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.
This is a preview of subscription content, access via your institution
Access Nature and 54 other Nature Portfolio journals
Get Nature+, our best-value online-access subscription
$29.99 / 30 days
cancel any time
Subscribe to this journal
Receive 51 print issues and online access
$199.00 per year
only $3.90 per issue
Rent or buy this article
Get just this article for as long as you need it
Prices may be subject to local taxes which are calculated during checkout
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.
The Eureka! (https://github.com/kevin218/Eureka) and PLATON (https://github.com/ideasrule/platon) codes used in this study are publicly available.
Pollack, J. B. et al. Formation of the giant planets by concurrent accretion of solids and gas. Icarus 124, 62 (1996).
Fortney, J. J. et al. A framework for characterizing the atmospheres of low-mass low-density transiting planets. Astrophys. J. 775, 80 (2013).
Venturini, J., Alibert, Y. & Benz, W. Planet formation with envelope enrichment: new insights on planetary diversity. Astron. Astrophys. 596, A90 (2016).
Thorngren, D. P., Fortney, J. J., Murray-Clay, R. A. & Lopez, E. D. The mass-metallicity relation for giant planets. Astrophys. J. 831, 64 (2016).
Sato, B. et al. The N2K Consortium. II. A transiting hot Saturn around HD 149026 with a large dense core. Astrophys. J. 633, 465 (2005).
Charbonneau, D. et al. Transit photometry of the core-dominated planet HD 149026b. Astrophys. J. 636, 445 (2006).
Winn, J. N., Henry, G. W., Torres, G. & Holman, M. J. Five new transits of the super-Neptune HD 149026b. Astrophys. J. 675, 1531 (2008).
Nutzman, P. et al. A precise estimate of the radius of the exoplanet HD 149026b from Spitzer photometry. Astrophys. J. 692, 229 (2009).
Carter, J. A., Winn, J. N., Gilliland, R. & Holman, M. J. Near-infrared transit photometry of the exoplanet HD 149026b. Astrophys. J. 696, 241 (2009).
Guillot, T. et al. Giant planets from the inside-out. Preprint at https://arxiv.org/abs/2205.04100 (2022).
Thorngren, D. P. & Fortney, J. J. Bayesian analysis of hot-Jupiter radius anomalies: evidence for Ohmic dissipation? Astron. J. 155, 214 (2018).
Thorngren, D. & Fortney, J. J. Connecting giant planet atmosphere and interior modeling: constraints on atmospheric metal enrichment. Astrophys. J. 874, L31 (2019).
Greene, T. P. et al. λ = 2.4 to 5 μm spectroscopy with the James Webb Space Telescope NIRCam instrument. J. Astronom. Telesc. Instrum. Syst. 3, 035001 (2017).
Bell, T. J. et al. Eureka!: an end-to-end pipeline for JWST time-series observations. J. Open Source Softw. 7, 4503 (2022).
Ahrer, E.-A. et al. Early Release Science of the exoplanet WASP-39b with JWST NIRCam, Nature 614, 653–658 (2023).
Rigby, J. et al. The science performance of JWST as characterized in commissioning. Publ. Astron. Soc. Pac. 135, 048001 (2023).
Zhang, M. et al. Phase curves of WASP-33b and HD 149026b and a new correlation between phase curve offset and irradiation temperature. Astron. J. 155, 83 (2018).
Stevenson, K. B. et al. Transit and eclipse analyses of the exoplanet HD 149026b using BLISS mapping. Astrophys. J. 754, 136 (2012).
JWST Transiting Exoplanet Community Early Release Science Team. Identification of carbon dioxide in an exoplanet atmosphere. Nature 614, 649–652 (2023).
Zhang, M., Chachan, Y., Kempton, E. M.-R. & Knutson, H. A. Forward modeling and retrievals with PLATON, a fast open-source tool. Publ. Astron. Soc. Pac. 131, 034501 (2019).
