Using the space telescope, a team of astronomers has detected with unprecedented precision two tails of helium 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 thanks to the James Webb Space Telescope (JWST). For the very first time, scientists have continuously monitored the atmosphere escaping from an exoplanet during a complete orbit.
The result: the gas giant WASP-121b is surrounded not by one, but by two immense tails of helium extending over more than half of its journey around its star. These observations, combined with numerical models developed at UNIGE, provide the most detailed portrait ever obtained of the phenomenon of atmospheric escape, a process capable of profoundly transforming a planet over time. The results are published in
Nature Communications.
This artist's impression of the exoplanet WASP-121 b shows its impressive double tail of helium which extends for nearly 60% of its orbit around its host star.
© B. Gougeon/UdeM A member of the ultra-hot Jupiters family, WASP-121b is a huge 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 such as 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 evolved.
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 persistent the helium escape was.
Two immense tails of gas
By tracking the absorption of helium atoms in the infrared, scientists discovered that the gas surrounding WASP-121b extends far beyond the planet itself. The signal persists for more than half of the orbit, constituting 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 stellar radiation and wind, and a leading tail, curved ahead of the planet, likely pulled towards the star by its gravity. Together, these two "outflows" cover a distance equivalent to more than 100 times the planet's diameter, or more than three times the distance separating the planet from its star.
"We were incredibly surprised to see how persistent the helium escape was," explains Romain Allart, a postdoctoral researcher at the University of Montreal, former PhD student at UNIGE 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 just beginning to discover the true complexity of these worlds."
The next steps for WASP-121b and beyond
Helium has become one of the most powerful tracers of atmospheric escape, and JWST's unique sensitivity now allows it to be detected over distances and durations never before achieved. Future JWST observations will be essential to determine whether the double-tail 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 limits of our numerical models, and push us to explore new physical mechanisms to push our understanding of these distant worlds ever further," concludes Vincent Bourrier, Senior Lecturer and Researcher in the Department of Astronomy at the Faculty of Science of UNIGE and co-author of the study.