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Evidence for morning-to-evening limb asymmetry on the cool low-density exoplanet WASP-107 b

Nature Astronomy (2024)Cite this article

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Abstract

The atmospheric properties of hot exoplanets are expected to be different between the morning and the evening limbs due to their global atmospheric circulation. Ground-based observations at high spectral resolution have detected this limb asymmetry in several ultra-hot (>2,000 K) exoplanets, but the prevalence of the phenomenon in the broader exoplanetary population remains unexplored. Here we use JWST/NIRCam transmission spectra between 2.5 and 4.0 μm to find evidence of limb asymmetry on exoplanet WASP-107 b. With its equilibrium temperature of 770 K and low density of 0.126 g cm−3, WASP-107 b probes a very different regime compared to ultra-hot giant planets and was not expected to exhibit substantial spatial heterogeneity according to atmospheric models. We infer instead a morning-to-evening temperature difference of the order of 100 K with a hotter evening limb. Further observations of other cooler exoplanets are needed to determine whether WASP-107 b is an outlier or whether the models have underestimated the presence of limb asymmetry in exoplanets.

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Fig. 1: Dynamic spectrum and broadband light curve from our JWST/NIRCam F322W2 transit observation of WASP-107 b.
Fig. 2: Separate transmission spectra of WASP-107 b’s morning and evening limbs.
Fig. 3: Signal of limb asymmetry in our JWST/NIRCam data.
Fig. 4: Atmospheric model fits to WASP-107 b’s evening and morning limb spectra.

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Data availability

The JWST/NIRCam data (JWST GTO programme 1185; PI Greene; Observations 8 and 9), JWST/MIRI data (JWST GTO programme 1280; PI Lagage; Observation 1) and TESS data (PI Caldwell; Observation hlsp_tess-spoc_tess_phot_0000000429302040-s0010_tess_v1) are publicly available from the Mikulski Archive for Space Telescopes (https://mast.stsci.edu). The Spitzer/IRAC data (Programme 13052; PI Werner; AORKEY 62712320) are publicly available from the Spitzer Heritage Archive (https://irsa.ipac.caltech.edu/applications/Spitzer/SHA/). The SOAR data (Programme N23A-840705; PI Murphy) are available from Zenodo at https://doi.org/10.5281/zenodo.12747273 (ref. 72).

Code availability

The three reduction pipelines used in this work (Eureka!, tshirt and Pegasus) are either already open source or planned to be made open source. The codes for each are hosted on GitHub, and there are links for each in Methods.

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Acknowledgements

We acknowledge N. Espinoza and K. Misselt for assistance in verifying the reliability of JWST’s instrument timestamps, P.-O. Lagage for providing the JWST/MIRI LRS data observed in JWST programme 1280, C. Beichman for suggesting we observe this planet as part of our JWST guaranteed time programme, K. Hardegree-Ullman for help with using the package TTVFaster and N. Espinoza for a helpful discussion about this paper. T.P.G. and T.J.B. acknowledge funding from NASA (WBS 411672.07.05.05.03.02). M.M.M., E.S. and M.R. acknowledge funding from NASA Goddard Space Flight Center (NASA Contract NAS5-02105). This work benefited from the 2023 Exoplanet Summer Program in the Other Worlds Laboratory at the University of California, Santa Cruz, a programme funded by the Heising–Simons Foundation and NASA.

Author information

Authors and Affiliations

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

    Matthew M. Murphy, Everett Schlawin & Marcia Rieke

  2. Department of Astronomy, University of Wisconsin–Madison, Madison, WI, USA

    Thomas G. Beatty

  3. Bay Area Environmental Research Institute, NASA’s Ames Research Center, Moffet Field, CA, USA

    Taylor J. Bell

  4. Space Science and Astrobiology Division, NASA’s Ames Research Center, Moffet Field, CA, USA

    Taylor J. Bell & Thomas P. Greene

  5. School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA

    Michael R. Line & Luis Welbanks

  6. Laboratoire Lagrange, Observatoire de la Côte d’Azur, CNRS, Université Côte d’Azur, Nice, France

    Vivien Parmentier

  7. Department of Astronomy, University of Michigan, Ann Arbor, MI, USA

    Emily Rauscher

  8. Department of Astronomy and Astrophysics, University of California Santa Cruz, Santa Cruz, CA, USA

    Jonathan J. Fortney

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Contributions

M.M.M. led this analysis including formulating the idea and performing the data and supporting analyses and was the primary author of the paper. T.G.B. contributed the Pegasus reduction of the NIRCam data, E.S. contributed the tshirt reduction, and T.J.B. contributed the Eureka! reduction of the NIRCam F322W2 spectra and the broadband reduction of the MIRI/LRS data. T.J.B., E.S., and T.G.B. also contributed to the interpretation of our results and the writing of the paper. M.R.L. contributed the modelling analysis using ScCHIMERA. T.P.G. selected the object, devised the observing strategy and contributed to the analysis. V.P., E.R., J.J.F., M.R.L. and L.W. each contributed to the methodology and interpretation of the results. M.R. designed the JWST/NIRCam instrument used for the observations and provided the observing time necessary for completing these observations.

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Correspondence to Matthew M. Murphy.

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Nature Astronomy thanks Leen Decin and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Uniform-limb Injection Test Results.

We generated ten realizations of synthetic light curve data with the same Gaussian light curve scatter and cadence of our real observations, based around a transit model with identical evening and morning limb radii corresponding to the observed combined transit depth. We fit these synthetic data with catwoman just as we did with the real data. Shown are the best-fit evening-limb (orange) and morning-limb (blue) spectra. Panels A - D show the results of four individual tests. Panel E shows all ten tests overlaid, and Panel F shows the average limb depth in each channel. In Panels A - E, error bars are the standard deviation (1-σ) of the transit depth posterior distributions. In Panel F, error bars are the standard error on the mean.

Extended Data Table 1 Joint-Fit Priors and Best-Fit Parameter Values
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Extended Data Table 2 Best-Fit Limb Transit Depths
Full size table

Supplementary information

Supplementary Information

Supplementary Figs. 1–4 and Table 1.

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Murphy, M.M., Beatty, T.G., Schlawin, E. et al. Evidence for morning-to-evening limb asymmetry on the cool low-density exoplanet WASP-107 b. Nat Astron (2024). https://doi.org/10.1038/s41550-024-02367-9

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