Nightside condensation of iron in an ultrahot giant exoplanet

Abstract

Ultrahot giant exoplanets receive thousands of times Earth's insolation^1,2. Their high-temperature atmospheres (greater than 2,000 kelvin) are ideal laboratories for studying extreme planetary climates and chemistry^3-5. Daysides are predicted to be cloud-free, dominated by atomic species⁶ and much hotter than nightsides^5,7,8. 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 observed^10-14, 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 transit^4,7,15. 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.

Extended Data Figure 1

Variations of the observing conditions during transit epoch 1 (a,b,c) and epoch 2 (d,e,f). 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 panels c and f indicate the airmass of 2.2 beyond which the data are discarded from the analysis.

Extended Data Figure 2

ESPRESSO radial velocities of WASP-76.a, Stellar radial velocities (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 ~2.8 m s^-1.

Extended Data Figure 3

MCMC chain 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.

Extended Data Figure 4

Doppler shadow of WASP-76.a,c, Local stellar CCFs behind the planet represented as a function of time for epoch 1 (a) and epoch 2 (c). The horizontal dashed lines represent (from bottom to top) the 2^nd contact, mid-transit and 3^rd contact. b,d, 1D view of the local stellar CCFs (black lines) with their Gaussian fits (red curves).

Extended Data Figure 5

Parameters of the stellar surface rotation model. 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 𝑣_eq sin i _★, the system scale a/R _★ and the planetary orbit inclination i_p. The posterior distribution medians and their 1σ uncertainties are represented by vertical dashed lines.

Extended Data Figure 6

Absorption signature of WASP-76b.a,b,c, On 2 September 2018 (epoch 1). d,e,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 Figure 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 (R_p/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 in the planet rest frame. The light grey region shows the FWHM associated to 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.

Extended Data Figure 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 reported in ref. []. a, Raw light curves with their best-fit models including systematic effects. b, Normalised light curves.

Figure 1. Rossiter-McLaughlin effect of WASP-76b.

a, “Classical” analysis of the effect showing the radial velocities integrated over the whole stellar disc for the ESPRESSO epoch 1 (orange), epoch 2 (green), both epochs combined (black thick curve) and 3 previous transits observed with HARPS (grey symbols; ref. ). b, “Reloaded” analysis of the effect showing the stellar surface velocities behind the disc of the planet. The red curve is a fit with a stellar surface model assuming solid-body rotation. Vertical dotted lines indicate the transit contacts and mid time. The hatched area delimits the times when the planet absorption signal crosses the Doppler shadow. The 1σ uncertainties have been propagated accordingly from the errors calculated by the ESPRESSO pipeline. Velocity scales are in the stellar rest frame. c, Sketch of the WASP-76 system (to scale) as seen from Earth. Arrows show the projected spin axes of the planetary orbit (green) and the star (black).

Figure 2. Planet absorption signature.

a, In the stellar rest frame, the planetary absorption signal appears close to the expected Keplerian of the planet, superimposed in white with its 1σ uncertainty. Transit contacts are shown by white horizontal dashed lines. The gap around 0 km s^-1 corresponds to the position of the Doppler shadow before its subtraction. b, In the planet rest frame, the shimmer is asymmetric and progressively blueshifts after ingress.

Figure 3. Polar view of the WASP-76 system.

a, The star WASP-76 and planet WASP-76b are represented to scale in size and distance. The planet is shown at different transit stages, with the transit contacts i, ii, iii and iv. During transit, the angle ζ between the planet terminator and the line of sight (dashed line in the middle) changes by 2 arcsin R_★/a = 29.4°, where a is the semi-major axis. b, Sketch of the absorption signature observed during transit, in the planet rest frame. The numbers refer to the insets. (1) During ingress, iron on the dayside is visible through the leading limb and creates an absorption around 0 km s^-1. The trailing limb enters the stellar disc and progressively blueshifts the signal. (2) The signal around 0 km s^-1 disappears as soon as no more iron is visible in the leading limb. Only the trailing limb contributes to the signal, which remain blueshifted around −11 km s^-1. (3) The signal remains at this blueshifted velocity until the end of the transit.