Exoplanet atmospheres - the atmospheres of worlds beyond the Solar System - have until now been depicted as smooth circles. But a study published in the journal Nature Astronomy has shown that the scorching gas giant WASP-17b has an atmosphere that, while round overall, is asymmetric - on one side it is more elongated. The discovery helps to understand how extreme heat and strong winds shape the appearance of such planets.
WASP-17b belongs to the class of "hot Jupiters" - gas giants that orbit very close to their star and are therefore strongly heated. The planet is located about 1,300 light-years from Earth. One of its sides always faces the star, while the other is in eternal shadow.
The study was carried out by a team led by Dr Lily Alderson, a postdoctoral researcher at Cornell University (USA). The work was done in collaboration with the University of Bristol (UK). The scientists were the first to apply the "transit strings" method, developed in Bristol by astrophysicists David Grant and Hannah Wakeford. The method uses data from the James Webb Space Telescope (JWST) - the largest observatory in orbit.
Unlike the traditional approach, which averages the signal across the entire edge of the atmosphere, the new method tracks the planet as it passes in front of the star and captures the composition of the light at different angles. This makes it possible to measure the distance from the planet's center to the edge of the atmosphere at different points, rather than assuming it is the same.
"It is as if we have gone from a ring to understanding where on that ring the gemstones are located," Alderson explained.
One such "stone" was found exactly where theory predicted: on the "evening" side of the equator. There, wind flows carry heat from the permanently illuminated day side to the night side, causing the atmosphere to swell more than on the cooler "morning" side. The observations also confirmed the predicted strong temperature difference between the morning and evening sides.
The authors emphasize that this is so far the first demonstration of the method on a single planet. The results are consistent with theoretical models and other methods of analysis, but this is a proof of concept, not ready-made "weather maps" for many worlds. Ahead lies the extension of the approach to other exoplanets, which will be a step toward mapping climate patterns on distant planets.