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On the launch pad at last, NASA’s Webb telescope will probe first stars and weather on alien worlds


The telescope Galileo Galilei first pointed at the heavens in 1609 had a lens no wider than a slice of cucumber. Yet with that modest tool, he saw the rings of Saturn, the moons of Jupiter—and sparked a scientific revolution that toppled Earth as the center of the cosmos. Astronomy has come a long way since then. But when the James Webb Space Telescope launches in December, with a 6.5-meter mirror that would tower over Galileo himself, it will open views of the universe’s first stars and galaxies, probe the atmospheres of planets around other stars—and launch another revolution. “James Webb will blow the lid off everything,” says exoplanet hunter Sasha Hinkley of the University of Exeter.

Webb’s mirror has more than five times the light-gathering power of the 31-year-old Hubble Space Telescope. Unlike Hubble, Webb will work in the infrared, allowing it to see the heavily “redshifted” light of distant objects and peer through clouds of obscuring dust. It will also be able to sift exoplanet atmospheres for gases whose infrared fingerprints are mostly off-limits to ground-based observatories. “We’ve never looked at the universe at these wavelengths and these depths and resolution,” says Steve Finkelstein of the University of Texas, Austin, who will lead several projects looking at distant galaxies in Webb’s first year. “I think we’ll be in for some surprises.”

More than 1000 teams of astronomers from across the globe applied for time on Webb in its first year, and 286 got lucky. They will task the telescope with a range of inquiries: looking for ice-covered oceans on Uranus’s 27 moons, searching for elusive medium-size black holes, and resolving conflicting measurements of the expansion rate of the universe. But broadly speaking, they will use Webb to pursue two overriding themes, at opposite extremes of time and distance: the early universe and nearby planetary systems.

Researchers want to glimpse the universe’s gargantuan first stars and study how messy clumps of them evolved into orderly, spiral galaxies. They want to witness the evolution of the giant black holes at the centers of many galaxies and chart their role in clearing a fog of neutral hydrogen gas that filled the early universe. In the nearby universe, Webb is expected to transform our knowledge of how gas and dust around young stars coalesce into planets, what conditions on those worlds are like, and whether they provide a welcoming habitat for life.

Just 40 light-years away, the seven known planets of the TRAPPIST-1 system (artist’s concept) are Earth-size. The James Webb Space Telescope will probe the composition of their atmospheres.NASA/JPL-CALTECH

It is a dream telescope, and one that has been many years—and $10 billion—in the making. Early on, Webb was expected to launch in 2011 for less than $2 billion. But the complexity of new technologies such as the segmented mirror and the cryocoolers that keep instruments cold enough to see far-infrared wavelengths led to delays and inflated costs. After Congress threatened to cancel the project in 2011, NASA set a new schedule and, for a while, kept to it. But more delays came when prime contractor Northrop Grumman discovered faulty welds, incorrect lubricants, missing bolts, and tears in the telescope’s giant sunshield.

Now, all that is fixed, but one last hurdle remains. In September, engineers in California packed Webb up in an environmentally controlled shipping container and sent it by sea to French Guiana, home to Europe’s spaceport. On 18 December, Webb will endure the roar and rattle of a 27-minute ride to orbit on a European Ariane 5 rocket, its folded-up mirror just barely fitting inside the fairing.

A month of critical maneuvers will follow as the telescope cruises deeper into space. “It’s 30 days of terror,” says Garth Illingworth of the University of California, Santa Cruz (UCSC), who was one of the project’s early architects. As soon as it’s in space, Webb will deploy its solar array and then, 2 hours later, its communications antenna. On day 3, as it passes the Moon, its huge sunshield will begin to unfurl. By day 11, the mirrors will start to unfold and swing into place. Finally, after 29 days, Webb’s boosters will make a final burn to put it in orbit around L2, a gravitational balance point 1.5 million kilometers from Earth. Unlike Hubble, Webb will be too far away to be repaired by visiting astronauts. It must work flawlessly straight out of the box.

The Webb telescope, folded and towering above the engineers around it.
After a sea journey to its launch site in French Guiana, the James Webb Space Telescope was extracted from its shipping container in October. It will remain folded when placed inside the fairing of an Ariane 5 rocket.NASA/Chris Gunn

Planning for Webb began as far back as 1989, before Hubble had even left the ground. It was originally nicknamed the First Light Machine, recalls Marcia Rieke, an astronomer at the University of Arizona who served on the project’s working group in the late 1990s. The goal was to peer back to the universe’s infancy.

