A 'super-puff' exoplanet is losing its atmosphere, and the James Webb Space Telescope had a look

Astronomers have spotted a distant world "shedding" its atmosphere into space in real time, creating a giant companion cloud of gas that travels ahead of the planet as it orbits its star.

Observations from the James Webb Space Telescope (JWST) have revealed an unprecedented view of helium gas evaporating from a distant, giant exoplanet called WASP-107b. The escaping gas stretches across a distance that's nearly 10 times the planet's radius and in fact precedes the planet along its path around its parent star. This discovery reveals new clues about how the atmospheres of giant planets like WASP-107b can slowly evaporate under intense radiation from their host star, according to a statement.

"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," Vigneshwaran Krishnamurthy, lead author of the study from McGill's Trottier Space Institute, said in the statement.

WASP-107b is classified as a "super-puff" exoplanet with a radius nearly as large as Jupiter's but only a fraction of its mass. Residing seven times closer to its star than Mercury is to the sun, WASP-107b's low density makes it especially vulnerable to atmospheric escape. The helium exosphere produced by the evaporated gas passes in front of the star 1.5 hours before WASP-107b's passage, or transit, begins — a phenomenon called pre-transit helium absorption, according to the statement.

Using the JWST's Near Infrared Imager and Slitless Spectrograph (NIRISS), the team detected the helium's infrared signature streaming ahead of the planet, effectively creating its own mini-transit, which is observed as a slight dimming of the star's brightness before the planet itself transits. While helium escape has been observed on other exoplanets, WASP-107b marks the first time astronomers have watched an exoplanet shedding its atmosphere in such real time and with such detail.

The findings also reveal clues about the planet's history. Alongside the helium, the JWST detected water vapor high in the atmosphere but no methane, suggesting vigorous mixing that brings hotter, methane-poor gas upward. This chemical pattern, combined with the extreme atmospheric escape, supports the idea that WASP-107b likely formed farther from its star and migrated inward, where intense heating began peeling away its outer layers, according to the statement.

"The amount of oxygen in the atmosphere of WASP-107b is larger than what we would expect if it formed on its current close-in orbit," Caroline Piaulet-Ghorayeb, co-author of the study, who modelled the NIRISS transmission spectrum, said in the statement. "The presence of another planet, WASP-107c, much farther out than WASP-107b, could have played a role in this migration."

Watching a planet actively losing its atmosphere provides insight into how worlds change over time and how some may be stripped down to a bare rocky or icy core.

Their findings were published Dec. 1 in the journal Nature Astronomy.