Astronomers used the NASA/ESA/CSA James Webb Space Telescope to obtain and analyze morning and evening transmission spectra of the hot Saturn exoplanet WASP-39b, which orbits a star about 700 light-years away in the constellation Virgo. The spectrum is best explained by a model in which the evening boundary is 177 K hotter than the morning boundary.

WASP-39b is a hot gas giant that orbits the G7 type star WASP-39 every 4.1 days.

First discovered in 2011, this alien world has roughly the same mass as Saturn, but is 50% larger.

The planet's extreme expansion is due in part to its high temperature (about 900 degrees Celsius or 1,652 degrees Fahrenheit).

“WASP-39b has become a kind of benchmark planet for the Webb mission's exoplanet atmospheric studies,” said Dr. Nestor Espinoza, an astronomer at the Space Telescope Science Institute.

“The planet has a bulging, puffy atmosphere, so the signal coming from the starlight passing through the planet's atmosphere is quite strong.”

Webb spectra of WASP-39b's atmosphere released so far have revealed the presence of carbon dioxide, sulfur dioxide, water vapor, and sodium, and show the entire day/night boundary, without any detailed attempt to distinguish one from the other.

Now, the new analysis constructs two different spectra from the terminator region, essentially splitting the day/night boundary into two semicircles: one from the evening and one from the morning.

According to Webb's data, evenings will be much hotter, reaching a scorching 800 degrees Celsius (1,450 degrees Fahrenheit), before easing into a cooler 600 degrees Celsius (1,150 degrees Fahrenheit) in the morning.

“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,” said Dr. Espinoza.

“Any movement of the instrument or the observatory while collecting data would severely limit our ability to make this detection. This has to be exceptionally precise, and Webb is just that precise.”

“We don't have a planet like this in our solar system, but most of the planets we observe orbiting distant stars have shorter orbits and are closer to us, like WASP-39b,” said Dr James Kirk, astronomer at Imperial College London.

“Now we can test theories about these planets and, for the first time, directly measure the morning and evening sides of an exoplanet across a wide range of wavelengths.”

This transmission spectrum, taken with Webb's NIRSpec (Near-Infrared Spectrometer) PRISM in Bright Object Time Series mode, shows the amount of different wavelengths (colors) of near-infrared stellar light blocked by WASP-39b's atmosphere. The spectrum shows clear evidence of water and carbon dioxide on the exoplanet, as well as morning and evening temperature variations. Image credit: NASA / ESA / CSA / Webb / R. Crawford, STScI.

Extensive modeling of the resulting data will also allow researchers to investigate the structure of WASP-39b's atmosphere, its cloud cover, and why it's hotter in the evenings.

While the team plans to next investigate how cloud cover affects temperature, and vice versa, the astronomers confirmed that the main cause of WASP-39b's temperature difference is the circulation of gas around the planet.

For highly irradiated exoplanets that orbit relatively close to their stars, like WASP-39b, researchers typically expect gas to shift as the planet rotates around the star: a strong equatorial jet stream should move hotter gas on the dayside from the evening to the nightside.

Because the temperature difference is so large, the pressure difference is also large, resulting in faster wind speeds.

Using the General Circulation Model, a 3-D model similar to those used to predict weather patterns on Earth, the astronomers found that on WASP-39b, prevailing winds tend to move from the nightside across the morning boundary, around the dayside, across the evening boundary, and then back 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 in the evening it is hit by winds of air heated by the day side.

The study suggests that wind speeds on WASP-39b could reach thousands of kilometers per hour.

“This analysis is particularly interesting because it provides previously unavailable 3D information about the planet,” Dr Espinoza said.

“The fact that it's hotter on the evening edge means that it's a bit bulging, which means that in theory there's a little swell at the boundary approaching the night side of the planet.”

of study Published in this week's journal Nature.

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N. Espinosa othersThe uneven boundary surface of exoplanet WASP-39b. NaturePublished online July 15, 2024; doi: 10.1038/s41586-024-07768-4

This article is based on press releases from NASA and Imperial College London.