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High atmospheric metal enrichment for a Saturn-mass planet

Nature volume 618, pages 43–46 (2023)Cite this article

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Abstract

Atmospheric metal enrichment (that is, elements heavier than helium, also called ‘metallicity’) is a key diagnostic of the formation of giant planets1,2,3. The giant planets of the Solar System show an inverse relationship between mass and both their bulk metallicities and atmospheric metallicities. Extrasolar giant planets also display an inverse relationship between mass and bulk metallicity4. However, there is significant scatter in the relationship and it is not known how atmospheric metallicity correlates with either planet mass or bulk metallicity. Here we show that the Saturn-mass exoplanet HD 149026b (refs. 5,6,7,8,9) has an atmospheric metallicity 59–276 times solar (at 1σ), which is greater than Saturn’s atmospheric metallicity of roughly 7.5 times solar10 at more than 4σ confidence. This result is based on modelling CO2 and H2O absorption features in the thermal emission spectrum of the planet measured by the James Webb Space Telescope. HD 149026b is the most metal-rich giant planet known, with an estimated bulk heavy element abundance of 66 ± 2% by mass11,12. We find that the atmospheric metallicities of both HD 149026b and the Solar System giant planets are more correlated with bulk metallicity than planet mass.

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Fig. 1: The thermal emission spectrum of HD 149026b.
Fig. 2: The TP profile and contribution functions for HD 149026b.
Fig. 3: The metallicity of HD 149026b in context.

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Data availability

The data used in this paper are associated with JWST program GTO 1274 (observation numbers 4 and 5) and are available from the MAST (https://mast.stsci.edu). Science data processing version (SDP_VER) 2022_2a generated the uncalibrated data that we downloaded from MAST. We used JWST calibration software version (CAL_VER) 1.6.0 with modifications described in the text. We used calibration reference data from context (CRDS_CTX) 1004. Source data are provided with this paper.

Code availability

The Eureka! (https://github.com/kevin218/Eureka) and PLATON (https://github.com/ideasrule/platon) codes used in this study are publicly available.

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Acknowledgements

This work is based on observations made with the NASA/ESA/CSA JWST. The data were obtained from the MAST at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. We thank K. Stevenson for help with the data reduction.

Author information

Authors and Affiliations

  1. Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL, USA

    Jacob L. Bean, Qiao Xue, Prune C. August & Michael Zhang

  2. School of Physics and Astronomy, Shanghai Jiaotong University, Shanghai, China

    Qiao Xue

  3. Institute of Physics, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland

    Prune C. August

  4. Department of Astronomy, Cornell University, Ithaca, NY, USA

    Jonathan Lunine

  5. Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD, USA

    Daniel Thorngren

  6. Department of Earth Sciences, University of California, Riverside, CA, USA

    Shang-Min Tsai

  7. Department of Physics, University of Oxford, Oxford, UK

    Shang-Min Tsai

  8. Department of Physics and Astronomy, Vanderbilt University, Nashville, TN, USA

    Keivan G. Stassun

  9. Steward Observatory, University of Arizona, Tucson, AZ, USA

    Everett Schlawin & Megan Mansfield

  10. Department of Physics, University of Warwick, Coventry, UK

    Eva-Maria Ahrer

  11. Department of Astronomy, University of Maryland, College Park, MD, USA

    Jegug Ih

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Contributions

J.L.B. led the project and wrote the manuscript. Q.X. performed the data analysis with assistance from M.Z., E.S., E.-M.A. and M.M. P.C.A. performed the atmospheric modelling with assistance from M.Z. and J.I. J.L. provided the JWST data and contributed text. D.T. performed the interior structure modelling. S.-M.T. did the photochemistry calculations. K.G.S. performed the analysis of the host star properties. All authors commented on the manuscript and aided in the interpretation.

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Correspondence to Jacob L. Bean.

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Extended data figures and tables

Extended Data Fig. 1 Spectral energy distribution (SED) of HD 149026.

