Atmospheric escape describes the loss of gas from a planet's upper atmosphere into space and operates on Earth at a rate of just over 3 kilograms per second, mainly as hydrogen. For planets that orbit very close to their stars and become strongly heated, this process can reshape their atmospheres over time.

WASP-107b, discovered in 2017, follows a tight orbit about seven times closer to its star than Mercury is to the Sun. The planet has a radius similar to Jupiter's but a mass only about one-tenth as large, giving it an extremely low density and placing it in the class of so-called super-puff exoplanets.

Using JWST's infrared capabilities, the researchers measured how the planet and its extended atmosphere absorb the light of the host star during transit. They detected a strong signal from helium in the planet's exosphere, forming giant clouds that escape the planet's gravity and surround it.

The helium absorption begins before the planet's disk passes in front of the star, showing that the escaping gas extends far ahead of the planet along its orbit. Helium is also detected after the transit, indicating a trailing component and revealing a broad, comet-like structure of the outflow.

"Our atmospheric escape models confirm the presence of helium flows, both ahead and behind the planet, extending in the direction of its orbital motion to nearly ten times the planet's radius," explains Yann Carteret, a doctoral student in the Department of Astronomy at the Faculty of Science of the University of Geneva and co-author of the study. The observations and modeling together provide a detailed picture of the geometry and scale of the escaping atmosphere.

Beyond helium, JWST spectra show signatures of water vapor as well as carbon monoxide, carbon dioxide and ammonia in WASP-107b's atmosphere. Methane is notably absent, even though JWST is sensitive enough to detect it if present in expected amounts, which points to atmospheric conditions that suppress methane relative to these other molecules.

The combination of detected species and the inflated, escaping atmosphere supports a formation scenario in which WASP-107b originally formed farther from its star in a colder region before moving inward. This migration, along with intense stellar irradiation at its current orbit, likely contributed to the planet's low density and enhanced atmospheric loss.

The team views WASP-107b as a benchmark object for understanding how atmospheric escape sculpts exoplanet populations. "Observing and modeling atmospheric escape is a major research area at the UNIGE Department of Astronomy because it is thought to be responsible for some of the characteristics observed in the exoplanet population," notes Vincent Bourrier, senior lecturer and research fellow in the Department of Astronomy at the UNIGE Faculty of Science and co-author of the study.

"On Earth, atmospheric escape is too weak to drastically influence our planet. But it would be responsible for the absence of water on our close neighbor, Venus. It is therefore essential to fully understand the mechanisms at work in this phenomenon, which could erode the atmosphere of certain rocky exoplanets," he concludes.

Research Report:Continuous helium absorption from both the leading and trailing tails of WASP-107 b

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