Ground-based telescopes at the time could barely see halfway back across the 13.8-billion-year history of the universe. Hubble, with no atmosphere to blur its view, could take astronomers much closer to the beginning. In its first “deep-field” exposure, in 1995, it stared at a seemingly empty patch of sky in Ursa Major for 140 hours. Almost every one of the 3000 objects that popped into view was a distant galaxy, shining from times as early as 1.5 billion years after the big bang.

Subsequent exposures went even deeper in time. Astronomers assumed the number of galaxies would fall off sharply at those early times because gravity had not yet pulled clouds of gas into stars, let alone assembled stars into galaxies. But Hubble showed the galaxies were there, albeit in dwindling numbers. In 2016, researchers using Hubble data found a small galaxy, dubbed GN-z11, that dated, astonishingly, from a time when the universe was just 400 million years old. “We had no idea you could see objects at that time,” Illingworth says.

At that distance, the expansion of the universe shifts a galaxy’s visible light—where stars tend to shine brightest—well into the infrared, which Hubble cannot see. GN-z11 was only visible because it shone brightly in ultraviolet light, seen as visible light after redshifting. Many more early galaxies, invisible to Hubble, may lurk in the infrared band. “It’s hard to be sure,” Rieke says.

Webb should be able to tell, thanks to its huge mirror and infrared detectors, which are insulated from heat that would degrade their sensitivity. The multilayer fabric sunshield, as big as a tennis court, will create a shadow deep enough to passively chill three of Webb’s instruments to –234°C, or 39 K. The cryocooler will refrigerate a fourth instrument, meant to peer further into the infrared, to 7 K.

Roberto Maiolino, an astronomer at the University of Cambridge, expects Webb to find 10,000 galaxies between cosmic dawn—when the first stars ignited about 200 million years after the big bang—and cosmic noon, the peak of star formation roughly 2 billion years later. The first galaxies probably started out small and disorganized, nucleating around clumps of dark matter, unseen stuff that makes up 85% of the matter of the universe. By compiling a census of early galaxies, Webb will show “how these blobs change to more organized structures,” says Rieke, who led the development of Webb’s near-infrared camera.

Mapping out how protogalaxies formed might also reveal something about the nature of dark matter, Rieke says. “Finding the first aggregations of stars may tell us more about what was leftover after the big bang,” she says. There are other fundamental questions: Did the galaxies grow simply by pulling in more gas, or through a series of mergers, known to spark bursts of star formation in more recent galaxies? “There is so much low-hanging fruit, so many obvious questions … but we didn’t have the technical ability to answer them yet,” Finkelstein says.

A major coup for Webb would be spotting evidence for first-generation stars, known as population III stars, formed from the primordial hydrogen and helium gas leftover from the big bang. Later generations of stars contain heavier elements, forged in stellar furnaces and scattered by supernovae, that radiate energy efficiently. Lacking these radiators, population III stars swell to enormous sizes, and are possibly up to 1000 times as massive as the Sun. Their size means they burn fast and furiously, exhausting their fuel in a few million years.

Webb will almost certainly not be able to see to these stars individually, but a galaxy’s spectrum can betray their presence. “If we see a galaxy with a spectrum of only hydrogen and helium, that would be a smoking gun” of population III stars, Maiolino says. The spectrum would also hold clues to the stars’ abundance and temperatures, giving astronomers a picture of early star formation and how these fast-burning giants, after exploding in supernovae, delivered the first smattering of heavy elements to the universe.

A diagram of Webb, an explanation of its imaging, and a timeline showing how it will see further into the past than Hubble.
(Data) Ryan MacDonald/Cornell University; NASA; (Graphic) C. Bickel/Science

Those discoveries could also bear on the mystery of what ionized the hydrogen gas that fills the universe, making it transparent to light. Roughly 400,000 years after the big bang, the universe had cooled enough for protons to hook up with electrons. The resulting neutral hydrogen acted as a cosmic fog, absorbing high energy photons. But half a billion years later, the fog began to clear as something split the hydrogen apart again. This “Epoch of Reionization” continued for another half-billion years until all the hydrogen was ionized.

Ultraviolet light from big and hot population III stars is an obvious ionizing source. But were there enough stars to ionize all of space? The small number of early galaxies discovered by Hubble would suggest not, but Webb’s more complete census may cement the role of starlight. If not, there is another possibility: the supermassive black holes at the heart of most galaxies. As material sucked into a black hole swirls down the cosmic drain, friction heats it to such enormous temperatures that it shines as a brilliant beacon: a quasar or active galactic nucleus (AGN).

