The James Webb Space Telescope will be astrochemists’ newest and most powerful tool

On Dec. 18, assuming all goes to plan, the world’s newest and largest space telescope will launch from French Guiana toward its destination, a point about 1.5 million km from Earth. The James Webb Space Telescope (JWST), the result of a more than 2-decade-long collaboration between NASA, the European Space Agency, and the Canadian Space Agency, will be the successor to the famed Hubble Space Telescope. But the instruments the JWST carries should allow the telescope to observe things that Hubble cannot, including the universe’s first galaxies and the chemical makeup of exoplanets and young solar systems.

The telescope will be unlike any other that humans have so far sent to space. For one thing, it’s much larger, with a mirror that is almost twice the diameter of the next largest, the Herschel Space Observatory, and almost three times that of Hubble.

The JWST also looks different from other space telescopes. Tube-shaped Hubble fits the typical image of a telescope, but the JWST will look more like a satellite dish. To fit the JWST aboard a rocket, the 6.5 m primary mirror that will collect and focus light from distant galaxies is made up of 18 hexagonal, gold-coated beryllium sections that will unfold into place. The observatory will position itself at a point in space where the gravitational pull of Earth and the sun will keep it almost stationary in relation to those two objects.

The JWST’s size, design, and distance from Earth will give it unprecedented resolution and sensitivity for observing faint objects. Its instruments will detect the near- and mid-infrared parts of the electromagnetic spectrum. Scientists designing the telescope initially chose those wavelengths to target high-redshift galaxies—those that are farthest from us and therefore the oldest—which Hubble’s instruments cannot see. Helping to answer astrophysical questions about the evolution of galaxies and the universe remains the JWST’s primary scientific mission. But the observatory’s focus on infrared light has also turned out to be a boon for astrochemists and exoplanet researchers.

The new telescope will carry three mid-infrared instruments. Small molecules like water, methane, and carbon dioxide have vibrational modes that absorb mid-infrared light. Scientists will look for their signatures in the atmospheres of planets circling distant stars to see if those exoplanets could support life. They will also scan the ice and dust clouds in protoplanetary disks—the objects that form around young stars and eventually become planetary systems—to try to learn about the molecules that hang around in those systems and whether they differ from what we find in our own, mature planetary system.

Much of the mid-infrared region of the light spectrum is invisible to ground-based telescopes because Earth’s atmosphere absorbs it. As a result, the JWST is a unique tool for astrochemists.

Scientists have already scheduled more than 250 research projects involving the JWST, and more will be approved over its life span. Thousands of scientists will take part in those missions, and we can expect hundreds of peer-reviewed research papers published on them each year the JWST operates. Here we look at a few examples of what scientists hope the telescope will teach them about the chemistry of the cosmos.

Planetary origins

When stars form from clouds of dust, gas, and ice, they attract a rotating disk of those same materials. That protoplanetary disk eventually separates into planets, asteroids, comets, and the other bodies found in a planetary system. This process can take billions of years.

As scientists have begun to find planetary systems beyond our own, they have realized that some look very different from ours. For example, many of the exoplanets astronomers have found are larger than Earth and smaller than Neptune—a class of planets called super-Earths—but our solar system has none. Why do some systems host these super-Earths and others don’t?

The JWST will let researchers map the chemical constituents of protoplanetary disks, possibly providing clues about how planetary systems form. And lurking beneath that somewhat esoteric inquiry is the more tantalizing question of whether other planetary systems have planets with a chemical makeup that could support life.

As part of that research, the JWST will provide scientists with new ways to study gas-phase molecules in protoplanetary disks that they’ve previously monitored with other telescopes, like the Spitzer Space Telescope and the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. Above Earth’s atmosphere, the JWST will observe vibrational signatures of molecules, like water, that ALMA cannot. And the JWST’s bigger mirror makes it more sensitive than Spitzer, which also could observe in the infrared. “Spitzer gave us a taste for this research,” astrochemist Karin Öberg of Harvard University says. “But its big limitation was size.”

Inventories of molecules like water, methane, and ammonia will tell scientists about the abundance and distribution of elements like oxygen, carbon, and nitrogen in these disks. That insight could help scientists understand how planets with the ingredients for life form, says astronomer Klaus Pontoppidan of the Space Telescope Science Institute, the contractor that will operate the JWST.

