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 major breakthrough using the James Webb Space Telescope (JWST). For the first time, researchers have followed gas escaping from an exoplanet’s atmosphere continuously over a full orbit around its star.
The observations revealed an unexpected and dramatic result. The gas giant WASP-121b is surrounded by not one, but two enormous streams of helium that stretch across more than half of its orbit. When combined with advanced computer models developed at UNIGE, the data offer the most detailed look yet at atmospheric escape, a powerful process that can reshape a planet over long periods of time. The findings are published in Nature Communications.
An Ultra Hot Jupiter Under Extreme Conditions
WASP-121b belongs to a class of planets known as ultra hot Jupiters. These massive gas giants orbit extremely close to their stars, and WASP-121b completes a full revolution in just 30 hours. Because of its proximity, intense radiation from the star heats the planet’s atmosphere to temperatures of several thousand degrees.
At such extreme heat, lightweight elements like hydrogen and helium can break free and drift into space. Over millions of years, this steady loss of atmospheric material may significantly change the planet’s size, composition, and long term evolution.
Why Continuous Observation Matters
Until now, astronomers could only study atmospheric escape during short planetary transits — the brief moments when a planet passes in front of its star from Earth’s perspective. These snapshots lasted only a few hours and provided limited information.
Without uninterrupted monitoring, scientists could not determine how far the escaping gas extended or how its structure changed over time.
A Full Orbit Tracked by James Webb
Using the Near-Infrared Spectrograph (NIRISS) aboard the James Webb Space Telescope, the research team observed WASP-121b for nearly 37 hours straight. This window covered more than one complete orbit, making it the most extensive continuous detection of helium ever recorded around a planet.
This prolonged observation allowed scientists to track atmospheric escape with unmatched detail and precision.
Two Massive Helium Tails Discovered
By measuring how helium absorbs infrared light, researchers found that gas around WASP-121b spreads far beyond the planet itself. The helium signal remains visible for more than half of the planet’s orbit, marking the longest continuous observation of atmospheric escape to date.
Even more striking, the helium does not form a single stream. Instead, it splits into two distinct tails. One trails behind the planet, pushed away by stellar radiation and winds. The other curves ahead of the planet, likely drawn forward by the star’s gravitational pull. Together, these gas flows stretch across a distance greater 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.”
Modeling the Limits of Current Theories
The Department of Astronomy at the University of Geneva (UNIGE) has long been a leader in the study of atmospheric escape. Numerical models developed there played a key role in interpreting the first helium detections made by the JWST.
While these models successfully describe simple, comet-like gas tails, they struggle to recreate the double-tailed structure observed around 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.
What Comes Next for Exoplanet Research
Helium has become one of the most effective tools for tracking atmospheric escape, and the JWST’s sensitivity now allows scientists to detect it over unprecedented distances and time spans. Future observations will help determine whether the twin-tail structure seen around WASP-121b is rare or common among hot exoplanets.
Researchers will also need to refine their theoretical models to better explain how gravity, radiation, and stellar winds interact to shape these escaping atmospheres.
“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.
