Water and carbon dioxide have been discovered in the atmosphere of a super-Neptune.

Located 368 light-years from Earth, on the edge of the so-called "hot Neptune desert"—a region near stars where Neptune-sized celestial bodies are rarely found—lies the exoplanet WASP-166b. Despite its extreme conditions, this unique world has managed to retain a substantial atmosphere, in which traces of water and carbon dioxide have been detected. This discovery significantly enhances our understanding of the evolution of exoplanets in cosmic "deserts."

2025-01-12 10:00:18https://naked-science.ru/article/astronomy/v-atmosfere-superneptuna

WASP-166b orbits an F9V-type star, which is approximately 20 percent larger and more massive than the Sun. The mass of the exoplanet is 32 times that of Earth, and its size is seven times greater. This super Neptune completes a full orbit around its star in just 5.44 days, and temperatures on its surface can reach up to 1000 °C (equivalent to about 1270 K).

To understand how WASP-166b has managed to survive and retain its atmosphere in the desert of hot Neptunes (a region where planets cannot grow to the typical size of hot Jupiters), an international team of astronomers led by Andrew W. Mayo from the University of California (USA) studied the exoplanet and its parent star using the James Webb Space Telescope. The findings of their research are presented on the Cornell University preprint server.

The researchers observed the planet's transit across the star's disk using the onboard spectrographs NIRSpec and NIRISS SOSS on the James Webb. It is worth noting that when a planet passes in front of a star, some of the emitted light is "filtered" by the atmosphere, and the absorption spectral lines help to determine the chemical composition and build an accurate model of the atmosphere.

By combining the data, the authors of the new study detected traces of water (H₂O), carbon dioxide (CO₂), a weak presence of ammonia (NH₃), as well as an abundance of heavy elements in the super Neptune's atmosphere. The metallicity (astronomers refer to all elements heavier than helium as metals) of the super Neptune's atmosphere was found to be higher compared to its parent star, while the carbon to oxygen ratio (C/O) was slightly lower than that of the star's atmosphere.

This, according to astronomers, suggests a possible formation of WASP-166b beyond the "snow line" — a region where simple volatile compounds freeze and condense — and its gradual migration inward into the desert of hot Neptunes.

The scientists also identified the presence of clouds at mid-altitudes, which, combined with the intense ultraviolet radiation from the parent star and the short orbital period (5.44 days), indicate photoevaporation (when the star's radiation "blows away" the planet's atmosphere), at least at this stage of evolution.

Although WASP-166b is one of several dozen super Neptunes that astronomers are studying using the James Webb Space Telescope, the results of this new research shed light on the formation and evolution of such worlds, providing scientists with insights into the key mechanisms that lead to the formation of hot Neptune deserts.