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WASP-43b - a gaseous exoplanet the size of Jupiter, but with double the mass - is located around two-hundred-and-sixty light-years away in the constellation of Sextans, and lies so close to its host star, the orange dwarf star WASP-43, that it completes an orbit in just 19.5 hours; it is also gravitationally locked so that it keeps one hemisphere facing the star. Although its nightside never receives any direct radiation from the star, strong eastward winds transport heat around from the dayside.
Since its discovery in 2011, WASP-43b has been observed with numerous telescopes, including the NASA/ESA Hubble Space Telescope and NASA’s now-retired Spitzer Space Telescope.
“With Hubble, we could clearly see that there is water vapour on the dayside. Both Hubble and Spitzer suggested there might be clouds on the nightside, 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.”
said Dr. Taylor Bell, an astronomer at the Bay Area Environmental Research Institute.
Although WASP-43b is too small, dim, and close to its star for a telescope to see directly, its short orbital period 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.
In their study, Dr. Bell and colleagues used Webb’s Mid-Infrared Instrument (MIRI) to measure light from the WASP-43 system every 10 seconds for more than 24 hours.
Dr. Bell went on to explain:
"By observing over an entire orbit, we were able to calculate the temperature of different sides of the planet as they rotate into view. 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 1,250 degrees Celsius (2,300 degrees Fahrenheit) — hot enough to forge iron. Meanwhile, the nightside is significantly cooler at 600 degrees Celsius (1,100 degrees Fahrenheit).
The data also help locate the hottest spot on the planet (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.
“The fact that we can map temperature in this way is a real testament to Webb’s sensitivity and stability,”
said Dr. Michael Roman, an astronomer at the University of Leicester.
To interpret the map, the astronomers used complex 3D atmospheric models like those used to understand 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 escaping to space. As a result, the night-side — 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 and methane 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 Dr. Joanna Barstow, an astronomer at the Open University.
The spectra show clear signs of water vapour on the night-side as well as the day-side of the planet, providing additional information about how thick the clouds are and how high they extend in the atmosphere; surprisingly, the data also show a distinct lack of methane anywhere in the atmosphere. Although the day-side is too hot for methane to exist (most of the carbon should be in the form of carbon monoxide), methane should be stable and detectable on the cooler night-side.
Dr. Barstow explained:
“The fact that we don’t see methane tells us that WASP-43b must have wind speeds reaching something like 5,000 miles per hour. If winds move gas around from the day-side to the night-side and back again fast enough, there isn’t enough time for the expected chemical reactions to produce detectable amounts of methane on the nigh-tside.”
“With the new observing power of Webb, WASP-43b has been unveiled in unprecedented detail,”
said Dr. Laura Kreidberg, director at the Max Planck Institute for Astronomy.
“We see a complex, inhospitable world, with furious winds, massive temperature changes, and patchy clouds likely made of rock droplets. WASP-43b is a reminder of the vast range of climates that are possible on exoplanets and the many ways in which Earth is special.”