Telescopes team up to probe distant worlds

In astronomy, many eyes are better than one. Take the case of WASP-17b, a Jupiter-size exoplanet with a scorching temperature of around 1,500 degrees Celsius. Measurements from the Hubble Space Telescope, which sees largely in the same wavelengths as human eyes, helped show that the giant planet’s atmosphere was chock-full of clouds. But when the James Webb Space Telescope (JWST) turned its infrared instruments on WASP-17b in March 2023, scientists got a shock: Those clouds were made of quartz crystal, something they’d never seen on any other world (1).

Measurements from the Hubble Space Telescope helped show that the giant exoplanet Wasp-17b, shown here in an artist’s conception, was chock-full of clouds. But it wasn’t until the JWST had a look that scientists got a surprise: Those clouds were made of quartz crystal, something never before seen on any other world. Image credit: NASA, European Space Agency, Canadian Space Agency, and R. Crawford (STScI).

Clearly, there’s value to having access to many parts of the electromagnetic spectrum. Each waveband provides unique insights into the cosmos. The JWST in particular has helped plug an important gap where astronomers previously had difficulty gathering light. Its enormous 6.5-meter mirror allows it to make unrivaled observations in the near-infrared, a segment of the spectrum mostly blocked by our atmosphere.

Researchers are now using the JWST in tandem with the Hubble and other telescopes to make multiwavelength observations of worlds outside our solar system. The work relies on simulations and lab experiments to investigate alien chemistries, atmospheric dynamics, and even the prospects for life. Such techniques are still in their early days as astronomers test out their new capabilities with the JWST. Soon, new telescopes on the ground and in space will provide even higher-quality measurements to assist in such ventures. And one day, an explicitly multiwavelength facility, the Habitable Worlds Observatory (HWO), could launch into orbit to study Earth-like exoplanets.

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.”

With the help of the JWST, astronomers identified sulfur dioxide in an exoplanet’s atmosphere for the first time. Sulfur dioxide on WASP-39b can be explained by photochemistry: Starlight triggers chemical reactions that lead to SO₂. Image credit: NASA/Jet Propulsion Laboratory–Caltech/Robert Hurt; Center for Astrophysics–Harvard & Smithsonian/Melissa Weiss.

Tough Upbringing

One overriding question often underlies many exoplanet observations: Could some planets out there host life? That may depend on complex interactions between planets and stars soon after they are formed. Planets are thought to be born from the debris disk surrounding young stars, but the details of such processes, including how they accumulate atmospheres and how those atmospheres evolve over time, are still unclear. Researchers are using observations at a wide range of wavelengths to better understand how activity from a newborn star affects its nascent exoplanets. This, in turn, can help them determine whether such places are potential abodes for life. “The early lives of stars are so chaotic that we need to study their behavior to understand how planets around them will evolve.” says Adina Feinstein, an astrophysicist at Michigan State University in East Lansing.

Feinstein has been looking at stars known as M-dwarfs, which are smaller and cooler than our sun but tend to be much more active, often spitting out massive plumes of particles and radiation. Since M-dwarfs are the most common stars out there, knowing how such conditions affect the habitability of planets around M-dwarfs is essential to understanding how common life might be. Such stellar activity can potentially both help and hinder life: Ultraviolet radiation is thought to be necessary for prebiotic chemistry to begin, but excessive bursts of energy from a star can strip a planet of its atmosphere.

As part of her work, Feinstein has been studying an M-dwarf named AU Microscopii. Born a mere 22 million years ago, the star is orbited by at least two worlds. She and her colleagues have already obtained data about the system using a suite of telescopes, including the optical and ultraviolet Hubble, the X-ray XMM-Newton space observatory, and several ground-based facilities that pick up signals at radio wavelengths (4).

The high-energy ultraviolet and X-ray observations capture activity such as flares bursting from the surface of AU Microscopii. The long-wavelength radio data help identify space weather caused by energetic particles traveling between the star and its planets. The team has combined these data with models to calculate that stellar activity bursting from AU Microscopii is powerful enough to affect planetary atmospheres. They calculate that the two young planets could lose up to 50 percent of their present-day atmospheres as they age.

To find out what’s actually happening on the planets, Feinstein and her team are using the JWST to observe the planetary atmospheres directly, while taking measurements in the X-ray, ultraviolet, and radio to monitor activity originating from the star. By comparing these observations, the researchers can find out, for instance, whether an interaction between the star and its planets is making their atmosphere heat up and become more distended. In this way, they hope to watch as the star refashions these young planets, potentially stripping away protective features such as an ozone layer—and that could offer insights into the likelihood of life arising on such worlds.

Wider Eyes

The search for exoplanet compositions and the peculiar ingredients of life is likely to get yet more help as new high-resolution and broad-wavelength telescopes come online in the near future. The Nancy Grace Roman Space Telescope, planned to launch as early as late 2026, will see in some of the same optical and infrared wavelengths as the Hubble but will have a field of view at least 100 times wider. This should allow it to spot small rocky worlds that are much farther from their parent stars in cooler, potentially more habitable orbits. Gigantic ground-based facilities such as the Extremely Large Telescope, being built in Chile and expected to start collecting data at the end of the decade, will also capture faint light coming from the atmospheres of exoplanets, allowing astronomers to pick out fine details on distant worlds.

And many in the field are hopeful that the HWO comes to fruition in the 2040s. The observatory has a hefty price tag of around $11 billion but broad support in the United States and across the international astronomy community. The HWO is being designed with a coronagraph—a star shade that can block light from a parent star and allow the telescope to directly image Earth-size exoplanets around sun-like stars. While its technical specifications are still being worked out, the project should have instruments that see in the ultraviolet, optical, and infrared, allowing for investigations across many wavelengths at the same time.

An upcoming program will use the JWST time to hunt for potential worlds that the future space-based observatory might study. “We’re asking: ‘What are the targets we’re going to be aiming [HWO] at to work out whether or not they have the possibility of being inhabited?’” Wakeford says. “That is the kind of profound way we’re designing telescopes now.”