Free Astronomy Magazine September-October 2024

25 SEPTEMBER-OCTOBER 2024 ASTRO PUBLISHING Using Webb’s NIRSpec (Near-In- frared Spectrograph), astronomers confirmed a temperature difference between the eternal morning and eternal evening on WASP-39 b, with the evening appearing hotter by roughly 300 Fahrenheit degrees (about 200 Celsius degrees). They also found evidence for different cloud cover, with the forever morn- ing portion of the planet being likely cloudier than the evening. Astronomers analyzed the 2- to 5- micron transmission spectrum of WASP-39 b, a technique that stud- ies the exoplanet’s terminator, the boundary that separates the planet’s dayside and nightside. A transmis- sion spectrum is made by comparing starlight filtered through a planet’s atmosphere as it moves in front of the star, to the unfiltered starlight detected when the planet is beside the star. When making that compar- ison, researchers can get informa- tion about the temperature, com- position, and other properties of the planet’s atmosphere. “WASP-39 b has become a sort of benchmark planet in studying the atmosphere of exoplanets with Webb,” said Néstor Espinoza, an exoplanet researcher at the Space Telescope Science Institute and lead author on the study. “It has an in- flated, puffy atmosphere, so the sig- nal coming 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 – there was no detailed attempt to differentiate between one side and the other. Now, the new analysis builds two different spectra from the termina- tor region, essentially splitting the day/night boundary into two semi- A light curve from NASA’s James Webb Space Telescope’s NIRSpec (Near-Infrared Spectrograph) shows the change in bright- ness from the WASP-39 star system over time as the planet transited the star. This observation was made using NIRSpec’s bright object time-series mode, which uses a grating to spread out light from a single bright object (like the host star of WASP- 39 b) and measure the brightness of each wavelength of light at set intervals of time. [NASA, ESA, CSA, Ralf Crawford (STScI)]