Mapping Weather Patterns on the Extreme World of WASP-94A b

WASP-94A b is a massive gas giant situated within a binary star system, located approximately 690 light-years from Earth. This planet orbits incredibly close to its host star, resulting in extreme thermal conditions. Crucially, the planet is tidally locked, a gravitational state in which one hemisphere permanently faces the star while the opposite side remains in perpetual darkness. In a study published in the journal Science, a research team led by Sagnick Mukherjee, an astrophysicist at Johns Hopkins University, utilized the James Webb Space Telescope (JWST) to conduct a rigorous analysis of this exoplanet’s atmosphere. The primary objective was to decipher the weather patterns on such extreme worlds, specifically to determine whether their atmospheres are static or dynamic and to investigate the presence and behavior of winds and clouds.

Astronomers typically study the atmospheres of exoplanets using a technique known as transmission spectroscopy. When a planet transits, or passes directly in front of its host star, a portion of the starlight filters through the planet’s atmospheric limb. By analyzing the spectrum of this transmitted light, scientists can identify the chemical composition of the atmosphere. However, this traditional methodology possesses a significant limitation: it averages the light collected from the entire circumference of the planet’s silhouette. This averaging process creates a homogenized view, effectively treating the atmosphere as a single, uniform sphere of gas rather than a complex, heterogeneous system.

For tidally locked planets such as WASP-94A b, this approach represents a massive oversimplification. On these worlds, the thermal disparity between the scorching day side and the freezing night side is extreme. These profound temperature gradients induce significant variations in atmospheric density. Furthermore, the planet’s slow rotation generates a Coriolis effect, leading to a phenomenon known as equatorial super-rotation. In this state, winds along the equator blow eastward at velocities exceeding the planet’s own rotational speed. Previous circulation models had predicted this specific behavior on WASP-94A b, but empirical verification remained elusive until now.

The leading edge of the planet’s disk during a transit is referred to as the morning limb. This is the boundary where the atmosphere rotates from the cold night side into the intense heat of the day side. Conversely, the trailing edge is the evening limb, where gases heated during the daytime cross over into the darkness of the night side. To observe this dynamic process in motion, Mukherjee and his colleagues employed a sophisticated technique called limb-resolved spectroscopy.

Because the planet requires a finite amount of time to fully cross the star’s disk, the telescope observes the morning limb blocking starlight slightly before the evening limb. Utilizing the Near Infrared Imager and Slitless Spectrograph (NIRISS) on the JWST, the team measured the light curves during the transit and mathematically separated the signal. This separation allowed them to extract two distinct chemical transmission spectra: one corresponding to the morning limb and another to the evening limb. The results revealed a striking divergence between the two sides.