Öberg is leading a team that plans to observe the chemical composition of the inner regions of several protoplanetary disks she had previously studied using ALMA. They will focus on the inner 1.5 billion km, which other telescopes have not had the instruments or resolution to observe. Her group will pair those observations with lab experiments and simulations to try to better understand the chemical reactions that happen as planetary systems form.

Meanwhile, other teams will analyze small molecules in protoplanetary disks’ ices. Existing telescopes can detect some of the spectral signatures of rotating gas-phase molecules but not the vibrational modes of solid-phase molecules in ices, Öberg says, which means scientists can see only about 1% of the small molecules in the disks. The JWST should give astrochemists access to the other 99%.

Melissa McClure of Leiden University will lead one of those projects. She wants to understand whether planets inherit their chemistry from the molecular clouds that birth protoplanetary disks or whether the molecular inventory changes as ices sublimate and refreeze in the disks before comets and planets start forming. She says that to answer that question, “we need to get detailed information about the ices in the cloud, the protostar, and the protoplanetary disk.” And only the JWST can do that, she adds. Like Öberg, McClure’s group will pair telescope observations with lab experiments and simulations to further understand this process.

The JWST also provides an opportunity to answer some astrochemical questions that scientists have debated. Scientists have seen some complex molecules, like methanol, in gas-phase observations of protostars and in studies of comets in our solar system. But lab experiments haven’t found efficient ways to make methanol in the gas phase. Other experiments have shown that methane could form on the surface of icy grains by hydrogenating carbon monoxide, says McClure’s collaborator and Leiden colleague Ewine van Dishoeck. So the question is, in actual protoplanetary disks, does methanol form on ice grains? McClure says scientists are seeing only half the story because current instruments are mostly limited to gas-phase observations. “We need JWST to find the smoking gun in ices,” she says. And if scientists can prove that complex organic molecules do form in the ice and dust where stars are born, she says, “it could be that each star system is seeded with all the starting points for life.”

Other worlds

Scientists had suspected for centuries that there are planets beyond our solar system, but it wasn’t until 1992—just 4 years before the first JWST design proposal—that researchers confirmed the existence of exoplanets. It’s unsurprising, then, that exoplanets were rarely mentioned in the early plans for the telescope to replace Hubble. In the decades since, however, astronomers have discovered thousands of distant planets, and exoplanet research has consequently gained a much more prominent position in the JWST’s mission.

Exoplanet research is “one of the most dynamic areas of astronomy, and it’s bringing in new areas like chemistry and astrobiology,” Sarah Kendrew of the European Space Agency says. She’s an expert on the JWST’s Mid-Infrared Instrument.

The JWST was initially scheduled to launch in 2007. The 14 years of delays have had a silver lining, says exoplanet researcher Laura Kreidberg of the Max Planck Institute for Astronomy. “For the exoplanet community, a lot of targets were not discovered until the last couple of years,” she says. “If it launched as scheduled, we would not have had a chance to observe all those.”

One of the main techniques scientists use to study exoplanet atmospheres is transit spectroscopy. When a planet passes in front of its star, molecules in the planet’s atmosphere absorb some of the star’s light in characteristic patterns. Other telescopes in space and on the ground have been using the technique for years, but Kendrew says the JWST will make the first such measurements in the mid-infrared, expanding the range of molecules that scientists can look for.

Scientists will be able to see exoplanet atmospheres’ major constituents, like carbon dioxide, methane, water, and ammonia. These molecules could indicate whether a planet is able to support life or even if life might already exist there. For larger planets that emit more radiation, often referred to as hot Jupiters, the JWST will have the resolution to map variations in atmospheric composition in 3D. “We’ll see those in unprecedented detail,” Kreidberg says. With those data, scientists could determine if parts of these planets’ atmospheres have enough water for life to take hold.

Dozens of transit spectroscopy projects have already been approved for the first years of the JWST’s operation. They will study a range of types of planets, such as hot, gaseous planets and rocky ones not much bigger than Earth.

But that technique is not the only way scientists can use the JWST to study exoplanets. Kreidberg is leading projects to observe two planets using radiation emitted by the planets themselves rather than the light of their stars shining through their atmospheres. That technique will let her team study these planets’ surfaces as well as their atmospheres and will help the scientists understand the interplay between the two. “My most fundamental question for right now is: When do rocky planets have an atmosphere, and when do they not?” she says.

