An international scientific team led by experts from the Université de Genève (UNIGE) and with the collaboration of other institutions from the NCCR PlanetS network published results on December 1, 2025, marking a scientific breakthrough in the study of exoplanets. Using the James Webb Space Telescope (JWST), huge clouds of helium escaping from the atmosphere of the exoplanet WASP-107b have been registered — a phenomenon not previously recorded with such precision and scope.

WASP-107b, discovered in 2017, cruises around a K-type star at a distance of about 210 light-years in the constellation Virgo. Although its size is comparable to Jupiter (radius about 0.96 Jupiter), its mass is only about 9–12% of Jupiter — classifying it in the rare class of so-called "super-puff" exoplanets, with extremely rarefied atmospheres and low density. Its orbit is extremely close to the star — about seven times closer than Mercury is to the Sun — which, along with strong stellar radiation, makes the atmosphere extremely vulnerable to gas loss.

First view of atmospheric leakage in real time

The team of astronomers used the NIRISS-SOSS instrument of the JWST telescope for spectroscopic measurement of the transit of WASP-107b in front of its star. What was recorded — pre-transit absorption of helium about 1.5 hours before the planet's actual entry into the transit disk — testifies that helium gas flows not only behind the planet but also in front of it, along the orbit. The strongest helium absorption was about 2.4% (with incredible statistical solidity of 36σ), while detection in the pre- and post-transit phase was at the level of 17σ. This confirms the continuous and strong escape of the atmosphere in the form of a huge, rarefied "envelope" or exosphere, stretching for tens of planetary radii.

Geometry of atmospheric escape: tail and leading cloud

Models of atmospheric behavior, developed at UNIGE, suggest that the helium mass is not limited only to the tail behind the planet but also shapes a leading cloud — giving the system an almost cometary appearance. This gas continuously absorbs starlight and makes brightness changes visible even before the planet enters transit, representing the most convincing example of "atmospheric escape" recorded to date in exoplanets.

In addition to helium, JWST also registered traces of water vapor (H₂O), carbon monoxide (CO), carbon dioxide (CO₂) and ammonia (NH₃). Interestingly, for WASP-107b — despite the sensitivity of the instruments — methane (CH₄) was not detected, which sheds light on the unusual chemical structure of the atmosphere and points to strong mixing processes and chemical disequilibrium.

Implications for planet origin and evolution

All these characteristics — large size with low mass, expanded atmosphere, appearance of helium, water and molecules, without the presence of methane — support the scenario according to which WASP-107b did not originate in its current orbit. It likely formed much further from the star, then migrated close to it, and due to intense radiation gained a puffed-up atmosphere which it is now losing into space.

This is key to understanding the evolution of exoplanets: it shows that atmospheric losses — especially in closely orbiting gaseous and semi-gaseous worlds — can dramatically change their structure and chemical composition over time. Studying such systems gives valuable insight into the expansion and disappearance of atmospheres, but also into the migrations of planets within their systems.

Why the discovery is revolutionary

Previously, helium was identified in the atmosphere of WASP-107b using the Hubble telescope (2018), and subsequent studies pointed to extended tails of gases in the exosphere. But with new JWST measurements, astronomers have obtained the first extremely detailed and comprehensive "photographic" view of atmospheric leakage — in real time and with spectroscopic precision. This represents a turning point in the ways we can monitor the evolution of the outer atmospheres of exoplanets.

Experts point out that such a process of atmospheric rarefaction, although slow for a planet like Earth — where only a few kilograms of gas escape per second — for super-puff exoplanets can mean the complete evaporation of the atmosphere over millions or billions of years, which can fundamentally change the fate of such planets.

Therefore, WASP-107b becomes a new benchmark for understanding how stellar radiation and gravitational weakness can shape the fates of exoplanets, influencing their mass, atmospheric composition and long-term stability.