Astronomers have found quasars less than 700 million years after the big bang, but many think they are too few and far between to evenly ionize all of space. Others are more hopeful. The quasars spotted so far are huge, with masses equivalent to 1 billion Suns, which means many smaller ones remain to be discovered, Finkelstein says. “There are probably a lot of 100-million-solar-mass black holes and even more 10-million-solar-mass black holes,” he says. “You just have to have the right tools to find them.”

If Webb does find the early universe swarming with black holes, astronomers will be forced to reckon with their speedy assembly. Did giant population III stars collapse into black holes, providing “seeds” that merged into supermassive ones? Or did black holes form directly from collapsing clouds of gas after the big bang? “It’s hotly debated,” Maiolino says.

quotation mark

We will see the first chemical fingerprints from Earth-sized planets.

  • Natalie Batalha
  • University of California, Santa Cruz

Besides probing the hectic birth of stars, galaxies, and black holes, Webb will study why the frenzy died down. “Forming stars and galaxies is a party that is over,” says Jane Rigby of NASA’s Goddard Space Flight Center, who is Webb’s project scientist for operations. Astronomers believe some process—supernovae explosions, blasts from AGNs, or stellar winds produced by giant stars—could have blown clouds of gas out of galaxies, starving star formation of its fuel.

One of Webb’s four instruments, the Near Infrared Spectrograph (NIRSpec), should be able to see these outflows and trace them to their source. The NIRSpec has an innovative mask with nearly 250,000 slitlike microshutters, each about the width of a human hair, that operators can open to let light through at specific locations. By analyzing light from each slit separately, the NIRSpec can dissect the spectrum of light from multiple points on the sky. It could, for instance, measure redshifts and calculate distances for 100 different distant galaxies, all at once. But it can also focus in on multiple slices of a single galaxy, such as one undergoing a slowdown in star formation. “It will get gorgeous spectra of each distinct region of a galaxy,” Rigby says, allowing observers to identify gas clouds and assess their temperatures and movement—along with what seems to be blowing them out. “We’ve never had that power before,” Rigby says.

After 10 years of concept studies and false starts, in 1999 NASA formally greenlighted what was then called the Next Generation Space Telescope. But by then, a new goal had emerged: understanding exoplanets, first discovered in the 1990s.

In the early years, exoplanet discoveries were few—“stamp collecting,” says Natalie Batalha of UCSC. But NASA’s Kepler mission, launched in 2009, stared at a patch of sky for years on end, continually monitoring 150,000 stars for the telltale dips in brightness that betray the passage, or transit, of an orbiting planet across the star’s face. Kepler found thousands of planets, showing exoplanets were not a galactic rarity, but the norm.

Webb isn’t aiming to discover lots of new planets; its time is too precious to sit and wait for brightness dips to occur. What it will do is get to know exoplanets better—first by witnessing their formation, something that researchers had previously only theorized about. The Atacama Large Millimeter/submillimeter Array (ALMA), a set of radio telescopes in Chile, has in recent years zoomed in on protoplanetary disks, the swirls of dust and gas around young stars that coalesce into planets. ALMA’s images show clear gaps in the disks, suggesting protoplanets are vacuuming up material as they orbit. But ALMA can only see the faint radio emissions from cold dust and gas in the outer parts of the disks—not the planets themselves.

At shorter infrared wavelengths, Webb will see newborn planets, which glow in the infrared. It will be able to dissect the chemical makeup of the planet-forming material and how it varies across the disk. “It’s important to know the environment in which the planet is formed,” says Isabelle Baraffe, a theorist at Exeter. “It helps to constrain the models.”

Webb will also dive into planetary atmospheres. Astronomers using Hubble and an infrared telescope far smaller than Webb, the Spitzer Space Telescope, pioneered the key technique, called transit spectroscopy. During a transit, the planet’s atmosphere absorbs a small amount of the starlight, resulting in tiny changes in the star’s spectrum. By watching transits, Hubble sensed water vapor in a few exoplanet atmospheres, and Spitzer found methane and carbon monoxide. Webb, sensitive to a large swath of the infrared, should find all those gases and more, including ammonia, acetylene, and hydrogen cyanide. “Webb will blow the door wide open,” says Nikole Lewis of Cornell University.

