After more than 18 months of nearly flawless operation, the James Webb Space Telescope (JWST) continues to deliver amazement, making unexpected discoveries, adding new wrinkles to known phenomena, and calling into question long-held theories of how the universe works.
“The instruments are working amazingly well, in essentially all cases better than expected,” said Garth Illingworth, professor emeritus of astronomy and astrophysics at the University of California, Santa Cruz (CA, USA), and one of the three originators of the mission over three decades ago. “It has exceeded every one of its performance requirements, which is truly amazing when you think about how complex it is.” That complexity has included three decades of planning, design, and construction, followed by launch and maneuvering 1.5 × 10
6 km from Earth to its second Lagrange (L
2) orbit, unfolding and locking into position the 18 segments of its 6.5 m diameter main mirror, and deploying 8 motors, 90 cables, and some 400 pulleys to unfurl its fragile sunshield [
1], [
2].
Among Webb’s latest discoveries is the detection of water and other molecules in the inner region of a disk of planet-forming gas and dust surrounding an infant star, suggesting for the first time that rocky, Earth-like planets can form in very extreme environments [
3]. Recent JWST observations have also led to the first detection of other remarkable molecules in space, including the oldest and most distant complex organic molecules ever discovered, located 12.3 billion light-years from Earth [
4]; the detection of the coldest ice molecules in the known universe, which harbor the building blocks for life [
5]; and evidence of frozen water in a near-Earth comet, which may help explain the origins of water on Earth [
6].
In addition to using JWST to make such unprecedented findings, astronomers have also used the observatory’s superior resolution and sensitivity to take a closer look at objects and phenomena already well-known to astronomy. For example, when the nearest and brightest supernova observed in nearly four centuries suddenly appeared in the sky in 1987, researchers used the world’s most powerful telescopes, including Webb’s predecessor, the Hubble Space Telescope (HST), to track the astonishing explosion. While Hubble’s observations led to numerous discoveries about stellar evolution, they could not resolve whether the object left in the supernova’s wake was a black hole or a neutron star [
7]. JWST’s extremely sensitive near- and mid-infrared spectroscopy instruments have now solved the mystery by identifying emissions of ionized argon and sulfur gas at the center of the supernova, which are tell-tale signs of the presence of a neutron star (
Fig. 1) [
8].
This example showcases one of JWST’s strengths. “Most of the great discoveries that came from Hubble were related to its images, which were just so remarkable. But its spectroscopic capability was quite limited,” Illingworth said. “And while Webb’s images have rightly garnered a lot of attention, we are now realizing that its spectroscopic capabilities are very, very powerful. You can think of Webb as a spectroscopic telescope that also makes great images.”
The telescope’s near-infrared imager and slitless spectrograph (NIRISS) and near-infrared spectrograph (NIRSpec) instruments have proven invaluable for investigating the atmospheres of exoplanets using transmission spectroscopy. Data collection for this purpose involves waiting for a planet to transit in front of a bright object such as a star, then measuring the absorption of specific wavelengths of the star’s light to infer the presence of different molecules and particles in the exoplanet’s atmosphere. In September 2023, astronomers reported they had used the telescope to detect methane and carbon dioxide in the atmosphere of exoplanet K2-18 b, along with potential traces of dimethyl sulfide, a substance created on Earth by living creatures. These findings suggest an ocean under a hydrogen-rich atmosphere on K2-18 b, an exoplanet 8.6 times the Earth’s mass, which orbits the dwarf star K2-18, 120 light-years away (
Fig. 2) [
9].
“JWST’s ability to detect individual elements or molecules in the atmospheres of these planets now is getting to the point of being very, very interesting,” said Wendy Freedman, professor of astronomy and astrophysics at the University of Chicago, IL, USA. “Being able to detect with high precision water and other molecules in the atmospheres tells us an enormous amount about how these planets are forming, what the conditions are—ultimately, perhaps, whether they are capable of harboring life.”
While many scientists have publicly said it is premature to point to these findings as evidence of life existing on other planets [
10], other researchers have crunched the numbers to determine just how well Webb might be able to detect life on distant planets [
11]. The exercise involved taking a spectrum of the Earth’s atmosphere, decreasing the data quality to mimic how it would look to an observer dozens of light-years away, and running it through a computer model replicating JWST’s instrument sensitivity. Their calculations suggest that the observatory could spot the signs of human civilization on Earth, such as methane, oxygen, nitrogen dioxide, and chlorofluorocarbons, if it were peering at us from another solar system within the Milky Way [
11].
