Using the James Webb Space Telescope (JWST), scientists observed the light coming from the exoplanet NGTS-10A b for 18 hours, corresponding to a full year for this planet orbiting very close to its star.
NGTS-10A b is a "hot Jupiter". On this type of planet, one side is permanently illuminated while the other remains in perpetual night, generating temperature differences of several hundred degrees. These contrasts fuel winds that can reach several miles per second (several kilometers per second), capable of transporting gases on a global scale.
Their observations reveal that carbon monoxide (CO) dominates the chemical composition of the atmosphere on both the day side and the night side, while methane (CH₄), expected on the night side, is strongly deficient. Under normal conditions, chemistry should favor CO at high temperatures and CH₄ at lower temperatures, as is the case in the atmospheres of giant planets in the Solar System.
The lack of methane on the night side can be explained by the very rapid transport of gas between the day side and the night side. The winds are strong enough that the chemical reaction converting CO into CH₄ simply does not have time to occur, thus preventing the atmosphere from reaching equilibrium. This phenomenon, long predicted, is here confirmed for the first time in a direct manner.
This discovery highlights the key role of atmospheric dynamics in understanding exoplanets and opens new perspectives for the study of these distant worlds.