Inhomogeneous terminators on the exoplanet WASP-39 b

Abstract

Transmission spectroscopy has been a workhorse technique used over the past two decades to constrain the physical and chemical properties of exoplanet atmospheres^1-5. One of its classical key assumptions is that the portion of the atmosphere it probes-the terminator region-is homogeneous. Several works from the past decade, however, have put this into question for highly irradiated, hot (T_eq ≳ 1,000 K) gas giant exoplanets, both empirically^6-10 and through three-dimensional modelling^11-17. While models have predicted clear differences between the evening (day-to-night) and morning (night-to-day) terminators, direct morning and evening transmission spectra in a wide wavelength range have not been reported for an exoplanet so far. Under the assumption of precise and accurate orbital parameters for the exoplanet WASP-39 b, here we report the detection of inhomogeneous terminators on WASP-39 b, which has allowed us to retrieve its morning and evening transmission spectra in the near-infrared (2-5 μm) using the James Webb Space Telescope. We have observed larger transit depths in the evening, which are, on average, 405 ± 88 ppm larger than the morning ones, and also have qualitatively larger features than the morning spectrum. The spectra are best explained by models in which the evening terminator is hotter than the morning terminator by $177_{-57}^{+65}$ K, with both terminators having C/O ratios consistent with solar. General circulation models predict temperature differences broadly consistent with the above value and point towards a cloudy morning terminator and a clearer evening terminator.

Fig. 1. Light-curve modelling and extraction of morning and evening depths.

a,b, Transit light-curve modelling of the 10 July 2022 transit at 4.38 μm (in the middle of the CO₂ spectral feature in the transmission spectrum) of WASP-39 b (grey datapoints) using both the catwoman framework (a, purple model) and the classic circular occulter model via batman (b, grey model). c,d, Residuals of the best-fit model for the catwoman (c) and batman (d) methodologies. The batman residuals (d) show, in turn, the difference between the catwoman and batman light-curve models (purple line), showcasing the catwoman model’s small (approximately a few hundred parts per million) light-curve asymmetries. Note that the residuals are of comparable magnitude. While it is difficult to conclude the effect is present in each individual light curve, the effect is detectable once all the wavelength-dependent light curves have been analysed (see Fig. 2 and text for details). e,f, Inferences enabled from the catwoman (e) and batman (f) methodologies. The catwoman methodology (e) allows the extraction of morning/evening transit depths (purple ellipses representing the 1, 2 and 3σ posterior contours; dashed grey line indicating equal morning and evening depths), whereas the circular occulter methodology (f) only allows the extraction of a single total transit depth from the light curve. All error bars represent 1-standard deviation.

Fig. 2. The morning and evening spectra of WASP-39 b from JWST NIRSpec/PRISM observations.

a,b, The total transit depth, from adding the morning and evening spectra (a), along with the individual morning (blue points) and evening (red points) spectra of WASP-39 b, as derived from our light-curve modelling (b). The large points are datapoints at a resolution (R) of 30, shown for illustration, the smaller points are at R = 100. The best-fit, chemically consistent models (solid black line in a, solid red and blue lines in b; fitted to the R = 100 spectra) are consistent with a hotter evening terminator (see text for details). c,d, Residuals from the best-fit model (red points for the evening (c), blue points for the morning (d)), the dashed line marks 0. All error bars represent 1-standard deviation.

Fig. 3. Comparison of morning and evening spectra using GCMs.

a–h, Comparison between the predictions from GCMs and the observed transit depths for the morning and evening terminators, as derived with catwoman. The left-hand column shows the spectra (a, c, e and g) and the right-hand column shows the difference between the morning and evening terminator (b, d, f and h). A vertical offset of −600 ppm was applied to the spectra in c, e and g to facilitate comparison with the observed spectra. a,b, Condensate cloud model together with the equilibrium chemistry taking into account elemental depletion by clouds compared against the morning (blue) and evening (red) spectra (a), and against the difference between the spectra (b, in black); the grey line indicates zero difference. This model qualitatively provides the best match to the morning terminator spectrum. c,d, Photochemical haze model based on the SPARC/MITgcm, with equilibrium chemistry gas-phase abundances. e,f, Clear-atmosphere equilibrium chemistry model. g,h, Clear-atmosphere model, including transport-induced disequilibrium chemistry (diseq. chem.). The clear-atmosphere models provide the best match to the evening terminator spectrum. All error bars represent 1-standard deviation.

