A distant exoplanet is leaking not one, but two spectacular helium tails, and the James Webb Space Telescope (JWST) has captured the phenomenon in unprecedented detail. This discovery marks the first time scientists have tracked atmospheric escape from an alien world across an entire orbit, offering the most complete look yet at how exoplanet atmospheres interact with their fierce stellar environments.
The planet in question is WASP-121b, nicknamed “Tylos,” a so-called ultrahot Jupiter roughly 858 light-years from Earth. This gas giant circles its Sun-like star in about 30 hours, enduring extreme heating that drives its atmospheric gases to blistering temperatures around 2,300°C (4,200°F).
Lead researcher Romain Allart of the University of Montreal expressed astonishment at the longevity of the helium outflow: the escape persisted for more than half of the planet’s orbit. “We were incredibly surprised to see how long the helium escape lasted,” he noted. The finding highlights how the physics governing distant atmospheres are more intricate than previously realized, and it suggests we’re only beginning to grasp the full complexity of these worlds.
Two tails, one mystery
Helium serves as a crucial tracer for tracking atmospheric loss from exoplanets, and JWST’s superb sensitivity makes this element detectable even across vast interstellar distances. By analyzing the light absorbed by helium atoms, the team mapped a gas envelope that extends far beyond WASP-121b itself. The helium signature endured for more than half of the planet’s orbit, making it the longest continuous detection of atmospheric escape recorded to date.
The most striking aspect of the study is that the escaping helium splits into two distinct tails. One tail is pushed backward by the combined effects of stellar radiation and winds, trailing behind the planet as it moves along its orbit. The other tail appears to surge forward, leading the planet and seemingly drawn toward the star by its gravity.
In total, the two tails span a distance roughly 100 times the planet’s own diameter and exceed three times the spacing between WASP-121b and its star. This dual-tail configuration defies current modeling and invites fresh questions about the mechanisms shaping such outflows.
As Vincent Bourrier of the University of Geneva’s Department of Astronomy put it, new observations often reveal gaps in our numerical models and drive the exploration of new physics needed to understand these distant worlds.
The research, published December 8 in Nature Communications, adds a vivid chapter to the story of how planets lose atmospheres under the intense influence of their stars.
If you’re curious about what this means for other close-in exoplanets, or how scientists reframe models when observations challenge expectations, the discussion is only beginning.
About the author: Rob Leairs is a science journalist based in the U.K. His work appears in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek, and ZME Science. He also covers science communication for Elsevier and the European Journal of Physics. Follow him on Twitter @sciencef1rst.
