Astronomers using the James Webb Space Telescope have observed the ultra-hot gas giant WASP-121b losing its atmosphere over a full orbit, revealing two enormous helium tails extending across more than half its path around its star. This marks the first continuous tracking of such atmospheric escape, providing unprecedented details on the process. The findings, published in Nature Communications, highlight the complexity of exoplanetary environments.
The James Webb Space Telescope (JWST) has provided astronomers with the most detailed view yet of an exoplanet shedding its atmosphere. Researchers 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) monitored WASP-121b, an ultra-hot Jupiter, for nearly 37 hours. This duration covered more than one complete orbit, which the planet completes every 30 hours due to its close proximity to its star.
WASP-121b experiences extreme conditions, with its atmosphere heated to several thousand degrees by intense stellar radiation. This causes lightweight elements like helium to escape into space, potentially altering the planet's size, composition, and evolution over millions of years. Using the Near-Infrared Spectrograph (NIRISS) on JWST, the team detected helium absorption in infrared light, showing the gas extending far beyond the planet.
The observations uncovered two distinct helium streams: one trailing behind the planet, propelled by stellar radiation and winds, and another curving ahead, likely pulled by the star's gravity. These tails span more than half the orbit, exceeding 100 times the planet's diameter and three times the distance to its star—the longest continuous detection of atmospheric escape recorded.
"We were incredibly surprised to see how long the helium escape lasted," said Romain Allart, a postdoctoral researcher at the University of Montreal and lead author. "This discovery reveals the complexity of the physical processes that sculpt exoplanetary atmospheres and their interaction with their stellar environment."
Advanced models from UNIGE helped interpret the data, but they struggled to replicate the double-tailed structure. "This indicates that the structure of these flows results from both gravity and stellar winds, making a new generation of 3D simulations essential," noted co-author Yann Carteret, a doctoral student at UNIGE.
The study challenges existing theories and underscores helium's value in studying atmospheric escape. Future JWST observations could determine if such twin tails are common among hot exoplanets. As Vincent Bourrier, a lecturer at UNIGE, concluded, "Very often, new observations reveal the limitations of our numerical models and push us to explore new physical mechanisms." The research appears in Nature Communications (2025; 16(1)), with DOI: 10.1038/s41467-025-66628-5.
