Short-period planets exhibit day–night temperature contrasts of hundreds to thousands of kelvin. They also exhibit eastward hotspot offsets whereby the hottest region on the planet is east of the substellar point1; this has been widely interpreted as advection of heat due to eastward winds2. We present thermal phase observations of the hot Jupiter CoRoT-2b obtained with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. These measurements show the most robust detection to date of a westward hotspot offset of 23 ± 4°, in contrast with the nine other planets with equivalent measurements3,4,5,6,7,8,9,10. The peculiar infrared flux map of CoRoT-2b may result from westward winds due to non-synchronous rotation11 or magnetic effects12,13, or partial cloud coverage, that obscure the emergent flux from the planet’s eastern hemisphere14,15,16,17. Non-synchronous rotation and magnetic effects may also explain the planet’s anomalously large radius12,18. On the other hand, partial cloud coverage could explain the featureless dayside emission spectrum of the planet19,20. If CoRoT-2b is not tidally locked, then it means that our understanding of star–planet tidal interaction is incomplete. If the westward offset is due to magnetic effects, our result represents an opportunity to study an exoplanet’s magnetic field. If it has eastern clouds, then it means that a greater understanding of large-scale circulation on tidally locked planets is required.

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L.D. thanks S. Carey, J. Ingalls and W. Glaccum from the Spitzer IRAC team for the helpful discussions that contributed to the reduction of the data. Funding for this work was provided in part by the Natural Sciences and Engineering Research Council of Canada (NSERC) discovery grant and the California Institute of Technology’s Infrared Processing and Analysis Center (Caltech/IPAC) Visiting Graduate Research Fellowship. Work by S.S. was funded by the Google Summer of Code programme. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech.

Author information


  1. Department of Physics, McGill University, Montréal, Québec, Canada

    • Lisa Dang
    • , Nicolas B. Cowan
    •  & Joel C. Schwartz
  2. McGill Space Institute (MSI), McGill University, Montréal, Québec, Canada

    • Lisa Dang
    • , Nicolas B. Cowan
    •  & Joel C. Schwartz
  3. Institut de Recherche sur les Exoplanètes (iREx), Université de Montréal, Montréal, Québec, Canada

    • Lisa Dang
    • , Nicolas B. Cowan
    •  & Joel C. Schwartz
  4. Department of Earth and Planetary Sciences, McGill University, Montréal, Québec, Canada

    • Nicolas B. Cowan
    •  & Joel C. Schwartz
  5. Department of Astronomy, University of Michigan, Ann Arbor, Michigan, USA

    • Emily Rauscher
  6. Division of Physics, Mathematics & Astronomy, California Institute of Technology, Pasadena, California, USA

    • Michael Zhang
  7. Division of Geological & Planetary Sciences, California Institute of Technology, Pasadena, California, USA

    • Heather A. Knutson
  8. School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA

    • Michael Line
  9. New York University Abu Dhabi, Abu Dhabi, UAE

    • Ian Dobbs-Dixon
  10. Department of Astronomy, University of Maryland, College Park, Maryland, USA

    • Drake Deming
  11. Department of Computer Science and Engineering, Amrita Vishwa Vidyapeetham, Amritapuri, Kerala, India

    • Sudarsan Sundararajan
  12. Department of Astronomy and Astrophysics, University of California, Santa Cruz, California, USA

    • Jonathan J. Fortney
  13. Department of Astronomy & Astrophysics, Pennsylvania State University, University Park, Pennsylvania, USA

    • Ming Zhao


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L.D. extracted the photometric measurements from the data, detrended the data, developed and fit the phase curve models, led the analysis and wrote the manuscript. N.B.C. is the prinicpal investigator of the successful Spitzer proposal from which we obtained the observations and contributed to the writing of the manuscript. J.C.S. contributed materials to the main text. E.R. contributed to the interpretation for the results and to the discussion. M.Z. and H.A.K. verified the robustness of the analysis and contributed to the interpretation of results. S.S. contributed to the photometric measurements pipeline. J.C.S., E.R., H.A.K., I.D.-D., M.L., D.D., J.J.F. and M.Z. are co-investigators of the successful Spitzer proposal from which we obtained the observations. All authors commented on the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Lisa Dang.

Supplementary information

  1. Supplementary Information

    Supplementary Tables 1–6, Supplementary Figures 1–15, Supplementary text, Supplementary references.

  2. Supplementary Dataset 1

    Binned data supporting Supplementary Figure 1.

  3. Supplementary Dataset 2

    Full data supporting Supplementary Figure 1.

  4. Supplementary Dataset 3

    Data supporting top panel of Supplementary Figure 13.

  5. Supplementary Dataset 4

    Data supporting bottom panel of Supplementary Figure 13.

  6. Supplementary Dataset 5

    Data supporting Supplementary Figure 4.

  7. Supplementary Dataset 6

    Data supporting Supplementary Figure 14.

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