For decades, astronomers studying transiting exoplanets have treated the atmosphere visible during a transit as a single, uniform ring. When a planet crosses in front of its star, a sliver of starlight filters through the gas at the planet’s edge, and molecules in that gas imprint telltale absorption signatures on the stellar spectrum. Traditional analysis assumes a circular planetary silhouette, producing one limb-averaged spectrum that blends together the morning and evening terminators, the equator and the poles. That approach can identify molecules, but it washes out the spatial variation that models of irradiated planets predict should be substantial, with horizontal temperature gradients reshaping both the scale height of the atmosphere and its chemistry from dayside to nightside and from equator to pole.

The transmission strings method, developed by David Grant and Hannah Wakeford of the University of Bristol, abandons the assumption of a circular limb. Instead, it parameterizes the planet’s transiting shape as a Fourier series of adjustable complexity, allowing the planetary radius to vary as a function of angle around the limb. The open-source light-curve fitting software Harmonica fits each wavelength-binned transit light curve with these non-circular shapes, regularized through a smoothing parameter and validated with a k-fold cross-validation technique that tests whether the fitted asymmetries represent a genuine physical signal rather than short-duration noise. The result is a function describing the planetary radius at every angle and wavelength, from which a transmission spectrum can be extracted for any slice of the atmosphere.

The target, WASP-17 b, is almost ideally suited to the challenge. It is an extremely inflated giant planet, roughly 1.9 times the radius of Jupiter but only about half its mass, orbiting with an equilibrium temperature near 1,760 kelvin. That combination of enormous atmospheric scale height and scorching temperatures gives it an exceptionally favorable signal-to-noise ratio for atmospheric characterization. The team drew on transit observations taken in March 2023 with three instruments aboard the James Webb Space Telescope: NIRISS in single-object slitless spectroscopy mode, NIRSpec with its G395H grating, and the Mid-Infrared Instrument’s low-resolution spectrograph, all obtained as part of the JWST Telescope Scientist Team’s DREAMS Guaranteed Time programme. Combined with optical data from the Hubble Space Telescope, the observations span wavelengths from 0.6 to 12 micrometers.

The extracted spectra changed dramatically with angle. At the western morning terminator, defined as the first point of contact with the stellar disk, the transmission spectrum was broadly flat, with muted spectral features. At the eastern evening terminator, transit depths rose steeply toward the infrared, with large absorption features dominated by water vapor bands between 1 and 3 micrometers and the strong carbon dioxide feature at 4.5 micrometers. Moving in increments between the two extremes, the spectra traced a gradual transition. The differences were statistically decisive: fitting the spectrum from one terminator with the model retrieved for the other yielded reduced chi-squared values of roughly 5 to 10, confirming that the two halves of the limb are genuinely distinct worlds.

To interpret the mapped spectra, the team applied two complementary modeling strategies. Forward model grids computed with the SANSAR and ATMO codes, using both self-consistent radiative-convective-thermodynamic-equilibrium profiles and isothermal profiles with equilibrium chemistry, provided broad trends around the limb. Free chemistry retrievals with the POSEIDON code, which fit abundances for individual species without assuming chemical equilibrium, quantified temperatures and composition at each angle. The retrievals revealed temperatures climbing continuously from about 850 kelvin at the morning limb to roughly 1,500 kelvin at the evening limb, an east-west contrast of about 664 kelvin. Because temperature controls the atmospheric scale height, the evening terminator’s atmosphere puffs up to nearly 1.8 times the scale height of the morning side, producing the larger and deeper absorption features observed there.

Notably, the self-consistent equilibrium models struggled. The radiative-convective models consistently failed to reproduce the amplitude of the infrared absorption features, and the best-fitting recirculation factor at every angle sat at the maximum the grid allowed, implying that even more heat redistribution than physically reasonable would be needed to make the evening limb hot enough under equilibrium assumptions. The isothermal models and free retrievals, which are agnostic about chemical equilibrium and sensitive to the changing thermal scale height, fared far better. The lesson, the authors argue, is that dynamically driven temperature contrasts, not equilibrium chemistry, dominate the structure of WASP-17 b’s limb.

Chemistry, however, left its fingerprints. The abundances of water and carbon dioxide, the two most reliably constrained species, remained broadly constant around the limb, consistent with predictions from general circulation models that equatorial superrotation should homogenize these tracers. Hints of greater complexity emerged at the poles: the retrieved methane abundance appeared to increase away from the equator, matching model predictions that polar regions, shielded from the eastward equatorial jet, are cooler and more stable for methane. Weak, poorly constrained signals of sulfur dioxide were also seen across the nightside portion of the limb, in line with earlier nightside emission measurements that detected the molecule. The overall picture of a supersolar metallicity and subsolar carbon-to-oxygen ratio survived the spatial dissection.

Perhaps most striking is the consistency across independent observational techniques. The limb temperatures span the value obtained from nightside emission measurements, and the east-west temperature contrast agrees with that derived from MIRI eclipse mapping of the dayside, even though the two techniques probe different pressure levels. Together, transmission mapping and eclipse mapping now provide a comprehensive three-dimensional thermodynamic portrait of a single transiting exoplanet, something no previous study has achieved. The contrast itself, larger than those measured for cooler hot Jupiters such as WASP-39 b and WASP-107 b, matches general circulation model predictions that hotter planets should exhibit stronger day-night heat transport asymmetries and an eastward hotspot offset.

The temperature range mapped around the limb, from about 850 to 1,500 kelvin, also spans the predicted transition between silicate and salt cloud regimes, raising the possibility that the bulk composition of aerosols changes around the limb even though the current data could not constrain aerosol properties. Future mid-infrared observations of WASP-17 b promise improved precision beyond 5 micrometers, where many aerosol species, including the quartz clouds already detected in the planet’s atmosphere, leave their strongest signatures. For now, the transmission strings technique has demonstrated that the edge of an exoplanet is not a uniform ring but a dynamic landscape, where morning clouds give way to evening heat and the chemistry of an alien atmosphere unfolds angle by angle.

Subject of Research: Three-dimensional transmission mapping of the atmospheric limb of the hot Jupiter WASP-17 b

Article Title: A spatial map of the atmospheric limb of a hot Jupiter

Article References: A spatial map of the atmospheric limb of a hot Jupiter. (n.d.). https://doi.org/10.1038/s41550-026-02986-4

Image Credits: AI Generated

DOI: 10.1038/s41550-026-02986-4

Keywords: exoplanets, hot Jupiter, WASP-17 b, JWST, transmission spectroscopy, atmospheric dynamics, atmospheric chemistry, transmission strings, terminator asymmetry, retrieval modelling, Nature Astronomy, spatial