While JWST has resolved many open questions, some of its observations have created headaches for astronomers. One observation many in the field had been eager for the telescope to provide data for is the Hubble constant, which measures how fast the universe is expanding. “Determining the Hubble constant is the biggest challenge for cosmology at the moment,” Illingworth said.
One means of calculating the constant is based on Planck satellite observations of fluctuations in the oldest light in the universe—the cosmic background microwave radiation—across the entire sky. These data indicate the universe is expanding at 67.4 km∙s
−1 per megaparsec (a megaparsec is about 3 million light-years), a result replicated in 2023 using the Atacama Cosmology Telescope in Chile [
12].
Alternatively, astronomers have used HST to measure the distance and velocity of flashing stars known as the Cepheids. This method pins the Hubble constant at 73 km∙s
−1 per megaparsec. Scientists have been hoping that Webb, with its higher sensitivity in the infrared range, would provide a sharper view of the Cepheids than Hubble and yield a Hubble constant more in line with the Planck observations. Instead, its findings are nearly identical to Hubble’s (
Fig. 3) [
13]. With JWST failing to relieve the so-called Hubble tension, astronomers may be faced with shoehorning new physics into the standard model of cosmology, possibly by adding a new form of dark energy or dark matter [
14].
Freedman, who was part of the group that derived the Hubble constant from HST data in 2001, said her group is currently working on three new ways of measuring distances to galaxies using JWST to determine whether the Hubble constant can be more accurately measured. “It will be interesting if those agree really well because it will give some sense that you have overcome any systematic effects,” Freedman said. “And if they do not agree, it tells you the overall uncertainty of the measurement.”
Another of Webb’s challenges to our understanding of the universe has come from observations that very early galaxies are unexpectedly bright [
15]. Because brightness is often a stand-in for mass, the implication is that these early galaxies grew too massive, too soon to be accounted for by current models. “If the result holds, it suggests that we do not understand how galaxies formed early in the universe,” Freedman said. “We are going to be debating for a long time what the implications of these results are.”
One reason the result may not hold is that a galaxy’s brightness, while related to mass, may be impacted by other factors and have a different relation than in the nearby universe. “The conversion of light to mass comes with a lot of assumptions,” said Kristen McQuinn, assistant professor of physics and astronomy at Rutgers University in Piscataway, NJ, USA. She said scientists are dealing with the unexpected result by either reevaluating the assumptions underlying the conversion from light to mass or adjusting their theories to accommodate the observations.
One team of theorists, for example, recently created a model that accommodates the standard model by suggesting that star formation in these precocious galaxies occurred in occasional bursts, rather than at the steady rate seen in older galaxies [
16]. Other posited explanations are that early galaxies contained much less dust than later galaxies, thereby confounding efforts to calculate their true mass; or star formation occurred more quickly in the early universe due to higher gas pressures and temperatures; or massive black holes formed earlier than thought and crushed clouds of dust and gas into stars more quickly [
17], [
18].
Freedman said that other astrophysical theories will surely be discussed and that astronomers will collect additional data to try to explain the unexpected observations. Still, many astronomers are intrigued by the implications of the initial results. “It is a useful thing when we find something that disagrees with simulations, because that means there are some physics we do not quite understand and we need to tweak our models,” said Jeyhan Kartaltepe, associate professor of physics and astronomy at the Rochester Institute of Technology (Rochester, NY, USA). “Exploring new ideas about the evolution of the universe has been quite exciting.”
Despite the astronomy community’s general enthusiasm, there have been a few glitches associated with the telescope’s rollout. “There has been an unexplained loss of sensitivity in the mid-infrared instrument at the longer wavelengths that has since stabilized,” Kartaltepe said. “That is probably the main disappointment, because it may not be as sensitive to faint things as hoped.”
Of more concern for many in the field was the software created to process JWST’s data, said Illingworth. Typically, idiosyncrasies inherent in telescopes like JWST, such as slightly misaligned mirrors or variation in detector sensitivities, must be accounted for before their data can be used. Illingworth said the software created for this purpose for JWST was inadequate, leading to unreliable results for the first several months of the telescope’s operation [
19]. “It was basically a failure, and many in the community anguished over it,” he said.
While Freedman agreed that the initial software release was problematic, she said that is expected to some degree, adding that, initially, HST’s data processing software had similar issues that had to be corrected. “What we are seeing with Webb are the usual kinds of teething problems,” she said. “But we are moving past them and getting really remarkable results.”
In any case, these issues have not kept scientists from requesting time on the telescope, with fewer than one in nine applications accepted in the most recent review cycle [
20]. “It is almost too big of a success for us,” McQuinn said, “because it is really hard to get time on it.”
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