Zhang, M., Chachan, Y., Kempton, E. M.-R., Knutson, H. A. & Chang, W. PLATON II: new capabilities and a comprehensive retrieval on HD 189733b transit and eclipse data. Astrophys. J. 899, 27 (2020).
Line, M. R. et al. A systematic retrieval analysis of secondary eclipse spectra. I. A comparison of atmospheric retrieval techniques. Astrophys. J. 775, 137 (2013).
Taylor, J. et al. Understanding and mitigating biases when studying inhomogeneous emission spectra with JWST. Mon. Not. R. Astron. Soc. 493, 4342 (2020).
Rustamkulov, Z. et al. Early Release Science of the exoplanet WASP-39b with JWST NIRSpec PRISM. Nature 614, 659–663 (2023).
Alderson, L. et al. Early Release Science of the exoplanet WASP-39b with JWST NIRSpec G395H. Nature 614, 664–669 (2023).
Tsai, S. M. et al. Photochemically produced SO2 in the atmosphere of WASP-39b. Nature https://doi.org/10.1038/s41586-023-05902-2 (2023).
Baxter, C. et al. A transition between the hot and the ultra-hot Jupiter atmospheres. Astron. Astrophys. 639, A36 (2020).
Mansfield, M. et al. A unique hot Jupiter spectral sequence with evidence for compositional diversity. Nat. Astron. 5, 1224 (2021).
Lodders, K. & Fegley, B. Atmospheric chemistry in giant planets, brown dwarfs, and low-mass dwarf stars. I. Carbon, nitrogen, and oxygen. Icarus 155, 393 (2002).
Zahnle, K., Marley, M. S., Freedman, R. S., Lodders, K. & Fortney, J. J. Atmospheric sulfur photochemistry on hot Jupiters. Astrophys. J. 701, L20 (2009).
Moses, J. I. et al. Compositional diversity in the atmospheres of hot Neptunes, with application to GJ 436b. Astrophys. J. 777, 34 (2013).
Kreidberg, L. et al. A precise water abundance measurement for the hot Jupiter WASP-43b. Astrophys. J. 793, L27 (2014).
Wakeford, H. R. et al. The complete transmission spectrum of WASP-39b with a precise water constraint. Astron. J. 155, 29 (2018).
Welbanks, L. et al. Mass-metallicity trends in transiting exoplanets from atmospheric abundances of H2O, Na, and K. Astrophys. J. 887, L20 (2019).
Tsiaras, A. et al. A population study of gaseous exoplanets. Astron. J 155, 156 (2018).
Fisher, C. & Heng, K. Retrieval analysis of 38 WFC3 transmission spectra and resolution of the normalization degeneracy. Mon. Not. R. Astron. Soc. 481, 4698–4727 (2018).
Pinhas, A., Madhusudhan, N., Gandhi, S. & MacDonald, R. H2O abundances and cloud properties in ten hot giant exoplanets. Mon. Not. R. Astron. Soc. 482, 1485–1498 (2019).
Min, M., Ormel, C. W., Chubb, K., Helling, C. & Kawashima, Y. The ARCiS framework for exoplanet atmospheres. Modeling philosophy and retrieval. Astron. Astrophys. 642, A28 (2020).
Feinstein, A. et al. Early Release Science of the exoplanet WASP-39b with JWST NIRISS. Nature 614, 670–675 (2023).
Ikoma, M., Guillot, T., Genda, H., Tanigawa, T. & Ida, S. On the origin of HD 149026b. Astrophys. J. 650, 1150 (2006).
Broeg, C. & Wuchterl, G. The formation of HD149026b. Mon. Not. R. Astron. Soc. 376, L62 (2007).
Dodson-Robinson, S. E. & Bodenheimer, P. Discovering the growth histories of exoplanets: the Saturn analog HD 149026b. Astrophys. J. 695, L159 (2009).
Asplund, M., Amarsi, A. M. & Grevesse, N. The chemical make-up of the Sun: a 2020 vision. Astron. Astrophys. 653, A141 (2021).
