Is the JWST Now an Interplanetary Meteorologist?

The JWST keeps one-upping itself. In the telescope’s latest act of outdoing itself, it examined a distant exoplanet to map its weather. The forecast?

An unending, blistering inferno driven by ceaseless supersonic winds.

WASP-43b is a hot Jupiter orbiting a main sequence star about 261 light-years away. It has a slightly larger radius than Jupiter and is about twice as massive. It orbits its star in under 20 hours and is only 1.3 million miles away from it. That means it is tidally locked to the star, with one side facing all the radiation and the other permanently dark.

This is not unusual for exoplanet gas giants. They’re often tight to their stars and don’t rotate.

WASP-43b’s discovery was announced in 2011. Since then, astronomers have studied it extensively. In 2019, researchers captured its spectrum and reported water in its clouds. Conversely, no methane, carbon dioxide, or carbon monoxide were detected. Further research showed that mineral particles dominate its clouds. The Hubble Space Telescope was largely responsible for these results; other telescopes like the Spitzer also contributed.

Scientists knew that when the JWST was launched, it would eventually turn its eye toward WASP-43b. “Having a short orbital period and being tidally locked makes WASP-43b an ideal candidate for JWST observations,” explained the authors of a 2020 paper. “Phase curve observations of an entire orbit will enable the mapping of the atmospheric structure across the planet, with different wavelengths of observation allowing different atmospheric depths to be seen.” Their paper anticipated what the JWST might find and how its observations might be understood.

Now, we’re in the future, and the JWST has taken a look at WASP-43b and captured more detailed observations than ever. The space telescope’s powerful infrared capabilities measured the heat on both sides of the planet and allowed the mapping of the planet’s atmospheric structure, just as the authors of the 2020 paper stated.

“The fact that we can map temperature in this way is a real testament to Webb’s sensitivity and stability.”

Michael Roman, University of Leicester.

A new paper in Nature Astronomy presents the results. It’s titled “Nightside Clouds and Disequilibrium Chemistry on the Hot Jupiter WASP-43b.” The lead author is Taylor Bell, a researcher from the Bay Area Environmental Research Institute.

“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,” explained lead author Bell. “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.”

Despite its power, the JWST can’t directly see WASP-43b. Instead, it utilizes phase curve spectroscopy. Phase curve spectroscopy measures the light from the planet and the star over time, sensing small changes in the light from both as the planet orbits the star. Since the JWST senses infrared light, which is emitted depending on an object’s heat, the telescope’s varying brightness data expresses the planet’s temperature.

Phase curve spectroscopy allows the JWST to sense the change in brightness as a planet orbits its star. This diagram shows the change in a planet’s phase (the amount of the lit side facing the telescope) as it orbits its star. Image Credit: NASA, ESA, CSA, Dani Player (STScI), Andi James (STScI), Greg Bacon (STScI)

The JWST’s MIRI spectrometer captured WASP-43b’s phase curve. The planet is hottest when it’s on the opposite side of the star and its lit-up side faces the telescope. The telescope sees the cooler dark side when the planet is on this side of the star and transiting in front of it.

This graph shows more than 8,000 measurements of mid-infrared light captured over a single 24-hour observation using the JWST’s low-resolution spectroscopy mode on its MIRI (Mid-Infrared Instrument). By subtracting the amount of light the star contributes, astronomers can calculate the amount coming from the visible side of the planet as it orbits. The telescope’s extreme sensitivity made this possible. Webb detected differences in brightness as small as 0.004% (40 parts per million). Image Credit: NASA, ESA, CSA, Ralf Crawford (STScI)

“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.”

To put the data into perspective, the researchers compared WASP-43b’s phase curve to General Circulation Model (GCM) simulations. The JWST phase curve data more closely matched a cloudy GCM than a cloudless GCM.

“The cloudy models are able to suppress the nightside emission and better match the data,” the authors explain in their paper.

This figure from the research shows the JWST’s phase curve data for WASP-43b (black dots) and what cloudless and cloudy GCM simulations predict. The data more closely matches a cloudy atmosphere. Image Credit: Bell et al. 2024.

The researchers used the detailed infrared data to construct a temperature map of the exoplanet. The dayside has an average temperature of about 1,250 Celsius (2,300 F), which is almost hot enough to forge iron. But the nightside likely has a thick layer of high-altitude clouds that trap some of the heat. Those clouds make the nightside appear cooler than it is. It’s much cooler at about 600 degrees Celsius (1,100 degrees Fahrenheit) but still hot enough to melt aluminum.

“The fact that we can map temperature 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.

This set of maps shows the temperature of the visible side of the hot gas-giant exoplanet WASP-43 b as the planet orbits its star. Image Credits: Illustration: NASA, ESA, CSA, Ralf Crawford (STScI). Science:
Taylor Bell (BAERI), Joanna Barstow (The Open University), Michael Roman (University of Leicester)

The researchers also mapped a hot spot in WASP-43b’s atmosphere, and it helped them gauge the exoplanet’s ferocious winds. The hot spot is east of the point receiving the most starlight. That means that powerful winds are moving the heated gas.

The JWST’s spectrum also allowed the researchers to measure the presence of water vapour (H2O) and methane (CH4.) “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.

Webb found water vapour on the dayside and the nightside, indicating cloud thickness and elevation. However, the telescope detected an absence of methane (CH4), which is unusual. The extreme heat on the dayside means carbon is in carbon monoxide (CO) form. But the cooler nightside should contain stable methane. Why isn’t it there? Powerful winds are responsible.

“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,” 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.”


Previous observations with the Hubble, Spitzer, and others revealed some aspects of WASP-43b’s atmosphere. But the JWST has taken it a step further. By determining the extremely high wind velocity on the exoplanet, scientists now believe the atmosphere is the same all around the planet.

“Taken together, our results highlight the unique capabilities of JWST/MIRI for exoplanet atmosphere characterization,” the authors write in their paper. They point out that there are still some discrepancies between the phase curve, the GCM simulations, and the chemical equilibrium in the atmosphere.

According to the researchers, more JWST exoplanet observations can help resolve them. “These remaining discrepancies underscore the importance of further exploring the effects of clouds and disequilibrium chemistry in numerical models as JWST continues to place unprecedented observational constraints on smaller and cooler planets,” they conclude.

Evan Gough

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