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A reflective, metal-rich atmosphere for GJ 1214b from its JWST phase curve

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Abstract

There are no planets intermediate in size between Earth and Neptune in our Solar System, yet these objects are found around a substantial fraction of other stars1. Population statistics show that close-in planets in this size range bifurcate into two classes on the basis of their radii2,3. It is proposed that the group with larger radii (referred to as ‘sub-Neptunes’) is distinguished by having hydrogen-dominated atmospheres that are a few percent of the total mass of the planets4. GJ 1214b is an archetype sub-Neptune that has been observed extensively using transmission spectroscopy to test this hypothesis5,6,7,8,9,10,11,12,13,14. However, the measured spectra are featureless, and thus inconclusive, due to the presence of high-altitude aerosols in the planet’s atmosphere. Here we report a spectroscopic thermal phase curve of GJ 1214b obtained with the James Webb Space Telescope (JWST) in the mid-infrared. The dayside and nightside spectra (average brightness temperatures of 553 ± 9 and 437 ± 19 K, respectively) each show more than 3σ evidence of absorption features, with H2O as the most likely cause in both. The measured global thermal emission implies that GJ 1214b’s Bond albedo is 0.51 ± 0.06. Comparison between the spectroscopic phase curve data and three-dimensional models of GJ 1214b reveal a planet with a high metallicity atmosphere blanketed by a thick and highly reflective layer of clouds or haze.

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Fig. 1: The white light phase curve of GJ 1214b.
Fig. 2: Temperature map of GJ 1214b.
Fig. 3: White light phase curve compared to GCM outputs.
Fig. 4: JWST MIRI dayside and nightside spectra of GJ 1214b.

Data availability

The raw data from this study will become publicly available by the STScI’s Mikulski Archive for Space Telescopes (https://archive.stsci.edu/) on 20 July 2023. The following Zenodo repository hosts secondary data products including the white light and spectral light curves, extracted fit parameters and ipython notebooks to calculate derived quantities: https://zenodo.org/record/7703086#.ZAZk1dLMJhESource data are provided with this paper.

Code availability

The primary data reduction code used in this paper (SPARTA) is available at https://github.com/ideasrule/sparta. The Eureka! code used for ancillary data analysis is available at https://github.com/kevin218/Eureka. We used adapted versions of the SPARC/MITgcm (https://github.com/MITgcm/MITgcm) and CARMA (https://github.com/ESCOMP/CARMA) for our GCM and 1D aerosol modelling, respectively. The 1D temperature-pressure profiles used to initialize the GCMs were generated by HELIOS (https://github.com/exoclime/HELIOS).

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Acknowledgements

This work is based on observations made with the NASA/ESA/CSA James Webb Space Telescope. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract no. NAS 5-03127 for JWST. These observations are associated with programme no. 1803. Support for this programme was provided by NASA through a grant from the Space Telescope Science Institute. This work benefited from the 2022 Exoplanet Summer Program in the Other Worlds Laboratory at the University of California, Santa Cruz, a programme supported by the Heising-Simons Foundation. E.M.R.K. acknowledges funding from the NSF CAREER programme (grant no. 1931736). M.Z. acknowledges support from the 51 Pegasi b Fellowship financed by the Heising-Simons Foundation. M. Mansfield and L.W. acknowledge support provided by NASA through the NASA Hubble Fellowship Program. J.T. acknowledges support from the John Fell Fund and the Candadian Space Agency.

Author information

Authors and Affiliations

Authors

Contributions

E.M.R.K. and J.L.B. proposed for the observations and co-led the project. E.M.R.K. led the writing of the paper. J.L.B. planned the observations and managed the data analysis. M.Z. performed the primary data reduction. M.E.S., I.M., M.T.R., V.P., E.R., A.B.S., K.E.A. and T.K. ran, postprocessed and analysed GCMs. A.A.A.P., J.T., M.C.N., J.I., L.W. and P.M. performed retrieval analyses. P.G. calculated 1D haze profiles and provided expertise on aerosol physics. M. Malik performed 1D forward models of GJ 1214b. Q.X. inverted the observations to generate the global temperature map shown in Fig. 2. K.B.S., T.J.B., S.Z., E.D., A.D. and P.-O.L. performed supplementary data reductions. K.B.S., M. Mansfield and G.F. aided in planning the observing strategy. S.K. provided expertise on the MIRI instrument. K.G.S. and T.B. characterized the star. G.W.H. performed photometric monitoring of the star. R.L. provided opacity tables for high mean molecular weight atmosphere modelling.

