Exoplanet Caught Shedding Its Atmosphere in Real Time 

Using the James Webb Space Telescope (JWST), a team led by McGill researcher Vigneshwaran Krishnamurthy has observed a giant cloud of helium gas evaporating from a distant giant exoplanet, called WASP-107b.

Vigneshwaran Krishnamurthy, the researcher behind the study.

Published on December 1, 2025 in Nature Astronomy, the findings represent a first: while helium escape has been observed before, this is the first time the JWST has witnessed a planet’s atmosphere evaporating so extensively that the escaping gas, stretching nearly 10 times the planet’s radius, precedes the planet along its path.

“The James Webb Space Telescope has captured helium escape from this planet for the first time, and it is the most confident detection of pre-transit helium absorption for any exoplanet,” said Krishnamurthy, a postdoctoral researcher at McGill’s Trottier Space Institute, member of the Trottier Institute for Research on Exoplanets (IREx), and lead author of the study.

This discovery sheds new light on how planetary atmospheres evolve, especially for giant planets like WASP-107b, which likely formed farther from their host stars and later migrated inward, where intense heat can strip away the gaseous envelopes.

A sprawling helium cloud

The researchers, based at McGill University, Université de Genève, the University of Chicago and Université de Montréal, exploited the powerful Canadian instrument of the James Webb Space Telescope (JWST), the Near Infrared Imager and Slitless Spectrograph (NIRISS).

Their target, WASP-107b, is seven times closer to its star than Mercury is to the Sun. Despite having a diameter 94% that of Jupiter, WASP-107b has just 12% of Jupiter’s mass, placing it among the so-called ‘super-puffs’, a category of exoplanets with extremely low densities. 

Detected in 2017, the planet has been extensively studied, notably in a 2021 program led by Caroline Piaulet-Ghorayeb, an IREx alumnus who was then a PhD student at Université de Montréal. 

With NIRISS, the team detected a sprawling helium cloud, known as an exosphere, that stretches nearly 10 times the radius of the planet and passes in front of the star 1.5 hours before the planet’s passage, or ‘transit,’ begins.

“Our atmospheric escape models confirm the presence of leading and trailing tails of helium streaming out of WASP-107b, both extending up to 10 times the planetary radius along the orbital motion,” says Yann Carteret, a co-author from the University of Geneva who is an atmospheric modelling expert. 

Additionally, the researchers reaffirmed that there is water on the planet, with a stronger confidence than what was previously announced, thanks to observations by the Hubble Space Telescope.

Planet migration and transformation

The detection of water and signs of chemical mixing in WASP-107b’s atmosphere provide clues about the planet’s formation and migration history. The evidence suggests that the planet formed far from its current orbit and only recently moved closer to its star, which could explain its puffy, escaping atmosphere. 

“The amount of oxygen in the atmosphere of WASP-107 b is larger than what we would expect if it formed on its current close-in orbit. The presence of another planet, WASP-107c, much farther out than WASP-107b, could have played a role in this migration,” says Caroline Piaulet-Ghorayeb, now a researcher at the University of Chicago, who modelled the NIRISS transmission spectrum. 

Further, in such a puffy planet, the exquisite sensitivity of NIRISS should have enabled researchers to easily detect not only water, but also methane. “The fact that we do not detect methane supports the conclusion that hotter, methane-poor gas from the deeper envelope is brought up to the upper atmosphere we observe, which indicates vigorous vertical mixing”, says Piaulet-Ghorayeb. 

Overall, these new data from the Webb Telescope provide a detailed picture of how WASP-107b’s atmosphere is shaped by its environment. This study serves as a valuable reference for better understanding the evolution and dynamics of these distant worlds and highlights Webb’s unique potential for observing atmospheric escape processes.

Press release written by Marie-Eve Naud

About the study

Continuous helium absorption from the leading and trailing tails of WASP-107 b, by Vigneshwaran Krishamurthy et al., was published in Nature Astronomy. In addition to Vigneshwaran Krishamurthy (McGill, TSI, IREx), Yann Carteret (Université de Genève) and Caroline Piaulet-Ghorayeb (University of Chicago, UdeM, IREx), the team includes Jared Splinter, Dhvani Doshi, Michael Radica, Louis-Philippe Coulombe, Romain Allart, Nicolas B. Cowan, David Lafrenière, Loïc Albert, Lisa Dang, Stefan Pelletier, Jason F. Rowe, Pierre-Alexis Roy of IREx, and seven co-authors from Switzerland, the United States, Canada and the UK.

Scientific Contact

Vigneshwaran Krishnamurthy
Researcher
Trottier Institute for Research on Exoplanets (IREx), McGill University
vigneshwaran.krishnamurthy@mcgill.ca

Media Contact

Nathalie Ouellette
JWST Outreach Scientist
Trottier Institute for Research on Exoplanets, Université de Montréal
613-531-1762
nathalie@astro.umontreal.ca

Useful Links

• Scientific article – Nature Astronomy (official publication)
• Scientific article – Open access version
• McGill’s press release – English, French
• University of Geneva press release