Until now, exoplanets with extremely hot atmospheres have been portrayed as having circular shapes. However, new research suggests that their atmospheres are not perfectly symmetrical spheres but instead have irregular, distorted contours.
The study, jointly led by Cornell University in the US and the University of Bristol in the UK, marks the first use of an innovative technique known as ‘transmission strings’. Developed at Bristol by astrophysicists Dr David Grant and Dr Hannah Wakeford, the method uses observations from JWST, the largest space telescope, to reveal the uneven outlines of an exoplanet’s atmosphere for the first time.
Published in the journal Nature Astronomy, the findings indicate that the atmosphere is broadly circular but asymmetrical, with one side extending farther than the other. These variations reveal how extreme temperatures and powerful winds shape the atmosphere of WASP-17b, a ‘hot Jupiter’ exoplanet located approximately 1,300 light-years from Earth.
Lead author Dr Lili Alderson, a Klarman Postdoctoral Fellow at Cornell University, embarked on the exoplanetary investigation at the University of Bristol, where she gained her doctoral degree in Physics.
Dr 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 the equator to the poles and between permanent day and night sides, are consistent with theoretical predictions and other modelling 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 Dr Hannah 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.”
How the Transmission Strings Technique Reveals Exoplanet Atmospheres
Previous research has used JWST’s game-changing 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 modelling 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 centre to the edge of the planet,” Dr 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 modelling 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 towards the permanently shadowed night side, inflating the atmosphere relative to the equator’s cooler “morning” side. Transit mapping verified other characteristics of WASP-17b.
Dr 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.”
New Research Opens the Way to Mapping Weather Patterns on Distant Planets
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 towards mapping weather-scale patterns in exoplanet climates.
Dr 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 analysed data collected by the JWST Telescope Scientist Deep Reconnaissance of Exoplanet Atmospheres through Multi-instrument Spectroscopy (TST DREAMS) Guaranteed Time Observations programme. It was funded by NASA, the UK Research and Innovation (UKRI) Science and Technology Facilities Council (STFC), and the Klarman Fellowship.
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