Webb Confirms Temperature Difference between Eternal Morning and Evening on WASP-39b

WASP-39b: A Puffy Hot Gas Giant in Virgo

WASP-39b is a hot-Saturn exoplanet orbiting a G7-type star in the constellation Virgo, about 700 light-years from Earth. Discovered in 2011, the planet has a mass similar to Saturn but is approximately 50% larger in size, making it notably inflated or “puffy.” This extreme puffiness is attributed to its high atmospheric temperature, which averages around 900 degrees Celsius (1,652 degrees Fahrenheit). The planet completes an orbit around its star every 4.1 days, placing it very close to its host star and subjecting it to intense stellar radiation.

Because of its inflated atmosphere, WASP-39b has become an important benchmark for atmospheric studies using the James Webb Space Telescope. Its large, extended atmosphere allows for strong signals when starlight passes through it, enabling detailed spectral analysis.

Breaking Down the Terminator: Morning vs. Evening Spectra

Previous observations with JWST captured the combined transmission spectrum of WASP-39b’s day-night boundary, known as the terminator, without distinguishing between the morning and evening sides. The new study, however, separates the terminator into two semicircles, allowing astronomers to analyze the morning and evening spectra independently.

The results reveal a striking temperature difference: the evening terminator is significantly hotter at about 800 degrees Celsius (1,450 degrees Fahrenheit), while the morning terminator is cooler, around 600 degrees Celsius (1,150 degrees Fahrenheit). This 177 K (approximately 320°F) difference is unprecedented in exoplanet atmospheric studies and was made possible by JWST’s exceptional sensitivity and stability in the near-infrared spectrum.

Atmospheric Composition and Spectral Insights

The transmission spectra from JWST’s Near-Infrared Spectrograph (NIRSpec) show clear signatures of water vapor, carbon dioxide, sulfur dioxide, and sodium in WASP-39b’s atmosphere. These molecules absorb starlight at specific wavelengths, allowing researchers to identify their presence and estimate atmospheric temperatures.

The spectral data not only confirm the chemical makeup but also provide clues about atmospheric structure and cloud cover. The hotter evening terminator appears more inflated or “puffed up,” consistent with higher temperatures causing atmospheric expansion.

Unraveling the Cause: Winds and Atmospheric Circulation

The temperature difference between the morning and evening sides is attributed primarily to atmospheric circulation driven by the planet’s rapid orbit and intense stellar heating. WASP-39b is tidally locked, meaning one side constantly faces the star (dayside) while the other remains in darkness (nightside).

Using three-dimensional General Circulation Models (GCMs), similar to Earth’s weather prediction tools, scientists simulated wind patterns on WASP-39b. These models suggest powerful equatorial jet streams transport hot gas from the dayside across the evening terminator to the nightside. Conversely, cooler gas flows from the nightside across the morning terminator toward the dayside.

This circulation results in the evening terminator being heated by recently sunlit air, while the morning terminator is cooled by air that has spent time on the nightside. The extreme temperature and pressure gradients generate wind speeds potentially reaching thousands of kilometers per hour.

Significance of the Findings and Future Research

This study represents the first direct measurement of temperature differences between the morning and evening sides of an exoplanet’s terminator over a broad wavelength range. It provides three-dimensional insights into atmospheric dynamics that were previously inaccessible.

Understanding these temperature contrasts and wind patterns is crucial for refining models of exoplanet atmospheres, especially for hot gas giants in close orbits. The findings also highlight the importance of JWST’s precision and sensitivity in enabling such detailed characterizations.

Future research will investigate how cloud cover interacts with temperature variations and atmospheric circulation, potentially influencing the observed spectra and thermal structure. Continued observations of WASP-39b and similar exoplanets will deepen our understanding of atmospheric physics under extreme conditions.

What this means

The James Webb Space Telescope’s observations of WASP-39b have opened a new window into the complex atmospheric behavior of exoplanets orbiting close to their stars. By distinguishing temperature differences between the morning and evening terminators, astronomers have gained unprecedented insight into the planet’s dynamic weather systems and wind patterns. These findings not only improve our understanding of WASP-39b’s atmosphere but also set a precedent for future studies of exoplanet climates. As JWST continues to deliver high-precision data, our knowledge of distant worlds will become increasingly detailed, bridging the gap between theoretical models and observational evidence in the quest to understand planetary atmospheres beyond our Solar System.