JWST transit data show WASP-17b's atmosphere is more extended on its evening side
Cornell and Bristol researchers applied transmission strings to JWST data and found WASP-17b's atmosphere more extended on one side. The shape matches theoretical predictions, so the study is a first test of the method on one planet.
The Scientist · Science desk
What happened
- The results appear in Nature Astronomy, and the method was developed at Bristol by astrophysicists David Grant and Hannah Wakeford.
- The more extended region sits on the evening side of the equator, where wind jets carry heat from the day side toward the night side and inflate the atmosphere relative to the cooler morning side.
- WASP-17b is a hot Jupiter orbiting a star about 1,300 light-years from Earth.
- Earlier JWST work pioneered eclipse mapping, which measures changes in light as a planet swings behind its host star.
Why it matters
- capability The method begins to separate conditions by latitude as well as by day and night side, and the write-up calls this a first step toward weather-scale maps of exoplanet climates.
- precedent WASP-17b becomes a reference case: the write-up calls the findings an important benchmark for comparison with the more than 6,000 other confirmed exoplanets.
- constraint Because the planet landed where the models predicted, this first application cannot show whether the method would flag a planet whose winds or heat transport the models get wrong.
Traditional modeling extracts a spectrum only as an average across the entire limb, the edge of the planet's atmosphere [7]. Transmission strings, also called transit mapping, tracks the planet as it crosses in front of its star and captures the light around the limb from different angles [6]. Lead author Lili Alderson, a Klarman postdoctoral fellow at Cornell, said: "Rather than assuming your atmosphere is a circle with the same radius all the way around, our technique allows you to pull out different distances from the center to the edge of the planet." [13][8]
WASP-17b landed where the models expected. Alderson said "Theory predicts that the morning and the evening sides should have a strong temperature difference, and WASP-17b fits really nicely in that trend" and added that the planet was "pretty much bang-on what we thought it would be" [9]. The composition and temperature results, including shifts from equator to poles and between day and night sides, also agree with theoretical predictions and other modeling techniques [10].
Alderson's own description of the result is modest in scope: "This is proof of concept that planets are in fact 3D objects" [11]. We think that is the right reading. A method that returns a structure the models already predicted has shown it can find that structure.
The thing this account doesn't tell you is how much more extended the evening side is. The phys.org report does not include a size for the asymmetry, an uncertainty, or the number of transits behind it.
The report also says the demonstration paves the way for mapping many more candidates [16]. Co-author Hannah Wakeford said: "The fact that all of these numbers came together and showed us this very structured, dynamical picture of this giant world over 1,000 light-years away was just really exciting." [12]
What to watch
- Transmission strings applied to other hot Jupiters, to see whether they also show an extended evening side.
- The paper's own figures for the size of the asymmetry and its uncertainty, which the phys.org report does not give.
- A planet where the method returns a structure the models did not predict.
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- [1]
The study, co-led by Cornell University and the University of Bristol, presents the first application of the 'transmission strings' method to JWST data.
- [2]
The results were published in the journal Nature Astronomy, and the transmission strings method was developed at Bristol by astrophysicists Dr. David Grant and Dr. Hannah Wakeford.
- [3]
The more extended region lies on the 'evening' side of WASP-17b's equator, where jets of wind carry heat from the permanently illuminated day side toward the permanently shadowed night side, inflating the atmosphere relative to the cooler 'morning' side; theory and previous modeling indicated it should be there.
- [4]
WASP-17b is a 'hot Jupiter' exoplanet orbiting a star some 1,300 light-years from Earth.
- [5]
Previous research used JWST's sensitivity to pioneer eclipse mapping, which measures changes in light as an exoplanet swings behind its host star.
- [6]
The transmission strings method, also called transit mapping, tracks the planet as it crosses in front of the star, capturing the composition of light around the limb of the planet's atmosphere from different angles.
- [7]
Traditional modeling extracts a spectrum only as an average across the entire limb.
- [8]
"Rather than assuming your atmosphere is a circle with the same radius all the way around, our technique allows you to pull out different distances from the center to the edge of the planet."
- [9]
"Theory predicts that the morning and the evening sides should have a strong temperature difference, and WASP-17b fits really nicely in that trend" and is "pretty much bang-on what we thought it would be."
- [10]
The findings about WASP-17b's atmospheric chemical composition and temperature shifts, from equator to poles and between permanent day and night sides, are consistent with theoretical predictions and other modeling techniques.
- [11]
"This is proof of concept that planets are in fact 3D objects, and that consistently, as they go around their star, what we predict should be happening is happening."
- [12]
"The fact that all of these numbers came together and showed us this very structured, dynamical picture of this giant world over 1,000 light-years away was just really exciting."
- [13]
Lead author Dr. Lili Alderson is a Klarman postdoctoral fellow at Cornell University who began the investigation at the University of Bristol, where she gained her doctoral degree in physics.
- [14]
The findings create one of the most comprehensive 3D views of an exoplanet's atmosphere and provide an important benchmark for comparison with the more than 6,000 other exoplanets confirmed to date.
- [15]
The process is starting to differentiate conditions across exoplanet latitudes, beyond just day and night sides, and provides a first step toward mapping weather-scale patterns in exoplanet climates.
- [16]
The successful demonstration of transmission strings on WASP-17b paves the way for mapping many more candidates.
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Sources
1 independent publisher whose own reporting we read for this story.
- phys.orgTrailblazing technique uncovers real shape of scorching-hot exoplanet atmospheres
1 article · October 14, 2026
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