On 28 October 2025, scientists published a breakthrough in Exoplanet Atmospheric Science: the first multi-dimensional spectroscopic eclipse map of an exoplanet’s atmosphere — namely the ultra-hot giant WASP-18b.
Instead of merely measuring the average light from an exoplanet’s dayside, researchers used the James Webb Space Telescope (JWST) instrument NIRISS to observe how temperature and chemistry vary across different longitudes and altitudes on the planet.
Setting the Scene: Meet WASP-18b.
Think of WASP-18b as an extreme world: a gas giant many times more massive than Jupiter, orbiting extremely close to its star, making it blisteringly hot — with an equilibrium temperature near 2,400 K. Because of this, the planet is classed as an ultra-hot Jupiter, meaning the dayside receives intense stellar irradiation and the atmosphere is expected to undergo dramatic changes in temperature and chemistry.
The Challenge: Beyond One-Dimensional Views.
Until now, most observations of exoplanet atmospheres have given us “hemisphere-integrated” spectra — meaning, the light summed over the visible dayside of the planet. We lose the ability to tell where on the planet the light is coming from (sub-stellar point, limb regions, nightside, etc.).
The genius in this study is the use of spectroscopic eclipse mapping: during a secondary eclipse (when the planet passes behind the star), variations in ingress and egress (the moments when the planet is being covered/uncovered) carry spatial information. Coupled with multiple wavelengths, this allows researchers to reconstruct how brightness (hence temperature) varies across the planetary disk and with altitude (pressure).
The Method: Eigenspectra & Mapping the Hotspot.
The team used the NIRISS Single-Object Slitless Spectroscopy (SOSS) mode on JWST, covering wavelengths ~0.85–2.85 µm.
They applied a mapping method called Eigenspectra, which essentially builds 2D brightness maps at each wavelength (latitude/longitude), then groups regions with similar spectra, yielding distinct “zones” on the planet (for example: a central hotspot vs. a ring region).
From this, they identified three “groups”: the hotspot (closest to sub-stellar point), a ring around it (dayside limb region), and an “outer” region (nearer nightside). They focused analysis on hotspot + ring.
What They Found: Temperature & Chemistry Mapped.
🔥 Hotspot Region.
- The hotspot is ~150 K hotter than the dayside average.
- It shows a thermally inverted vertical profile (temperature increasing with decreasing pressure) in the near-IR photosphere.
- Water (H₂O) is present, but slightly lower in abundance than the dayside average — consistent with thermal dissociation (breaking up of molecules due to extreme heat).
- Signatures of optical absorbers (such as H⁻, TiO, VO) were detected at high significance (~5.1σ). These absorbers help drive the thermal inversion.
🌑 Ring Region.
- The ring region is ~400 K cooler than the hotspot region.
- The retrievals (models to infer temperature‐pressure and chemistry) for the ring region were more puzzling: the data suggested very low H₂O abundances and no strong optical absorber detection — physically unexpected. The authors caution these may be due to limitations of 1D retrieval models applied to a 2D/3D reality.
🌀 Horizontal vs Vertical Gradients.
- Surprisingly, the longitudinal (east–west) temperature gradients were weaker than many theoretical models predicted.
- The hotspot offset (distance of hottest point from the substellar point) was negligible (between –5° to +7°) across wavelengths.
- The temperatures near the limbs (day-side edges) were warmer than predicted by many general circulation models (GCMs). Two possible explanations: increased heat transport via hydrogen dissociation/recombination, or nightside clouds warming the limb region.
Why It Matters: Opening the 3D Window.
This study marks a big shift: from one-dimensional averages of exoplanet atmospheres to multidimensional maps that reveal how energy, chemistry and dynamics play out across a planet. For worlds like WASP-18b (and many others), this means:
- Better constraints on atmospheric dynamics: how winds, jets and magnetic drag shape temperature distribution.
- Clues to chemical transitions: like dissociation of molecules at high temperatures, and how that affects observed abundances.
- Pathways to compare observed structure vs theoretical models (GCMs), refining our understanding of exoplanet climate.
- A template for future observations: JWST and future telescopes can apply this approach across a variety of planets (cooler hot Jupiters, smaller planets) and build a comparative atlas of exoplanet atmospheres. From the paper: “Similar future analyses will reveal the three-dimensional thermal, chemical and dynamical properties of a broad range of exoplanet atmospheres.”
Imagine standing on the dayside of WASP-18b: at the substellar point you’re baking under ~3000 K, molecules tear apart, optical absorbers trap heat causing a thermal inversion, the ‘hotspot’ glows fiercely. Move sideways toward the limb and you find yourself in a cooler “ring” (~2500 K), where the chemistry looks very different. The heat doesn’t just radiate evenly — the planet’s atmospheric behavior is rich, complex, and now we’re finally starting to map it.