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A map of the large day–night temperature gradient of a super-Earth exoplanet

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Abstract

Over the past decade, observations of giant exoplanets (Jupiter-size) have provided key insights into their atmospheres1,2, but the properties of lower-mass exoplanets (sub-Neptune) remain largely unconstrained because of the challenges of observing small planets. Numerous efforts to observe the spectra of super-Earths—exoplanets with masses of one to ten times that of Earth—have so far revealed only featureless spectra3. Here we report a longitudinal thermal brightness map of the nearby transiting super-Earth 55 Cancri e (refs 4, 5) revealing highly asymmetric dayside thermal emission and a strong day–night temperature contrast. Dedicated space-based monitoring of the planet in the infrared revealed a modulation of the thermal flux as 55 Cancri e revolves around its star in a tidally locked configuration. These observations reveal a hot spot that is located 41 ± 12 degrees east of the substellar point (the point at which incident light from the star is perpendicular to the surface of the planet). From the orbital phase curve, we also constrain the nightside brightness temperature of the planet to 1,380 ± 400 kelvin and the temperature of the warmest hemisphere (centred on the hot spot) to be about 1,300 kelvin hotter (2,700 ± 270 kelvin) at a wavelength of 4.5 micrometres, which indicates inefficient heat redistribution from the dayside to the nightside. Our observations are consistent with either an optically thick atmosphere with heat recirculation confined to the planetary dayside, or a planet devoid of atmosphere with low-viscosity magma flows at the surface6.

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Figure 1: 55 Cancri e Spitzer/IRAC 4.5-μm phase curve.
Figure 2: Longitudinal brightness maps of 55 Cancri e.

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Acknowledgements

We thank D. Deming, D. Apai and A. Showman for discussions as well as the Spitzer Science Center staff for their assistance in the planning and executing of these observations. 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. M.G. is a Research Associate at the Belgian Funds for Scientific Research (FRS-FNRS). V.S. was supported by the Simons Foundation (award number 338555, VS).

Author information

Authors and Affiliations

Authors

Contributions

B.-O.D. initiated and led the Spitzer observing programme, conducted the data analysis and wrote the paper. M.G. performed an independent analysis of the dataset. E.B. carried out the simulations assessing the amplitude of tidal heating in the interior of 55 Cancri e. J.d.W. performed the longitudinal mapping of the planet. N.M. wrote the interpretation section with inputs from E.B., K.H., V.S., R.H., N.L. and T.K. J.K., B.B., S.K. and D.Q. contributed to the observing programme. All authors commented on the manuscript.

Corresponding author

Correspondence to Brice-Olivier Demory.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 55 Cancri e raw photometry.

a–c, The raw data for time series acquired on 15 June 2013 (a), 18 June 2013 (b) and 21 June 2013 (c). The best-fit instrumental + astrophysical model is superimposed in red. Grey filled circles are data binned per 30 s. Black filled circles are data binned per 15 min. The error bars are the standard deviation of the mean within each time bin. BJD, barycentric Julian date.

Source data

Extended Data Figure 2 Continuation of Extended Data Fig. 1.

a–c, The raw data for time series acquired on 29 June 2013 (a), 3 July 2013 (b) and 8 July 2013 (c).

Source data

Extended Data Figure 3 Continuation of Extended Data Fig. 1.

a–c, The raw data for time series acquired on 11 July 2013 (a, b) and 15 July 2013 (c).

Source data

Extended Data Figure 4 55 Cancri e corrected photometry.

a–c, The detrended data for time series acquired on 15 June 2013 (a), 18 June 2013 (b) and 21 June 2013 (c). The best-fit instrumental + astrophysical model is superimposed in red. Grey filled circles are data binned per 30 s. Black filled circles are data binned per 15 min. The error bars are the standard deviation of the mean within each time bin. BJD, barycentric Julian date.

Source data

Extended Data Figure 5 Continuation of Extended Data Fig. 4.

a–c, The detrended data for time-series acquired on 29 June 2013 (a), 3 July 2013 (b) and 8 July 2013 (c).

Source data

Extended Data Figure 6 Continuation of Extended Data Fig. 4.

a–c, The detrended data for time-series acquired on 11 July 2013 (a, b) and 15 July 2013 (c).

Source data

Extended Data Figure 7 Photometric r.m.s. versus bin size for all data sets.

a–i, Black filled circles indicate the photometric residual r.m.s. for different time bins. Each panel corresponds to each individual data set (ai, increasing observing date). The expected decrease in Poisson noise normalized to an individual bin (30 s) precision is shown as a red dotted line.

Extended Data Figure 8 Polynomial-detrended phase-folded photometry.

Photometry for all eight data sets combined and folded on the orbital period of 55 Cancri e. a, Fit results using the entire time series as input data. b, Fit results obtained by splitting the times series in two. Data in a and b represent the planet-to-star flux ratio (Fplanet/Fstar) variation in phase and are binned per 15 min; the error bars are the standard deviation of the mean within each orbital phase bin. The best-fit model is shown in red. Contrary to Fig. 1, these fits are obtained using polynomial functions of the centroid position and the FWHM of the PRF.

Extended Data Figure 9 Tidal heating constraints for 55 Cancri e.

The planet-to-star flux ratio (Fp/F) is shown as a function of the orbital eccentricity for different values of dissipation (relative to the Earth’s σ; indicated by the different colours) and albedos (‘A’, indicated by the different line styles, from 0.0 (solid) to 1.0 (long-dashed)). The pink and orange bands represent the occultation depth values measured in 2012 and 2013 with Spitzer, respectively. Vertical lines indicate the plausible range of the eccentricity of 55 Cancri e as determined from the N-body simulations for each dissipation value. The 2012 occultation depth can be matched for high albedos and a high dissipation, while the deeper 2013 occultation depth can be matched for the highest dissipation (10σ) and the whole albedo range.

Extended Data Table 1 55 Cancri e Spitzer data set

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Demory, BO., Gillon, M., de Wit, J. et al. A map of the large day–night temperature gradient of a super-Earth exoplanet. Nature 532, 207–209 (2016). https://doi.org/10.1038/nature17169

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