Ultrahot giant exoplanets receive thousands of times Earth’s insolation1,2. Their high-temperature atmospheres (greater than 2,000 kelvin) are ideal laboratories for studying extreme planetary climates and chemistry3,4,5. Daysides are predicted to be cloud-free, dominated by atomic species6 and much hotter than nightsides5,7,8. Atoms are expected to recombine into molecules over the nightside9, resulting in different day and night chemistries. Although metallic elements and a large temperature contrast have been observed10,11,12,13,14, no chemical gradient has been measured across the surface of such an exoplanet. Different atmospheric chemistry between the day-to-night (‘evening’) and night-to-day (‘morning’) terminators could, however, be revealed as an asymmetric absorption signature during transit4,7,15. Here we report the detection of an asymmetric atmospheric signature in the ultrahot exoplanet WASP-76b. We spectrally and temporally resolve this signature using a combination of high-dispersion spectroscopy with a large photon-collecting area. The absorption signal, attributed to neutral iron, is blueshifted by −11 ± 0.7 kilometres per second on the trailing limb, which can be explained by a combination of planetary rotation and wind blowing from the hot dayside16. In contrast, no signal arises from the nightside close to the morning terminator, showing that atomic iron is not absorbing starlight there. We conclude that iron must therefore condense during its journey across the nightside.
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The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available owing to the proprietary status of data obtained in the framework of the ESPRESSO Guaranteed Time Observations. At the end of the proprietary period, the data will be publicly available in the ESO archive (https://archive.eso.org).
The ESPRESSO DRS is public software available from ESO at https://www.eso.org/sci/software/pipelines/espresso/espresso-pipe-recipes.html. The main analysis routines have been written by the authors in Interactive Data Language and are available upon reasonable request from the corresponding author.
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We thank G. Fu for sharing information regarding the binary companion of WASP-76A and D. Kitzmann for discussion about how iron can condense. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (project FOUR ACES; grant agreement no. 724427). It has also been carried out in the frame of the National Centre for Competence in Research PlanetS supported by the Swiss National Science Foundation (SNSF). This work was supported by FCT/MCTES through national funds and by FEDER (Fundo Europeu de Desenvolvimento Regional) through COMPETE2020 (Programa Operacional Competitividade e Internacionalização) by these grants: UID/FIS/04434/2019; PTDC/FIS-AST/32113/2017 and POCI-01-0145-FEDER-032113; PTDC/FIS-AST/28953/2017 and POCI-01-0145-FEDER-028953. V.A. and S.S. acknowledge support from FCT through Investigador FCT contracts IF/00650/2015 and IF/00028/2014, and POPH/FSE (EC) by FEDER funding through the programme “Programa Operacional de Factores de Competitividade—COMPETE”. This work of C.J.A.P.M. was financed by FEDER funds through the COMPETE 2020—Operational Programme for Competitiveness and Internationalisation (POCI), and by Portuguese funds through FCT (Fundação para a Ciência e a Tecnologia) in the framework of the projects POCI-01-0145-FEDER-028987 and UID/FIS/04434/2019. O.D. is supported by a work contract (DL 57/2016/CP1364/CT0004). M.R.Z.O. acknowledges financial support from AYA2016-79425-C3-2-P from the Spanish Ministry for Science, Innovation and Universities (MICIU). J.I.G.H. acknowledges financial support from the MICIU under the 2013 Ramón y Cajal programme MICIU RYC-2013-14875. J.I.G.H., R.R., C.A.P. and A. Suárez Mascareño also acknowledge financial support from the MICIU for project AYA2017-86389-P. This publication makes use of The Data and Analysis Center for Exoplanets (DACE), which is a facility based at the University of Geneva (CH) dedicated to extrasolar planet data visualization, exchange and analysis. DACE is a platform of the Swiss National Centre of Competence in Research (NCCR) PlanetS, federating the Swiss expertise in Exoplanet research. The DACE platform is available at https://dace.unige.ch. This paper is based on observations made at the ESO Very Large Telescope (Paranal, Chile) under programme 1102.C-744 and at the ESO 3.6-m telescope (La Silla, Chile) under programmes 090.C-0540 and 100.C-0750.