Helled, R. et al. Revelations on Jupiter’s formation, evolution and interior: challenges from Juno results. Icarus 378, 114937 (2022).
Line, M. et al. A solar C/O and sub-solar metallicity in a hot Jupiter atmosphere. Nature 598, 580 (2021).
Stassun, K. G. & Torres, G. Parallax systematics and photocenter motions of benchmark eclipsing binaries in Gaia EDR3. Astrophys. J. 907, L33 (2021).
Stassun, K. G. & Torres, G. Eclipsing binaries as benchmarks for trigonometric parallaxes in the Gaia era. Astron. J. 152, 180 (2016).
Stassun, K. G., Collins, K. A. & Gaudi, B. S. Accurate empirical radii and masses of planets and their host stars with Gaia parallaxes. Astron. J. 153, 136 (2017).
Stassun, K. G., Corsaro, E., Pepper, J. A. & Gaudi, B. S. Empirical accurate masses and radii of single stars with TESS and Gaia. Astron. J. 155, 22 (2018).
Paunzen, E. A new catalogue of Strömgren-Crawford uvbyβ photometry. Astron. Astrophys. 580, A23 (2015).
Brewer, J. M., Fischer, D. A., Valenti, J. A. & Piskunov, N. Spectral properties of cool stars: extended abundance analysis of 1,617 planet-search stars. Astrophys. J. Supp. S. 225, 32 (2016).
Schlegel, D. J., Finkbeiner, D. P. & Davis, M. Maps of dust infrared emission for use in estimation of reddening and cosmic microwave background radiation foregrounds. Astrophys. J. 500, 525 (1998).
Torres, G., Andersen, J. & Giménez, A. Accurate masses and radii of normal stars: modern results and applications. Astron. Astrophys. Rev. 18, 67 (2010).
Findeisen, K., Hillenbrand, L. & Soderblom, D. Stellar activity in the broadband ultraviolet. Astron. J. 142, 23 (2011).
Mamajek, E. E. & Hillenbrand, L. A. Improved age estimation for solar-type dwarfs using activity-rotation diagnostics. Astrophys. J. 687, 1264 (2008).
Chabrier, G., Mazevet, S. & Soubiran, F. A new equation of state for dense hydrogen-helium mixtures. Astrophys. J. 872, 51 (2019).
Saumon, D., Chabrier, G. & van Horn, H. M. An equation of state for low-mass stars and giant planets. Astrophys. J. Supp. S. 99, 713 (1995).
González-Cataldo, F., Wilson, H. F. & Militzer, B. Ab initio free energy calculations of the solubility of silica in metallic hydrogen and application to giant planet cores. Astrophys. J. 787, 79 (2014).
Horne, K. An optimal extraction algorithm for CCD spectroscopy. Publ. Astron. Soc. Pac. 98, 609 (1986).
Kreidberg, L. batman: basic transit model calculation in Python. Publ. Astron. Soc. Pac. 127, 1161 (2015).
Foreman-Mackey, D. et al. emcee v3: a Python ensemble sampling toolkit for affine-invariant MCMC. J. Open Source Softw. 4, 1864 (2019).
Cubillos, P. et al. On correlated-noise analyses applied to exoplanet light curves. Astronom. J. 153, 3 (2017).
Visscher, C., Lodders, K. & Fegley, B. Atmospheric chemistry in giant planets, brown dwarfs, and low-mass dwarf stars. III. Iron, magnesium, and silicon. Astrophys. J. 716, 1060 (2010).
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.
The authors declare no competing interests.
Peer review information
Nature thanks Michael Line and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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.
Rights and permissions
Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.
About this article
Cite this article
Bean, J.L., Xue, Q., August, P.C. et al. High atmospheric metal enrichment for a Saturn-mass planet. Nature (2023). https://doi.org/10.1038/s41586-023-05984-y
By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.