Researchers use NASA’s Webb to map weather of planet 280 light-years away

An international team of researchers, including astronomers from the University of Leicester, has successfully used NASA’s James Webb Space Telescope to map the weather on the hot gas-giant exoplanet WASP-43 b. 

Precise brightness measurements over a broad spectrum of mid-infrared light, combined with 3D climate models and previous observations from other telescopes, suggest the presence of thick, high clouds covering the nightside, mostly clear skies on the dayside, and equatorial winds upwards of 5,000 miles per hour mixing atmospheric gases around the planet.

The investigation is just the latest demonstration of the exoplanet science now possible with Webb’s extraordinary ability to measure temperature variations and detect atmospheric gases trillions of miles away.

WASP-43 b is a “hot Jupiter” type of exoplanet: similar in size to Jupiter, made primarily of hydrogen and helium, and much hotter than any of the giant planets in our own solar system. Although its star is smaller and cooler than the Sun, WASP-43 b orbits at a distance of just 1.3 million miles – less than 1/25th the distance between Mercury and the Sun.

With such a tight orbit, the planet is tidally locked, with one side continuously illuminated and the other in permanent darkness. Although the nightside never receives any direct radiation from the star, strong eastward winds transport heat around from the dayside.

Since its discovery in 2011, WASP-43 b has been observed with numerous telescopes, including NASA’s Hubble and now-retired Spitzer space telescopes. The new JWST observations are the most precise measurements yet, allowing the researchers to study the planet’s atmosphere in greater detail.

Dr Michael Roman from the University of Leicester School of Physics and Astronomy contributed to the new study by analysing different computer simulations of weather conditions on WASP-43 b. The researchers used a combination of different numerical models to interpret the JWST observations in context of theoretical expectations. Dr Roman said: “The models help fill in missing pieces to our understanding and help provide a fuller picture of what the atmosphere of WASP-43 b is like.”

For the paper, several groups around the world simulated the atmosphere using different numerical weather models, called General Circulation Models. Each simulation used different assumptions or approaches to deal with clouds and weather.  

Dr Roman explains: “We took these different model atmospheres and essentially asked, ‘What would we see if we were to observe these simulated atmospheres with JWST? What would their data look like, and how would they compare to what we see in the actual observations of WASP-43b?’  So we simulated the light emitted from these different models and compared those simulated observations to the real observations.

“The analysis showed that the nightside of the planet was shrouded in thick clouds, while the dayside was mostly clear. HST and Spitzer observations had shown evidence of these nightside clouds, but with the power of JWST, we were able to place some new constraints on the location and thickness of the clouds for the first time, along with new insights into the chemistry. Such details are key to helping us to better understand the processes shape hot Jupiter atmospheres and how they affect our observations.”

“With Hubble, we could clearly see that there is water vapor on the dayside. Both Hubble and Spitzer suggested there might be clouds on the nightside,” explained Taylor Bell, researcher from the Bay Area Environmental Research Institute and lead author of a study published today in Nature Astronomy. “But we needed more precise measurements from Webb to really begin mapping the temperature, cloud cover, winds, and more detailed atmospheric composition all the way around the planet.”

Although WASP-43 b is too small, dim, and close to its star for a telescope to see directly, its short orbital period of just 19.5 hours makes it ideal for phase curve spectroscopy, a technique that involves measuring tiny changes in brightness of the star-planet system as the planet orbits the star. 

Since the amount of mid-infrared light given off by an object depends largely on how hot it is, the brightness data captured by Webb can then be used to calculate the planet’s temperature.

The team used Webb’s MIRI (Mid-Infrared Instrument) to measure light from the WASP-43 system every 10 seconds for more than 24 hours. “By observing over an entire orbit, we were able to calculate the temperature of different sides of the planet as they rotate into view,” explained Bell. “From that, we could construct a rough map of temperature across the planet.”

The measurements show that the dayside has an average temperature of nearly 2,300 degrees Fahrenheit (1,250 degrees Celsius) – hot enough to forge iron. Meanwhile, the nightside is significantly cooler at 1,100 degrees Fahrenheit (600 degrees Celsius). The data also helps locate the hottest spot on the planet (the “hotspot”), which is shifted slightly eastward from the point that receives the most stellar radiation, where the star is highest in the planet’s sky. This shift occurs because of supersonic winds, which move heated air eastward.

Dr Sarah Casewell, STFC Ernest Rutherford Fellow at the University of Leicester, said: “It feels like we have come full circle here with these MIRI results - MIRI was a large part of our instrument building for so long here at the University, and so it’s really fantastic to have University of Leicester researchers leading part of the work on some of the first results.”

The broad spectrum of mid-infrared light captured by Webb also made it possible to measure the amount of water vapor (H2O) and methane (CH4) around the planet. “Webb has given us an opportunity to figure out exactly which molecules we’re seeing and put some limits on the abundances,” said Joanna Barstow, a co-author from the Open University in the U.K.

While the spectra show clear signs of water vapor on both the dayside and nightside of the planet, the data show a surprising lack of methane anywhere in the atmosphere, despite theoretical expectations. Although the dayside is too hot for methane to exist (most of the carbon should be in the form of carbon monoxide), methane should, in theory, be stable and detectable on the cooler nightside. This surprising finding might be explained by the planet’s strong winds.

“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 detectable 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 apparent 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 phenomena. 

The James Webb Space Telescope is the world's premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and the Canadian Space Agency.