Trailblazing technique uncovers real shape of scorching-hot exoplanet atmospheres

Until now, the alien white-hot worlds of exoplanets have been depicted as circular, but new research has revealed the true shape of their atmospheres to be a deformed sphere rather than a perfect circle.

The study, co-led by Cornell University in the U.S. and the University of Bristol in the U.K., presents the first application of a method called “transmission strings,” developed at Bristol by astrophysicists Dr. David Grant and Dr. Hannah Wakeford, which unravels for the first time an exoplanet’s uneven contours using insights from the largest telescope in space, JWST.

Its results, published in the journal Nature Astronomy, show the atmosphere is circular, but also asymmetrical with one more extended side. Such dimensions expose how intense heat and whipping winds influence the atmosphere of the “hot Jupiter” WASP-17b exoplanet, orbiting a star some 1,300 light-years from planet Earth.

Lead author Dr. Lili Alderson, a Klarman postdoctoral fellow at Cornell University, embarked on the extraplanetary investigation at the University of Bristol, where she gained her doctoral degree in physics.

Building a three-dimensional atmospheric picture

Alderson said, “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. To move beyond computational exercises and see it for real is a different matter.”

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. Together they 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.

Co-author Wakeford, associate professor in astrophysics at the University of Bristol, said, “Planets are temperamental, their atmospheres complex and chaotic. 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.”

Tracing heat around the planet’s edge

Previous research has used JWST’s sensitivity to pioneer a technique called eclipse mapping, which measures changes in light as an exoplanet swings behind its host star. The new ‘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 edge of the planet’s atmosphere, or limb, from different angles. By contrast, traditional modeling extracts a spectrum only as an average across the entire limb.

“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,” Alderson said. “We’re able to go around the edge. So, we’ve gone from a ring to understanding where on the ring the gemstones are, so to speak.”

The researchers revealed a “gem” where theory and previous modeling indicated one should be on WASP-17b, on the “evening” side of its equator. There, jets of wind transport heat from the planet’s permanently illuminated day side around toward the permanently shadowed night side, inflating the atmosphere relative to the equator’s cooler “morning” side. Transit mapping verified other characteristics of WASP-17b.

Alderson added, “Theory predicts that the morning and the evening sides should have a strong temperature difference, and WASP-17b fits really nicely in that trend—pretty much bang-on what we thought it would be.”

Toward weather maps of distant worlds

This successful demonstration of transmission strings on WASP-17b paves the way for mapping many more candidates. The process is starting to differentiate conditions across exoplanet latitudes, beyond just day and night sides, and providing a first step toward mapping weather-scale patterns in exoplanet climates.

Wakeford said, “Having this very robust mathematical model that we can apply to this state-of-the-art data is incredibly exciting. The results are a really beautiful demonstration of the dynamics that are going on in these multidimensional and multifaceted worlds.”

The research analyzed data collected by the JWST Telescope Scientist Deep Reconnaissance of Exoplanet Atmospheres through Multi-instrument Spectroscopy (TST DREAMS) Guaranteed Time Observations program.

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