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JWST Catches a 'Super-Puff' Planet Actively Losing Its Atmosphere — A Helium Cloud Leads the Way

JWST's NIRISS instrument detected helium streaming from the low-density "super-puff" exoplanet WASP-107b, forming an exosphere nearly 10 times the planet's radius that crosses the star about 1.5 hours before the planet's transit. The team also found water vapor but no methane, indicating strong vertical mixing. These observations support the idea that WASP-107b formed farther out and migrated inward, where intense stellar heating is now driving substantial atmospheric escape. The findings were published Dec. 1 in Nature Astronomy.

12:30 AM, 12/04/2025Science
JWST Catches a 'Super-Puff' Planet Actively Losing Its Atmosphere — A Helium Cloud Leads the Way

A team of astronomers using the James Webb Space Telescope (JWST) has directly observed a distant, low-density "super-puff" exoplanet shedding its atmosphere in real time. The evaporating gas forms an enormous helium cloud that travels ahead of the planet WASP-107b as it orbits its star, producing a faint dimming of the starlight well before the planet itself transits.

The observations were made with JWST's Near Infrared Imager and Slitless Spectrograph (NIRISS), which detected the infrared signature of helium stretching to nearly 10 times the planet's radius. Remarkably, this helium exosphere passes in front of the star about 1.5 hours before the planet begins its transit — a phenomenon known as pre-transit helium absorption.

What the detection tells us

WASP-107b is classed as a "super-puff": its radius approaches Jupiter's while its mass is only a fraction of Jupiter's, giving it an extremely low density. The planet orbits very close to its star — roughly seven times closer than Mercury is to the Sun — exposing it to intense stellar radiation that drives atmospheric escape. The new JWST data provide a detailed look at how such irradiation can peel away a giant planet's outer layers over time.

"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 Vigneshwaran Krishnamurthy (McGill University's Trottier Space Institute), lead author of the study.

Chemistry and planetary history

In addition to helium, the team detected water vapor high in the planet's atmosphere but found no methane. This chemical combination suggests vigorous vertical mixing that brings hotter, methane-poor gas upward. Coupled with the extreme atmospheric loss, the composition supports a scenario in which WASP-107b formed farther from its star and migrated inward, where heating began stripping away its outer layers.

"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," said Caroline Piaulet-Ghorayeb, co-author who modeled the NIRISS transmission spectrum. She added that a more distant companion, WASP-107c, may have influenced the planet's inward migration.

Watching a planet actively lose its atmosphere gives astronomers a real-time window into planetary evolution and the processes that can transform gas giants into smaller, stripped cores. The full results were published Dec. 1 in the journal Nature Astronomy.

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