Extended Data Fig. 1. Different approaches at detecting limb asymmetries from NIRSpec/PRISM data.

a-b. Evening (top) and morning (middle) depths as extracted from three independent analyses of our NIRSpec/PRISM lightcurves; one using the catwoman framework with limb-darkening as free parameters with a prior (NE), a framework leaving those fixed (MM) and a framework on which half-ingress and half-egress are fitted independently using a batman lightcurve model (Tiberius); see text for details. Note the agreement between approaches for both terminators, and how the amplitude of the features seem to be smaller in the morning terminator c. An independent look at limb asymmetries by fitting for a wavelength-dependent time-of-transit center to each wavelength-dependent lightcurve. As with the top and middle panels, differences between the limbs as tracked by the time-of-transit center seem to be largest between 2–3.5 μm, i.e., around the water bands. All errorbars represent 1-standard deviation.

Extended Data Fig. 2. Impact of less accurate and precise orbital parameters on the detection of inhomogeneous terminators on WASP-39 b.

In our experiments, we inflated the errorbars on the orbital parameters (e.g., impact parameter, a/R_*, etc.) by different factors, and performed wavelength-dependant catwoman lightcurve fits on our NIRSpec/PRISM data using normal priors for each parameter along with the other wavelength-dependant parameters described in the main text and in the Methods section such as the planet-to-star radius ratio, limb-darkening, etc. a. Error inflation exercise assuming a circular orbit (i.e., eccentricity fixed to zero) and b. same exercise but assuming an eccentric orbit — with uncertainties on all parameters, including $e\cos \omega$ and $e\sin \omega$, inflated by 3 (left), 5 (middle) and 10-fold (right). Dashed line marks the non-detection threshold (i.e., equal evening and morning depths). All bold errorbars represent 1-standard deviation. Thin errorbars are 3-standard deviations.

Extended Data Fig. 3. Robustness of limb asymmetry detection to transit parameters and assumptions.

To study the robustness of our extracted morning and evening spectra for the NIRSpec/PRISM observations, we simulated transit lightcurves using batman and then fitted those using catwoman with different assumptions, leaving all parameters fixed but the depths of the morning and evening limbs. a-b. Null case on which the true model has no limb asymmetries and the input transit parameters are unchanged; the catwoman fits correctly recover the same morning (blue) and evening (red) spectra (top). The difference Δ between the morning and the evening spectra are consistent with zero, as expected from this null case (bottom). c. Same experiment as in a., but generating a light curve with a time-of-transit center 3-σ away from the fixed value, which amounts to an offset of 3.4 seconds. The difference Δ is consistent with zero, suggesting our inferences are robust against this parameter. d. Same experiment, but generating a light curve that had limb-darkening coefficients of the quadratic law offset by 0.01. Note how this injects a systematic offset in the difference between the morning and evening spectra; depending on the direction of this offset, this can lead to mornings having larger depths than evenings or viceversa. e. Same experiment, but generating a transit lightcurve with a non-zero eccentricity consistent at 3-σ with the white-lightcurve fits of Carter & May et al. (in review; e = 0.035, ω = 10 deg). Note how this slight eccentricity can generate significantly larger mornings than evenings, due to the asymmetry an eccentric orbit imprints on the transit lightcurve. For WASP-39 b, this eccentricity effect cannot generate larger evenings than mornings, which is what we observe. This suggests our results are also robust against this parameter (see text for details). All errorbars represent 1-standard deviation.

Extended Data Fig. 4. Quadratic limb-darkening coefficients from catwoman WASP-39 b transit light curve fits.

Solid lines (purple for u₁, blue for u₂) are the theoretical limb-darkening coefficients obtained by, first, using the limb-darkening library using ATLAS models to extract limb-darkening coefficients, and then passing those through the SPAM algorithm of to obtain the model predictions. Points with errorbars are retrieved limb-darkening coefficients from our catwoman (NE) transit light curve fits. Note the apparent offset between model and retrieved coefficients between about 2.5 to 4.5 μm for u₁. All errorbars represent 1-standard deviation.

Extended Data Fig. 5. Posterior distribution of some of the retrieved CHIMERA parameters.

Corner plot of the morning/evening temperatures, (log) C/O ratios and (log) cloud-top pressures. Only the evening cloud-top pressure is constrained by our retrievals. The purple line in the morning/evening temperature posterior samples showcases the line of equal temperatures; as can be observed, our posterior samples imply a significantly different morning-to-evening temperature.