A telescope image of many galaxies.
A disc surrounding a star.
The James Webb Space Telescope will reach deeper in time than Hubble, which captured “deep field” images (first image), revealing galaxies that shone 800 million years after the big bang. It will also zoom in on gaps in disks of gas and dust around some young stars, seen in radio telescope images (second image), which are thought to mark planetary orbits. Images: (First) NASA/ESA/S. Beckwith (STSCI) and the HUDF team; (Second) ALMA (NRAO/ESO/NAOJ)

The absorption signatures are strongest when the starlight passes through a thick atmosphere, so Hubble and Spitzer focused on big, puffy planets that orbit close to their star—so-called hot Jupiters. With its heightened sensitivity, Webb will be able to sniff out molecules in the thinner atmospheres of rocky planets. “We will see the first chemical fingerprints from Earth-sized planets,” says Batalha, who has assembled a team of about 60 collaborators to test transit spectroscopy in one of Webb’s largest early programs.

Few think Webb will have sharp enough eyes to sense signs of life in an exoplanet’s atmosphere, such as oxygen, ozone, or chlorophyll. But it will go a long way toward understanding the variety of atmospheres and how habitable they are. “Webb will give us the demographics of atmospheres,” Batalha says. “It’s like moving from silent movies in black and white to color TV.”

So intense is the interest in Earth-size planets and their suitability for life that in 2018 NASA launched a “spotter” for Webb: a space mission to identify the best candidates for atmospheric studies—rocky planets around dim red dwarf stars, which do not swamp a planet’s light as much as other stars. The Transiting Exoplanet Survey Satellite (TESS) has in its first 3 years found 159 confirmed planets and has another 4500 awaiting confirmation.

A first task for Batalha’s team will be to solve a mystery posed by the planetary inventory Kepler and TESS have assembled: The most common exoplanets are a type that doesn’t exist in the Solar System, with a mass between that of Earth and Neptune. “We don’t really understand what they are,” Batalha says. Webb should help. If the mystery planets are smaller versions of Neptune, a gas giant, they should have a thick atmosphere dominated by primordial hydrogen mixed with water, carbon dioxide, and carbon monoxide. If they are rocky bodies—super-Earths—their atmosphere should reflect geological processes, like volcanism, that often spew molecules such as methane or ammonia.

Transiting planets also periodically pass behind their star; Webb will compare the star’s light just before and during these eclipses. Prior to the eclipse, the light includes both the star’s emissions and the much fainter light from the planet, whereas the eclipse itself blots out the planet’s light. By comparing the two signals, astronomers can infer the planet’s own glow, which betrays information about temperatures and cloud cover.

The combined light of the star and orbiting planet should change subtly as observers receive light from the planet’s dayside, nightside, and varying mixtures of the two. The resulting “phase curve” reveals the dynamics of an atmospheric system: how the atmosphere shuffles heat from the hot dayside to the cooler nightside, which in turn affects winds and cloud movement. “We will also be able to tease out 2D and 3D effects, such as clouds on one side and not the other,” Lewis says. “It’ll keep us busy for a long time.”

Webb will also take pictures of exoplanets directly, with the help of a coronagraph, a mask within the telescope that blocks the overwhelming glare of the star so that the tiny dim planets around it can be seen. The largest telescopes on Earth can only discern the largest planets, more than twice Jupiter’s mass, in wide orbits. Furthermore, only young planets have been imaged, ones glowing with the leftover heat of their formation, not just the much dimmer reflected light of the star.

Webb, designed to catch this infrared glow without the blurring interference of Earth’s atmosphere, will be able to see smaller planets later in life, orbiting closer to their stars. “Something like Jupiter would be great,” Baraffe says; researchers wonder whether the composition and temperature of an alien Jupiter will be anything like the homegrown version. “Will it share general characteristics or be very different because it formed in different conditions?” Baraffe asks. But directly imaging an Earth-like planet and searching for signs of life will have to wait for an even larger and more capable space telescope.

For many, the first data from Webb will prompt a shift from one tension to another: from anxiety over its launch and deployment to pressure to make the most of its limited life. Whereas Hubble could be refueled and upgraded by visiting astronauts, extending its life for decades, the clock is ticking for Webb: It’s designed to last 5 or 10 years, but beyond that nothing is certain.

The limiting factor is propellant. Webb needs its thrusters to maintain its orbit around L2 and to occasionally dump angular momentum from the reaction wheels that point the telescope and keep the sunshield in position. Without propellant, Webb will drift from L2 and eventually fall into a fatal spin. It won’t be able to charge its solar-powered batteries or communicate with Earth. Webb will see no more.

Finkelstein will rue that day, but he is confident he will be plenty busy in the meantime. “I’ve basically cleared the next decade from my schedule.”



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