A study published in Nature Astronomy on June 10, 2026 uses JWST's NIRSpec to directly detect that the morning and evening terminator regions of the tidally locked gas giant have measurably different temperatures and chemical composition.
NASA’s James Webb Space Telescope has directly measured, for the first time, that the morning and evening twilight zones of the ultra-hot gas giant WASP-121 b have meaningfully different temperatures and atmospheric chemistry - a result that confirms long-standing theoretical models about how tidally locked planets circulate heat, according to a study published in Nature Astronomy on June 10, 2026.
The finding, led by Cyril Gapp, a doctoral candidate at the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, is the first observational confirmation that the two terminator regions of a tidally locked gas giant are atmospherically distinct. The research used JWST’s Near-Infrared Spectrograph (NIRSpec) and a technique called rotational transit spectroscopy to isolate signals from the planet’s morning and evening atmospheric edges as it crossed in front of its host star.
WASP-121 b is classified as an ultra-hot Jupiter - a gas giant that orbits its host star so closely, and so rapidly, that it is gravitationally locked, permanently presenting the same hemisphere to its star. The planet completes one orbit every approximately 30 hours, at a distance roughly 40 times closer than Earth is to the Sun. Its permanent dayside reaches average temperatures of around 2,770 Kelvin - approximately 2,500 degrees Celsius - hot enough for iron and other metals to exist in vapour form. The cooler nightside sits at around 1,000 Kelvin.
Because the same face is always lit, “dawn” and “dusk” on WASP-121 b are fixed features rather than daily events: permanent bands at the boundary between the eternal day hemisphere and the eternal night hemisphere.
Prior observations of WASP-121 b with Hubble and earlier JWST instrument modes characterised its atmospheric chemistry as an averaged signal across the full atmospheric limb - the ring visible during transit. The June 2026 paper resolves that averaged signal into its morning and evening components by using the planet’s own rotation as an additional spectroscopic signature.
The results show that the dusk, or evening, terminator absorbs more stellar light than the dawn, or morning, terminator. The evening atmosphere is hotter and more vertically extended. According to the Max-Planck-Gesellschaft press release accompanying the study, “fierce winds” originating on the permanent dayside carry thermal energy around the planet, arriving first and with greater intensity at the dusk side.
The chemical consequences are measurable. In the hotter dusk region, water molecules break apart into their atomic constituents - hydrogen and oxygen - rather than remaining intact. Carbon monoxide detection also varies between the two terminators, with the signal stronger on the warmer dusk side. The morning terminator retains more intact water.
The morning-evening asymmetry had been predicted by atmospheric circulation models for well over a decade. General circulation models of tidally locked gas giants indicated that superrotating equatorial jet streams would carry heat eastward from the permanent noon point, warming the evening limb relative to the morning before the unlit nightside can cool it down. The MPIA describes the Gapp et al. result as the first direct observational confirmation of this predicted asymmetry via transit spectroscopy.
Co-author Dr. Tom Evans-Soma of the University of Newcastle noted that the technique opens new avenues for probing atmospheric structure that had previously been accessible only through theoretical modelling.
Researchers note that WASP-121 b’s atmosphere remains an active subject of investigation, and whether the morning-evening asymmetry observed in the June 2026 data is stable across different orbital epochs is an open question.
WASP-121 b has served as a benchmark for exoplanet atmospheric physics since its detailed characterisation began around 2016. The Gapp et al. study demonstrates that JWST’s NIRSpec can resolve sub-planetary-scale atmospheric variation during transit - without requiring direct imaging, which is not currently feasible for gas giants at stellar distances of this kind.
The method is, in principle, applicable to other transiting exoplanets in JWST’s observation queue. The confirmed asymmetry does not directly address the question of habitability - WASP-121 b is far too hot and close to its star for that - but it validates the observational toolkit needed to characterise the atmospheric structures of smaller, potentially rocky worlds where the same fundamental circulation dynamics, at far lower energies, are also expected to apply.
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