Free Astronomy Magazine July-August 2024

JULY-AUGUST 2024 “The fact that we can map temper- ature in this way is a real testament to Webb’s sensitivity and stability,” said Michael Roman, a co-author from the University of Leicester in the U.K. To interpret the map, the team used complex 3D atmospheric models like those used to under- stand weather and climate on Earth. The analysis shows that the night- side is probably covered in a thick, high layer of clouds that prevent some of the infrared light from es- caping to space. As a result, the nightside – while very hot – looks dimmer and cooler than it would if there were no clouds. The broad spectrum of mid-infrared light captured by Webb also made it possible to measure the amount of water vapor (H 2 O) and methane (CH 4 ) around the planet. “Webb has given us an opportunity to figure out exactly which mole- cules we’re seeing and put some lim- its on the abundances,” said Joanna Barstow, a co-author from the Open University in the U.K. The spectra show clear signs of water vapor on the nightside as well as the dayside of the planet, provid- ing additional information about how thick the clouds are and how high they extend in the atmosphere. Surprisingly, the data also shows a distinct lack of methane anywhere in the atmosphere. Although the dayside is too hot for methane to exist (most of the carbon should be in the form of carbon monoxide), methane should be stable and de- tectable on the cooler nightside. “The fact that we don’t see methane tells us that WASP-43 b must have wind speeds reaching something like 5,000 miles per hour,” explained Barstow. “If winds move gas around from the dayside to the nightside and back again fast enough, there isn’t enough time for the expected chemical reactions to produce de- tectable amounts of methane on the nightside.” The team thinks that because of this wind-driven mixing, the atmospheric chemistry is the same all the way around the planet, which wasn’t ap- parent from past work with Hubble and Spitzer. The MIRI observation of WASP-43 b was conducted as part of the Webb Early Release Science programs, which are providing researchers with a vast set of robust, open-access data for studying a wide array of cosmic phe- nomena. T his light curve shows the change in brightness of the WASP-43 system over time as the planet orbits the star. This type of light curve is known as a phase curve because it includes the entire orbit, or all phases of the planet. Because it is tidally locked, different sides of WASP-43 b rotate into view as it or- bits. The system appears brightest when the hot dayside is facing the telescope, just before and after the secondary eclipse when the planet passes behind the star. The system grows dimmer as the planet continues its orbits and the nightside rotates into view. After the transit when the planet passes in front of the star, blocking some of the starlight, the system brightens again as the dayside rotates back into view. This graph shows more than 8,000 measurements of 5- to 12-micron mid-infrared light captured over a single 24-hour observation using the low-resolution spectroscopy mode on Webb’s MIRI. By subtract- ing the amount of light contributed by the star, astronomers can calculate the amount coming from the visible side of the planet as it orbits. Webb was able to detect differences in brightness as small as 0.004% (40 parts per million). Since the amount of mid-infrared light given off by an object is directly related to its temperature, astronomers were able to use these measurements to calculate the average temperature of different sides of the planet. [Illustration: NASA, ESA, CSA, Ralf Crawford (STScI). Science: Taylor Bell (BAERI), Joanna Barstow (The Open University), Michael Roman (University of Leicester)] !