One of their target planets is tidally locked to its star, meaning it doesn’t rotate on its axis like Earth. Instead, one side is in permanent daylight and reaches 770 °C, hot enough to melt aluminum. Scientists don’t think this planet has an atmosphere. Another planet on the group’s list is about the size and temperature of Venus and is thought to have a thick atmosphere.

Scientists struggle to use our solar system to understand how planetary atmospheres evolve, Kreidberg says. We have only a small sample size local to us, she says, and the processes that can form or destroy atmospheres are varied and complex. Studying these two exoplanets, she says, could provide some answers.

Van Dishoeck says the JWST’s potential to link our understanding of protoplanetary disks with exoplanets and planetary systems is exciting. “Can we trace back the location of mature planets to where they formed in disks? What is the history of the formation of planets? That’s something I’m looking forward to.”

Closer to home

For a small fraction—about 5%—of the JWST’s observation time, the telescope will also study objects in our solar system. NASA Goddard Space Flight Center’s Stefanie N. Milam has been working to develop the telescope’s planetary science mission since 2014. Like exoplanet research, solar system planetary science was not originally part of the JWST’s mission of studying high-redshift galaxies, and Milam’s job has been to work with researchers interested in learning more about our neighborhood to make sure they can use the telescope too.

A team led by NASA researchers will look at the icy moons Enceladus of Saturn and Europa of Jupiter to try to determine the composition of plumes that jet from their interiors. Scientists think these moons host reservoirs of water beneath their surfaces, and researchers wonder if they could host life, possibly at submerged hydrothermal vents.

Other groups hope they will have a chance to point the JWST at Saturn or Jupiter after a meteor impact to watch how their atmospheres respond. Another team will watch for interstellar objects like ‘Oumuamua, which passed through the solar system in 2017.

Milam wants to study our solar system’s comets. Like other researchers, she hopes to inventory the chemicals on these comets, especially molecules that ground telescopes can’t spot, like carbon dioxide. With her colleagues, she will also look for large, complex organic molecules like polycyclic aromatic hydrocarbons, whose presence on comets has not been proved. And Milam wants to study how comets’ chemical compositions change as they near the sun.

But despite scientists’ excitement about the research that the telescope will make possible, the JWST project has not escaped criticism. People inside and outside the astronomy community have demanded that the telescope be renamed. They point to evidence that James Webb, a NASA administrator in the 1960s, participated in—or was at least complicit in—efforts within the US government to fire LGBTQ+ public servants because of their identity. “The time for lionizing leaders who acquiesced in a history of harm is over. We should name telescopes out of love for those who came before us and led the way to freedom—and out of love for those who are coming up after,” wrote a group of four astronomers and physicists in Scientific American earlier this year. More than 1,200 people signed the group’s petition.

In September, NASA announced it had finished its own investigation of Webb’s role in those discriminatory efforts. “We have found no evidence at this time that warrants changing the name,” NASA administrator Bill Nelson told NPR. NASA has not released a report on its investigation. One member of a NASA advisory panel, Lucianne Walkowicz, an astronomer at the Adler Planetarium and an author of the Scientific American article, announced they were resigning from that committee in protest of the decision.

Others have also questioned the project’s cost. The launch next month comes after a very long and very expensive journey. When the telescope was first conceived around 1996, the initial estimated price tag was $500 million. Now, 25 years and $10 billion later, US lawmakers have lambasted the telescope’s manufacturer, Northrop Grumman, for its role in delays and cost overruns, and they have asked whether Congress should keep funding the telescope or if it should fine Northrop Grumman. The US Government Accountability Office has criticized NASA officials and other contractors for their roles in the cost and delay issues.

Nevertheless, the JWST is now ready. Still, before the telescope can start churning out exciting data, a few things still have to go right. The launch has to be successful, the telescope has to reach its proper destination, and its mirrors and instruments need to get set up correctly. There is not much room for error either. After Hubble launched, astronauts had to correct its out-of-focus mirror, but the JWST will be too far away from Earth for repair missions.

So, are scientists nervous?

“Basically terrified,” Kendrew says.

“Of course I’m worried,” Kreidberg says.

“Oh yes. Oh my. Very nervous,” says van Dishoeck, who has been alive for more telescope launches than any other scientist C&EN contacted.

But NASA’s Milam, who has had a front-row seat to the many tests that ensure the JWST is ready for launch, was able to answer with a more reassuring “Yes and no.” She admits she thinks about all the things that could go wrong. But she knows that the scientists and technicians who built and tested the telescope have given us the best chance of success that they can.