First detection of morning and evening on distant exoplanet using James Webb Space Telescope
The James Webb Space Telescope has revealed a morning and evening side on WASP-39b, an exoplanet 700 light-years away from Earth.
![First detection of morning and evening on distant exoplanet using JWST First detection of morning and evening on distant exoplanet using JWST](https://services.meteored.com/img/article/first-detection-of-morning-and-evening-on-distant-exoplanet-using-jwst-1721747552658_1024.jpeg)
The atmospheres of exoplanets have been studied using powerful telescopes for some time but have always been treated as being uniform all over the planet.
Now, the James Webb Space Telescope (JWST) has revealed the different atmospheric conditions of the ‘morning’ and ‘evening’ sides of WASP-39b – a distant exoplanet, where the morning side is cloudier than the evening side.
Continuous day and endless night
Scientists learn about a planet’s atmosphere by measuring the light received as a planet passes in front of its star; this light is changed by filtering through the atmosphere, with specific signatures revealing different molecules in the atmosphere.
This technique assumes the atmosphere of the exoplanet is the same across the entire surface. However, WASP-39b is ‘tidally locked’; the planet completes a one ‘day’ rotation in the same time it takes to complete one orbit. This means the same side of the planet always faces its star, creating a continuous ‘day’ on one side and an endless ‘night’ on the other side of the planet; in between there are also ‘morning’ and ‘evening’ sides.
As the day side of WASP-39b always faces the star, its much hotter than the night side. Because of the way the planet spins, the huge temperature difference between the two is thought to create a strong equatorial wind that moves in one direction, creating a ‘morning’ side – where cooler night wind travels into the day side – and a ‘evening’ side, where the hot day wind carries to the night side. Data shows the evening can reach 800°C, while the morning is a relatively cooler 600°C.
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The way clouds form depends on temperature, so it was predicted the morning and evening sides would have different amounts of cloud cover. Researchers used several methods of analysing JWST data to test this, finding that the morning was cloudier than the evening, as expected.
Dr James Kirk from the Department of Physics at Imperial College London, says most planets observed orbiting distant stars are like WASP-39b: “Now, we have been able to test our theories about these planets and, for the first time, directly measure an exoplanet’s morning and evening side over a wide wavelength range.”
Néstor Espinoza, exoplanet researcher at the Space Telescope Science Institute and lead author says this analysis is “particularly interesting because you’re getting 3D information on the planet that you weren’t getting before. Because we can tell that the evening edge is hotter, that means it’s a little puffier. So, theoretically, there is a small swell at the terminator approaching the nightside of the planet.”
Future work
Kirk developed an analysis technique that focused only on the light received as the planet transits across the star’s surface; since only the edges of the planet ‘touch’ the star’s disc at these moments, light would only be filtered through the morning or evening sides of the atmosphere respectively. His results matched well with other analyses completed by other members of the team.
The team hope to expand their analysis to include data from more instruments aboard JWST, including those that analyse light in a range of wavelengths, to reveal more detail about the atmospheric differences on WASP-39b.
Dr Kirk says: “Now we’ve demonstrated the feasibility of this method with JWST, and the precision of JWST is so immense, it really opens up a new avenue into understanding and measuring atmospheric circulation for exoplanets that we were previously largely insensitive to.”
News reference
Espinoza, N. et al (2024) Inhomogeneous terminators on the exoplanet WASP-39b, Nature.