Webb investigates eternal sunrises, sunsets on distant world

Using the James Webb Space Telescope, researchers have finally confirmed what models have predicted: exoplanets have a permanent morning atmosphere and a permanent night atmosphere. WASP-39 b is a giant planet with a diameter 1.3 times that of Jupiter and a mass similar to Saturn, orbiting a star about 700 light-years from Earth and is tidally locked to its parent star. This means that it has a permanent dayside and a permanent nightside, with one side of the planet always exposed to the star and the other always in darkness.

Using Webb’s NIRSpec instrument, the astronomers confirmed that there is a temperature difference between WASP-39 b’s eternal morning and eternal evening, with the evening appearing to be roughly 300 degrees Fahrenheit (about 200 degrees Celsius) hotter. They also found evidence of different cloud cover, suggesting that the eternal morning parts of the planet are likely cloudier than the evenings.

Image A: Artist concept WASP-39 b

Astronomers analyzed WASP-39 b’s 2-5 micron transmission spectrum, a technique for studying exoplanet boundaries — the boundaries that separate a planet’s dayside from its nightside. A transmission spectrum is created by comparing the starlight that passes through the planet’s atmosphere as it moves in front of it with the starlight that isn’t transmitted when the planet is next to the star. This comparison allows researchers to learn information about the temperature, composition, and other properties of the planet’s atmosphere.

“WASP-39 b serves as a kind of benchmark planet for the Webb mission’s exoplanet atmospheric studies,” said Nestor Espinosa, an exoplanet researcher at the Space Telescope Science Institute and lead author of the study. “The planet’s atmosphere is puffy and fluffy, so the signal from the star’s light passing through the planet’s atmosphere is very strong.”

A web spectrum of WASP-39b’s atmosphere revealed the presence of carbon dioxide, Sulfur dioxidewater vapor, and sodium represent the entire boundary between day and night, and no detailed attempt is made to distinguish one from the other.

Now, in a new analysis, they created two different spectra from the boundary region, splitting the day/night boundary into two semicircles: one taken from the evening and one from the morning. The data shows that the evening was significantly hotter, reaching a scorching 1,450 degrees Fahrenheit (800 degrees Celsius), while the morning was a relatively cool 1,150 degrees Fahrenheit (600 degrees Celsius).

Image B: Transmission spectrum

“It’s really amazing that we were able to resolve this small difference, and this is thanks to Webb’s sensitivity to near-infrared wavelengths and its extremely stable photometric sensor,” Espinoza said. “Any movement of the instrument or the observatory while collecting the data would severely limit our ability to detect this. This requires extraordinary precision, and Webb is exactly that.”

Extensive modeling of the resulting data will also allow the researchers to investigate the structure of WASP-39 b’s atmosphere, its cloud cover, and why it’s hotter in the evenings. The team will next investigate how cloud cover affects temperature and vice versa, but the astronomers confirmed that the circulation of gas around the planet is the main cause of WASP-39 b’s temperature difference.

For highly irradiated exoplanets that orbit relatively close to their stars, like WASP-39 b, researchers typically expect gas to move as the planet rotates around its star. This means that hotter gas on the dayside should be transported from the evening to the nightside by the strong equatorial jet stream. The extreme temperature difference would lead to large pressure differences, which would result in faster wind speeds.

Image C: Transit light curve

Using a general circulation model, a 3D model similar to that used for the forecast Weather patterns On Earth, the researchers found that on WASP-39 b, the prevailing winds tend to move from the nightside, across the morning boundary, around the dayside, across the evening boundary, and then around the nightside.

As a result, the morning side of the boundary is cooler than the evening side. In other words, the morning side is hit by winds of air cooled by the night side, and the evening side is hit by winds of air heated by the day side. The study suggests that wind speeds on WASP-39 b could reach thousands of miles per hour.

“This analysis is particularly interesting because it gives us previously unavailable 3D information about the planet,” Espinoza added. “The fact that the evening edge is hotter means that it’s bulging out a bit, which means that in theory there’s a little undulation at the boundary approaching the planet’s night side.”

The team results Published in Nature.

The researchers plan to use the same analysis to study differences in the atmospheres of other tidally locked hot Jupiters in the future as part of the Webb Cycle 2 General Observation Program 3969.

WASP-39 b was one of the first targets analyzed when Webb begins regular science observations in 2022. Data for this study were collected under Early Release Science Program 1366, designed to help scientists quickly learn how to use the telescope’s instruments and realize its full scientific potential.