The authors declare no competing interests.
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Extended data figures and tables
a–c, Epoch 1. d–f, Epoch 2. The seeing (a, d), signal-to-noise ratio per pixel at 550 nm (b, e) and airmass (c, f) are shown as a function of the time in transit. Vertical dotted lines represent the transit contacts. The horizontal dashed lines in c and f indicate the airmass of 2.2 beyond which the data are discarded from the analysis.
a, Stellar radial velocities (RV; blue points) and the maximum-likelihood fit using values from Extended Data Table 3. The transit occurs at the inferior conjunction (0 h). In-transit data have been removed as they are affected by the Rossiter–McLaughlin effect and the atmospheric absorption from the planet. b, Residuals of the radial velocities after subtraction of the maximum-likelihood fit. The standard deviation of the residuals is about 2.8 m s−1. Error bars correspond to 1σ uncertainties and include both parameterized noise terms.
Shown is the corner plot for the orbital parameters representing the posterior distribution of variables used for the MCMC computations of the orbital parameters. The posterior distribution medians are reported in Extended Data Table 3.
a, b, Data for epoch 1; c, d, data for epoch 2. a, c, Local stellar CCFs behind the planet represented as a function of time. The horizontal dashed lines represent (from bottom to top) the second contact, mid-transit and third contact. b, d, 1D view of the local stellar CCFs (black lines) with their Gaussian fits (red curves).
The corner plot shows the posterior distributions of the four free parameters of the model, the projected spin–orbit angle λ, the projected equatorial stellar rotational velocity veqsini⁎, the system scale a/R⁎ and the planetary orbit inclination ip. The posterior distribution medians and their 1σ uncertainties are represented by vertical dashed lines.
a–c, On 2 September 2018 (epoch 1); d–f, on 30 October 2018 (epoch 2). The planetary absorption signal is shown in the stellar rest frame (a, d), the planet rest frame (b, e) and is time-averaged in the planet rest frame to produce the atmospheric absorption profile integrated over the whole limb (c, f). An indicative Gaussian fit (red curves) is overplotted on the absorption profiles. Both epochs show compatible results.
Extended Data Fig. 7 Measured properties of the planetary absorption signature as a function of time.
Data from epoch 1 (orange), epoch 2 (green) and both epochs combined (binned by 2; black curve with 1σ uncertainty in dark grey) are shown. They result from Gaussian fits to the planetary absorption signal in the residual maps of Fig. 2b and Extended Data Figs. 5b and e. A factor of (Rp/R⁎)2/(1−ΔF/F(t)) was applied to the residual maps before the fit, where ΔF/F(t) is the model light curve used to extract the Doppler shadow. a, Radial velocity of the planetary signal (called the planet ‘shimmer’) in the planet rest frame. The light grey region shows the FWHM associated with each point. b, The FWHM of the signal. The weighted mean (horizontal dashed line) is 8.6 ± 0.7 km s−1. Horizontal dotted lines indicate the standard deviation of the values. c, Amplitude of the shimmer representing the differential transit depth. The weighted mean is 494 ± 27 ppm. The hatched area in all panels represents the overlap between the Doppler shadow and the planetary signal; data between −0.2 h and +0.7 h from mid-transit are excluded from the analysis. Error bars in all panels correspond to 1σ uncertainties.
Extended Data Fig. 8 Photometric transit light curve of WASP-76b obtained with the EulerCam instrument on the Swiss Euler 1.2-m telescope in La Silla, Chile.
The last three transits (bottom rows) have been previously reported19. a, Raw light curves with their best-fit models including systematic effects. b, Normalized light curves. Error bars represent 1σ uncertainties.
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Ehrenreich, D., Lovis, C., Allart, R. et al. Nightside condensation of iron in an ultrahot giant exoplanet. Nature 580, 597–601 (2020). https://doi.org/10.1038/s41586-020-2107-1
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