space Agency (ASI) under contract 2018-24-HH.0. S.B. gratefully acknowledges support from the Italian Space Agency (ASI) under contract 2018-16-HH.0. M.B. and S.G. acknowledge support from the UK Science and Technology Facilities Council (STFC) research grant ST/S000631/1. A.S.B., G.G., A.M., G.M., and A.S. acknowledge financial contributions from the agreement ASI-INAF number 2018-16-HH.0. A.S.B., R. Claudi, G.L., A.M., V.N., L.P., A.S. and G.S. acknowledge support from PRIN INAF 2019. These results are based on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated by the Fundación Galileo Galilei (FGG) of the Istituto Nazionale di Astrofisica (INAF) at the Observatorio del Roque de los Muchachos (La Palma, Canary Islands, Spain). S.N.Y. acknowledges STFC Project number ST/R000476/1. The research leading to these results received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement number 679633: Exo-Atmos).

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Contributions

P.G., M.B. and G.G. carried out the primary data reduction and data analysis. S.G. and P.C. ran theoretical models for the planet’s atmosphere and transmission spectra. P.G., M.B., A.S.B., L.F., S.G., and P.C. contributed to the writing of the manuscript. P.G., M.B., S.G., A.S.B., A.S., and L.F. planned the tests to assess the reliability of the molecular detections through cross-correlation techniques. The underlying observation programme was conceived and organized by A.B., E.C., R. Claudi, S.D., S.B., A.F.L., A.M., E.M., G.M., I.P., E.P., G.P., and A.S. A.S.B. and V.N. planned the observations. R. Claudi is in charge of the schedules of the observations. Observations with GIANO-B were carried out by M.R., K.B., M.P., and I.C. M.R. and A.H. wrote, maintained and updated the reduction pipeline. A.B. maintained and updated the observation archive. R. Cosentino, A.F., A.H., M.P., E.P. and A.G. maintained and upgraded the GIANO-B instrument at the TNG. D.F., A.T., N.S., M.L., and A.G. contributed to the design and construction of the GIANO-B spectrograph. K.L.C. and S.N.Y. have provided molecular data. All authors contributed to the interpretation of the data and the results.

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Correspondence to Paolo Giacobbe.

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Extended data figures and tables

Extended Data Fig. 1 Detection and analysis of the H2O signal in the atmosphere of HD 209458b.

a, Map of the significance of the cross-correlation values as a function of the planetary radial-velocity semi-amplitude KP and the planet rest-frame velocity Vrest. b, Values of the cross-correlation function in the planet rest-frame as a function of the orbital phase and Vrest. The horizontal dashed lines denote the transit ingress and egress, while the vertical dashed line indicates the expected position of the planetary signal. c, Distribution of CCF values in-trail and out-of-trail. d, Significance of the cross-correlation values as a function of Vrest. e, Significance of the cross-correlation values as a function of KP. The dashed line indicates the peak position, while the dotted line shows the 1σ confidence interval.

Extended Data Fig. 2 Theoretical pressure−temperature profiles of the atmosphere of HD 209458b.

The panels show the temperature variation with pressure for different atmospheric C/O ratios and metallicities ([M/H]) under the assumption of radiative and thermochemical equilibrium.

Extended Data Fig. 3 Theoretical pressure−abundance profiles of the atmosphere of HD 209458b.

The panels show the abundance profiles (mole fraction) of atomic and molecular species for different atmospheric C/O ratios and metallicities ([M/H]) under the assumption of radiative and thermochemical equilibrium. Each colour corresponds to a different species (see key at top-right).

Extended Data Fig. 4 Comparison of atmospheric models in radiative and thermochemical equilibrium.

The six panels show the goodness of fit for the mixed models containing all the detected species as a function of C/O ratio, metallicity and presence of clouds. The filled circles represent the models with clouds, while the empty circles indicate the clear models with no clouds. The best model is found for a cloudy atmosphere with C/O = 1.05 and subsolar metallicity of 0.001 × solar (top-left panel). The goodness of fit of the models is shown with respect to the best model in units of standard deviations σ (the higher σ, the more disfavoured the model). The horizontal dashed lines indicate the 3σ level adopted as a threshold to distinguish different scenarios. Note that for display purposes the y-axis scale is linear between 0σ and 1σ, and logarithmic elsewhere.

Extended Data Fig. 5 Selection of the GIANO-B spectral orders.

The Earth’s telluric spectrum and the theoretical transmission spectra of H2O, HCN, NH3, C2H2, CO and CH4 are shown from top to bottom in relative flux units. The colours display for each molecule the orders selected for the cross-correlation procedure, while the grey vertical bands denote the orders excluded owing to the failure of the spectral alignment and/or of the wavelength calibration procedure.

Extended Data Fig. 6 Stages of the analysis of GIANO-B spectra.

Example of our data reduction process over a small wavelength interval. a, Extracted spectra; b, residuals after normalization of each spectrum (each row) by its median value (throughput correction); c, residuals after ‘standardization’ of each spectral channel (each column) by mean subtraction; d, residuals after PCA telluric removal; e, residuals after division of each spectral channel by its variance and multiplication of the final spectral matrix by the median of the variances of the individual spectral channels, in order to conserve the flux (not applied in the likelihood framework).

Extended Data Table 1 Log of the HD 209458b GIANO-B observations
Extended Data Table 2 HD 209458 system parameters
Extended Data Table 3 Line list databases for the search for molecular species
Extended Data Table 4 Line lists and VMRs of isothermal atmospheric models

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Giacobbe, P., Brogi, M., Gandhi, S. et al. Five carbon- and nitrogen-bearing species in a hot giant planet’s atmosphere. Nature 592, 205–208 (2021). https://doi.org/10.1038/s41586-021-03381-x

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