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Thanks to the space telescope, a team of astronomers detected with unprecedented precision two helium tails escaping from the exoplanet WASP-121b.
Astronomers from the University of Geneva (UNIGE), the National Centre of Competence in Research PlanetS, and the Trottier Institute for Research on Exoplanets (IREx) at the University of Montreal (UdeM) have made a striking discovery using the James Webb Space Telescope (JWST). For the very first time, scientists have continuously monitored the atmosphere escaping from an exoplanet throughout a complete orbit. The result: the gas giant WASP-121b is surrounded not by one, but by two immense helium tails extending over more than half of its orbit around its star. These observations, combined with numerical models developed at UNIGE, provide the most detailed portrait ever obtained of the atmospheric escape phenomenon, a process capable of profoundly transforming a planet over time. The results are published in Nature Communications.
A member of the ultra-hot Jupiter family, WASP-121b is a massive gas giant that orbits so close to its star that its revolution lasts only 30 hours. The star's intense radiation heats its atmosphere to several thousand degrees, allowing light gases like hydrogen and helium to escape into space. Over millions of years, this slow atmospheric escape can alter the planet's size, composition, and future evolution.
Until now, scientists had only obtained brief glimpses of these atmospheric flows during planetary transits—those few hours when the planet passes in front of its star. Without continuous monitoring, it was impossible to know how far these flows extended or how they changed.
Using the Near-Infrared Spectrograph (NIRISS) on the James Webb Space Telescope, scientists observed WASP-121b for nearly 37 consecutive hours, covering more than one complete orbit. This is the most comprehensive continuous observation ever made of the presence of helium on a planet.
We were incredibly surprised to see how long the helium escape lasted.
Two huge tails of gas
By tracking the absorption of helium atoms in the infrared, scientists discovered that the gas surrounding WASP-121b extends well beyond the planet itself. The signal persists for more than half of its orbit, making it the longest continuous detection of atmospheric escape ever observed.
Even more remarkable: the helium particles form two distinct tails. A trailing tail, pushed back by radiation and the stellar wind, and a leading tail, curved in front of the planet, likely pulled toward the star by its gravity. Together, these two flows cover a distance equivalent to more than 100 times the planet's diameter, or more than three times the distance between the planet and its star.
"We were incredibly surprised to see how long the helium escape lasted," explains Romain Allart, a postdoctoral researcher at the University of Montreal, former doctoral student at the University of Geneva, and lead author of the paper. "This discovery reveals the complexity of the physical processes that sculpt exoplanetary atmospheres and their interaction with their stellar environment. We are only beginning to discover the true complexity of these worlds."
The digital models of the University of Geneva
The Department of Astronomy at the University of Geneva (UNIGE) is at the forefront of atmospheric escape research. The numerical models developed there, for example, enabled the interpretation of the first helium observations with the JWST. These models can explain simple, comet-shaped tails, but they struggle to reproduce the double structure observed on WASP-121b. "This discovery indicates that the structure of these flows results from both gravity and stellar winds, making a new generation of 3D simulations essential for analyzing their physics," explains Yann Carteret, a doctoral student in the Department of Astronomy at the Faculty of Science of UNIGE and co-author of the study.
The next steps for WASP-121b and beyond
Helium has become one of the most powerful tracers of atmospheric escape, and the JWST's unique sensitivity now allows it to be detected over unprecedented distances and durations. Future JWST observations will be crucial in determining whether the double-tailed structure observed around WASP-121b is unique or common among hot exoplanets. Scientists also need to refine their theories to better understand this structure.
"Very often, new observations reveal the limitations of our numerical models and push us to explore new physical mechanisms to further our understanding of these distant worlds," concludes Vincent Bourrier, lecturer and researcher in the Department of Astronomy at the Faculty of Science of the University of Geneva and co-author of the study.
