Having a plethora of wavelengths at once is a boon for astronomers, says Sarah Moran, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “I think people are starting to realize how powerful that is.”
Hard Rain
The complexity of signals presents a major challenge for astronomers studying distant worlds. Telescopes are almost never able to pick out the tiny blip of a planet next to its blazing parent star. Instead, observatories like the JWST are mostly taking measurements as an exoplanet crosses in front of its star, receiving the combined light of both objects. “You’re not looking at a planet; you’re looking at a planet and a star,” says Hannah Diamond-Lowe, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland. “You’re trying to pull apart what is coming from the star and what is coming from the planet.”
Multiwavelength data are starting to help researchers reveal more about exoplanets and their alien features. With WASP-17b, for instance, multiple different telescope observations over more than a decade had suggested that clouds could be swirling around in the enormous world’s atmosphere. But it was only when a team obtained data using the Hubble’s ultraviolet and optical instruments and then combined them with infrared information from the Spitzer Space Telescope in 2022 that they were able to confirm that the clouds were there. The scientists compared their multiwavelength data with different models of WASP-17b’s potential atmosphere and showed that the combination could best be explained if there was a cloud deck on the planet (2).
The next obvious question: What are those clouds made of? Given the sweltering conditions on WASP-17b, chemists suspected either magnesium silicate or aluminum oxide. But determining which one required seeing in wavelengths outside what the Hubble or Spitzer could capture, specifically those that the JWST was designed for.
In 2023, the JWST spent 10 hours staring at WASP-17b and its parent star and found a characteristic bump in the spectrum corresponding to something else entirely: silicon dioxide (SiO2). Models that combined the Hubble, Spitzer, and JWST data showed a good fit if this silicon dioxide was in the form of tiny crystals, a mere 10 nanometers in length, rather than amorphous globs. The Hubble was instrumental for the finding because the small size of particles meant that they scattered more light in the blue part of the visible spectrum. Tiny changes in the structure of the material making up the clouds also altered the overall shape of the silicon dioxide bump in the JWST’s observations. By measuring different materials in a lab and comparing their spectra to those from the combined telescopes’ data, researchers were able to constrain the structure and size of the SiO2 particles (1).
In much the same way that water droplets condense in Earth’s atmosphere and start sticking together to form rain, “we expect these quartz crystals to essentially be grouping together and raining out glass in the atmosphere [of WASP-17b],” says Hannah Wakeford, an astrophysicist at the University of Bristol in the United Kingdom. So WASP-17b has a quartz cycle, somewhat analogous to Earth’s water cycle, whereby silicon dioxide evaporates and condenses into clouds, from which it then rains out.
“Without the simulations, you can’t interpret the observations that you’re getting. They’re all working together to help us understand planets.”
—Sarah Moran
Sulfurous Mystery
With another peculiar giant world, this multiangle approach ended up taking researchers on a wild ride of surprising findings. In 2023, the JWST trained its powerful infrared eyes on a hot puffball named WASP-39b, an exoplanet with a mass similar to Saturn but a much bigger diameter—about 1.3 times that of Jupiter. When WASP-39b passes in front of its parent star, the starlight dims slightly and filters through the distended atmosphere. Different chemicals present on the exoplanet absorb certain wavelengths, creating signatures that the JWST’s spectroscopic instruments can detect.
Prior observatories, such as the Hubble and Spitzer, had discerned the existence of water vapor, sodium, and potassium in WASP-39b’s atmosphere. With the JWST, researchers were able to see many other features, including a tiny bump in the light indicating the presence of carbon dioxide, the first time the molecule had been unambiguously detected outside our solar system. More unexpected was another signature in the spectrum that, at first, scientists had trouble identifying.
Hoping to solve the mystery, theorists created simulations based on laboratory data and their understanding of chemistry, attempting to match the JWST information with the expected signatures from a dozen carbon-bearing molecules, various bromides, fluorides, and chlorides, several sulfur-based gases, and many other chemicals. Eventually, they realized that the best fit was sulfur dioxide (SO2). But this was a headscratcher. The gas is known to be emitted by volcanoes on rocky planets in the solar system, and WASP-39b is a gas giant with little or no rocky core to host volcanoes (3).
Further analysis involving chemical modeling indicated that there was an interplay between the exoplanet and its parent star, whereby starlight breaks down water and hydrogen sulfide (H2S) in WASP-39b’s atmosphere. Their constituents recombine to form sulfur dioxide. Combining the multiwavelength data with modeling was vital to unpacking this exoplanetary mystery. “Without the simulations, you can’t interpret the observations that you’re getting,” Moran says. “They’re all working together to help us understand planets.”
