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A unique hot Jupiter spectral sequence with evidence for compositional diversity

Abstract

The emergent spectra of close-in, giant exoplanets (‘hot Jupiters’) are expected to be distinct from those of self-luminous objects with similar effective temperatures because hot Jupiters are primarily heated from above by their host stars rather than internally from the release of energy from their formation1. Theoretical models predict a continuum of dayside spectra for hot Jupiters as a function of irradiation level, with the coolest planets having absorption features in their spectra, intermediate-temperature planets having emission features due to thermal inversions and the hottest planets having blackbody-like spectra due to molecular dissociation and continuum opacity from the H ion2,3,4. Absorption and emission features have been detected in the spectra of a number of individual hot Jupiters5,6, and population-level trends have been observed in photometric measurements7,8,9,10,11,12,13,14,15. However, there has been no unified, population-level study of the thermal emission spectra of hot Jupiters as there has been for cooler brown dwarfs16 and transmission spectra of hot Jupiters17. Here we show that hot Jupiter secondary eclipse spectra centred around a water absorption band at 1.4 μm follow a common trend in water feature strength with temperature. The observed trend is broadly consistent with model predictions for how the thermal structures of solar-composition planets vary with irradiation level, but is inconsistent with the predictions of self-consistent one-dimensional models for internally heated objects. This is particularly the case because models of internally heated objects show absorption features at temperatures above 2,000 K, whereas the observed hot Jupiters show emission features and featureless spectra. Nevertheless, the ensemble of planets exhibits some degree of scatter around the mean trend for solar-composition planets. The spread can be accounted for if the planets have modest variations in metallicity and/or elemental abundance ratios, which is expected from planet formation models18,19,20,21.

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Fig. 1: Planet-to-star flux ratio as a function of wavelength for all observed hot Jupiters.
Fig. 2: Temperature–pressure profiles, resulting dayside planet fluxes, and opacity ratio as a function of equilibrium temperature.
Fig. 3: HST water feature strength diagram.
Fig. 4: Change in HST water feature strength from models with different parameters.

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Data availability

Data that support this paper’s findings and its plots are available on GitHub at https://github.com/meganmansfield/HSTeclipse. The full model grid can be found at https://www.dropbox.com/sh/gfsmqlxs6l1p0st/AABXyRA9RlZawpsknXc9Ya7ra?dl=0. Source data are provided with this paper.

Code availability

Software used for this work included batman44, emcee47, Matplotlib74, NumPy75, pysynphot76 and SciPy77. All code used to produce findings in this paper is available on GitHub at https://github.com/meganmansfield/HSTeclipse.

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Acknowledgements

The work was based on observations made with the NASA/ESA Hubble Space Telescope that were obtained from the data archive at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555. M.M. acknowledges funding from a NASA FINESST grant. M.R.L. acknowledges funding from NSF AST-165220, and NASA NNX17AB56G. M.R.L. also acknowledges opacity information from R. Lupu. M.R.L., J.L.B. and J.J.F. acknowledge funding for this work from STScI grants GO-13467 and GO-14792. J.J.F. and M.R.L. acknowledge the support of NASA grant 80NSSC19K0446. J.-M.D. acknowledges support from the Amsterdam Academic Alliance Program and from the European Research Council European Union’s Horizon 2020 research and innovation programme (grant no. 679633; Exo-Atmos). This work is part of the research programme VIDI New Frontiers in Exoplanetary Climatology with project number 614.001.601, which is (partly) financed by the Dutch Research Council.

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Authors

Contributions

M.M. reduced and analysed the new data sets, led the data–model comparison and wrote the manuscript. M.R.L. created the self-consistent 1D exoplanet model grids and contributed to the writing of the manuscript. J.L.B. contributed to the conception of the population study and the writing of the manuscript. J.J.F. contributed to the interpretation of the results and the writing of the manuscript. L.W. created the self-consistent 1D self-luminous object model grids. V.P., E.M.-R.K., C.B. and J.-M.D. contributed to the interpretation of the results. E.G.-N. generated the opacities and absorption cross-sections for the 1D model grids. D.K.S. and M.L.-M. are principal investigators of the HST program GO-14767 from which we obtained the new observations that were analysed in this work. M.R.S. and G.M.R. contributed to the conception of the population study. All authors commented on the manuscript.

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Correspondence to Megan Mansfield.

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The authors declare no competing interests.

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Peer review information Nature Astronomy thanks Elena Manjavacas, Amaury Triaud and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

Supplementary Figs. 1–4 and Tables 1–6.

Source data

Source Data Fig. 1

.zip archive containing 19 .txt files, each of which contains one of the secondary eclipse spectra displayed in Fig. 1

Source Data Fig. 3

.zip archive containing four .txt files. Two of the .txt files contain the data points for hot Jupiters and brown dwarfs shown in Fig. 3. The other two .txt files contain the models for hot Jupiters and self-luminous objects shown in Fig. 3.

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Mansfield, M., Line, M.R., Bean, J.L. et al. A unique hot Jupiter spectral sequence with evidence for compositional diversity. Nat Astron 5, 1224–1232 (2021). https://doi.org/10.1038/s41550-021-01455-4

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