Red symbols represent the observed photometric measurements, where the horizontal bars represent the effective width of the passband. Blue symbols are the model fluxes from the best-fit Kurucz atmosphere model (black). Error bars in flux are 1σ uncertainties; error bars in wavelength give the bandpass of the measurement.

Extended Data Fig. 2 Mass - radius diagram for giant planets like HD 149026b.

The points that are bolded represent planets that are within 20% of HD 149026b’s mass and 50% of its irradiation. Saturn’s position in the diagram is indicated by the “S”. HD 149026b is indicated by the diamond-shaped point. It stands out as being among the smallest planets for its mass despite being highly irradiated. The most similar planet is K2-60b (the point to the right and slightly down from HD 149026b), but it receives just one third of the flux that HD 149026b does. HD 149026b is smaller than Saturn despite receiving >105 times more irradiation. Error bars are 1σ uncertainties.

Extended Data Fig. 3 Results of the Bayesian retrieval for the interior structure of HD 149026b.

Mass is in Jupiter masses and age is in gigayears. Zp and ε are unitless and refer to the bulk metallicity of the planet and the log10 of the hot Jupiter heating relative to the incident flux, respectively. The retrieval shows that the planet is very metal rich irrespective of the exact age or heating rate. The structure models are from ref. 11 and the statistical model is from ref. 12 Note that the planet radius (accounting for uncertainty) was a vital observation which the model seeks to explain, but it is not a model parameter and so is not shown in the posterior.

Extended Data Fig. 4 Extraction of the long-wavelength data.

Panels a and c show the spectral trace of the median frame after correcting curvature and subtracting background. Optimal spectral extraction is performed between the orange-dashed lines, and background subtraction is performed in the region above and below the blue lines. Panels b and d show the interpolated cubic function of flux along detector y axis over the 850th to 860th detector columns. Panels a and b show the F322W2 data. Panels c and d show the F444W data.

Extended Data Fig. 5 White light curves for the long-wavelength data.

Panels a and b show the data with no corrections and overplotted by best-fit model. Panels c and d show the normalized data with systematics divided out and overplotted by the transit model. Panels e and f show the residuals to the fit. Panels a, c, and e show the F322W2 data. Panels b, d, and f show the F444W data. Error bars are 1σ uncertainties.

Extended Data Fig. 6 Allan deviation for the spectroscopic light curves obtained in both visits.

The black lines show the root mean square error from each channel, which are normalized by the value of the unbinned data. The red line shows the expected behavior for white noise.

Extended Data Fig. 7 Error analysis for the short-wavelength data.

We compute the residuals between the best-fitting model and the light curve, shift the residuals one point at a time, add the residuals back to the model (in order to preserve the time-correlated structure in the data), and find the best fit again (i.e., a “prayer bead” analysis). The histogram shows the distribution of the eclipse depths for the two visits. The results from the first visit (blue distribution) match the expectation for white noise, but the results for the second visit (green distribution) are highly non-Gaussian, indicating correlated noise in the data.

Extended Data Fig. 8 Corner plot of the retrieved parameters for HD 149026b’s atmosphere.

From left to right: the metallicity [M/H], the C/O ratio, the 5-parameter T-P profile model from ref. 22 (thermal opacity, visible to thermal opacity ratio of the first and second visible streams, percentage apportioned to the second visible stream, and effective albedo), and the dilution coefficient as described by ref. 23.

Extended Data Fig. 9 Abundances of key chemical species retrieved for HD 149026b’s atmosphere.

These abundances are for the best-fit chemical equilibrium model shown in Fig. 2.

Extended Data Fig. 10 Chemistry Jacobians for HD 149026b.

The spectrum’s sensitivity to key molecules is evaluated by computing its deviations from the main fit with respect to small changes in the chemical abundance (in log units). The different coloured lines correspond to different wavelength channels, as shown in the legend.

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Bean, J.L., Xue, Q., August, P.C. et al. High atmospheric metal enrichment for a Saturn-mass planet. Nature 618, 43–46 (2023). https://doi.org/10.1038/s41586-023-05984-y

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