Corresponding author

Correspondence to Eliza M.-R. Kempton.

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Extended data figures and tables

Extended Data Fig. 1 MIRI spectroscopic light curves from 5 to 12 μm.

Black lines are the best-fit astrophysical model to the data, assuming a second-order sinusoid functional form for the phase variation. Colored points are the data binned every 5 degrees in orbital phase, plotted without error bars for clarity. Wavelength ranges for each light curve are as indicated. Note the differing y-axis scale on each sub-panel.

Source Data

Extended Data Fig. 2 The observed emission spectrum of GJ 1214b at various orbital phases.

The upper left and upper right-hand panels correspond to the nightside and dayside emission spectrum, respectively. Colored lines denote blackbody planetary emission at temperatures of 400, 500, and 600 K, as indicated in the upper right-hand panel. Black points with 1σ error bars are the wavelength-binned phase curve data.

Source Data

Extended Data Fig. 3 Raw white light curve for GJ 1214b.

All the individual integrations are shown in blue. A median filtered (64 points) version of the light curve is shown in orange. For our analysis we discard the 550 integrations (63 min) before the vertical black line. Note the higher discrepant integrations, some of which correspond to HGA moves (vertical dashed lines); the ramp at the start of observations; and the pre-transit brightening.

Extended Data Fig. 4 Phase curve amplitudes and offsets vs. wavelength.

a, The phase curve amplitude is defined as (Fmax − Fmin)/Fmax, where Fmax and Fmin are the maximum and minimum planet/star flux ratios from the best-fit phase curve model, respectively. b, The peak offset is defined as the number of degrees in phase away from secondary eclipse at which the peak planet/star flux ratio is achieved. Negative values denote the peak occurring prior to secondary eclipse, meaning that the maximum planetary flux is eastward of the sub-stellar point. In both panels, colored lines are the GCM-derived values for the same set of models shown in Fig. 4 (see that figure’s legend). Models with higher metallicity (i.e., ≥ 100 × solar) tend to provide a qualitatively better fit to the data. All error bars are 1σ.

Source Data

Extended Data Fig. 5 The transmission spectrum of GJ 1214b.

a, The MIRI data are shown compared to GCM-derived spectra from the same set of GCMs as in Fig. 4 (see the legend in Fig. 4). b, The same set of models are shown over a broader wavelength range, with the HST/WFC3 transmission spectrum from ref. 11 also over-plotted (smaller symbols with error bars). The WFC3 data have been offset by 76 ppm to match the weighted-average transit depth of the MIRI observations in order to account for a mismatch in the system parameters applied in analyzing these two data sets and the potential for other epoch-to-epoch changes in the stellar brightness profile. Models with higher metallicity and thicker haze provide a qualitatively better fit to the transmission spectrum, in line with our findings from the thermal emission data. A more detailed interpretation of the MIRI transmission spectrum will be presented in Gao et al. (submitted). All error bars are 1σ.

Source Data

Extended Data Fig. 6 Dayside and nightside spectrum retrieval results obtained using the HyDRo atmospheric retrieval framework.

a,d, The best-fit retrieved spectra, and b,e the best-fit retrieved temperature profiles from the dayside and nightside, respectively. Dark red lines show the median retrieved spectrum and temperature profile, while dark/light shading shows the 1σ and 2σ contours, respectively. The blue points and 1σ error bars in panels a and d show the observed spectra. c,f The posterior probability distributions for the abundances of H2O, CO2, CH4 and HCN on the dayside and nightside, respectively. The black squares and error bars show the median retrieved abundances and 1 σ uncertainties for cases in which a bounded constraint was obtained. Only data at wavelengths <10.5 μm were used in the retrievals to avoid potential systematics at longer wavelengths. The retrievals are able to fit the slight absorption feature at  8 μm on the dayside (panel a) with opacity from H2O. The large absorption feature on the nightside at 8 μm (panel d) is best fit with opacity from H2O, CH4 and HCN.

Extended Data Fig. 7 Observed stellar spectrum, compared to the PHOENIX model we adopted.

The top panel shows the modelled and observed spectra. The bottom panel shows the residuals as a ratio.

Extended Data Table 1 Derived phase curve parameters
Extended Data Table 2 Transit parameters inferred from white light curve
Extended Data Table 3 Overview of GCM simulations

Source data

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Kempton, E.MR., Zhang, M., Bean, J.L. et al. A reflective, metal-rich atmosphere for GJ 1214b from its JWST phase curve. Nature 620, 67–71 (2023). https://doi.org/10.1038/s41586-023-06159-5

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