Webb investigates eternal sunrises and sunsets on a distant world

Researchers using the James Webb Space Telescope have finally confirmed what models had predicted so far: an exoplanet has differences between its eternal atmosphere in the morning and in the evening. WASP-39 b, a giant planet 1.3 times the diameter of Jupiter but similar in mass to Saturn, orbits a star about 700 light-years from Earth. It is therefore constantly rotating around its star. It therefore has a constant dayside and nightside: one side of the planet is always exposed to its star, while the other is always plunged into darkness.

Using Webb’s Near Infrared Spectrograph (NIRSpec), astronomers confirmed a temperature difference between eternal morning and eternal evening on WASP-39 b, with the evening appearing about 300 degrees Fahrenheit (about 200 degrees Celsius) warmer. They also found evidence of different cloud cover, with the part of the planet where eternal morning occurs likely being cloudier than the part where evening occurs.

Image A: WASP-39 b concept art

The astronomers analyzed WASP-39 b’s 2- to 5-micron transmission spectrum, a technique that allows them to study the exoplanet’s terminator, the boundary that separates the planet’s day and night sides. A transmission spectrum is constructed by comparing starlight filtered by a planet’s atmosphere as it moves in front of the star to unfiltered starlight detected when the planet is next to the star. By making this comparison, researchers can obtain information about the temperature, composition, and other properties of the planet’s atmosphere.

“WASP-39 b has become a kind of reference planet in the study of exoplanet atmospheres along with Webb,” said Néstor Espinoza, an exoplanet researcher at the Space Telescope Science Institute and lead author of the study. “It has a puffy, bloated atmosphere, so the signal from starlight filtered through the planet’s atmosphere is quite strong.”

Previously published Webb spectra of WASP-39b’s atmosphere, which revealed the presence of carbon dioxide, sulfur dioxide, water vapor, and sodium, represent the entire day/night boundary. No detailed attempt has been made to differentiate one side from the other.

The new analysis now constructs two different spectra from the terminator region, essentially splitting the day/night boundary into two semicircles, one for the evening and one for the morning. The data reveal that the evening is significantly warmer, with a scorching 800 degrees Celsius, and the morning is relatively cooler, with 600 degrees Celsius.

Image B: Transmission spectra

“It’s really amazing that we’re able to analyze this small difference, and it’s only possible because of Webb’s sensitivity in near-infrared wavelengths and its extremely stable photometric sensors,” Espinoza said. “Any movement of the instrument or the observatory while the data were being collected would have severely limited our ability to make this detection. It has to be extraordinarily precise, and Webb is exactly that.”

Extensive modeling of the resulting data also allows the researchers to study the structure of WASP-39 b’s atmosphere, cloud cover, and why the evening is warmer. While the team’s future work will investigate how cloud cover can affect temperature, and vice versa, the astronomers confirmed that the circulation of gas around the planet is the main culprit for the temperature difference on WASP-39 b.

On a highly irradiated exoplanet like WASP-39 b that orbits relatively close to its star, researchers typically expect gas to move as the planet rotates around its star: hotter gas on the dayside should move to the nightside in the evening via a powerful equatorial jet stream. Because the temperature difference is so extreme, the difference in atmospheric pressure would also be significant, which in turn would lead to strong winds.

Image C: Transit light curve

Using general circulation models, three-dimensional models similar to those used to predict weather on Earth, the researchers found that on WASP-39 b, the prevailing winds likely move from the night side through the morning terminator, around the day side, through the evening terminator, and then around the night side.

As a result, the morning side of the terminator is colder than the evening side. In other words, the morning side is hit by winds of air that has been cooled from the night side, while the evening side is hit by winds of air that has been heated from the day side. Research suggests that wind speeds on WASP-39 b can reach thousands of kilometers per hour.

“This analysis is also particularly interesting because it allows us to obtain 3D information about the planet that we did not have before,” Espinoza added. “Since we can say that the evening edge is warmer, that means it is a little bit more swollen. So, theoretically, there is a little swell at the terminator that is approaching the night side of the planet.”

The team’s results were published in Nature.

The researchers will now seek to use the same analysis method to study atmospheric differences on other tidally locked hot Jupiters, as part of the Webb Cycle 2 General Observers 3969 program.

WASP-39 was one of the first targets analyzed by Webb as it began regular science operations in 2022. Data for this study were collected as part of the Early Release Science 1366 program, designed to help scientists quickly learn how to use the telescope’s instruments and exploit its full scientific potential.