The James Webb Space Telescope will transform our understanding of alien worlds

Kourou, French GuianaBehind glass, sealed inside a clean room, the James Webb Space Telescope looks like a museum exhibit—an artifact meant to be preserved and revered. Yet its journey has hardly begun. An army of technicians is preparing the telescope for its upcoming million-mile voyage into space, where the observatory’s golden honeycomb eye will gaze back in time, peering at the earliest beginnings of planets, stars, and galaxies.

For now that 21-foot-wide eye is closed, the telescope folded like a clamshell. Gleaming in shades of gold, silver, and crinkled lavender, the $10-billion instrument is too big to fit inside one of the world’s biggest rockets, the Ariane 5, without being folded up.

NASA is footing the bulk of the mission’s bill, but the European Space Agency, which contributed to two of the four on-board science instruments, is responsible for launching the telescope. That’s why, as soon as December 24, JWST is slated to take flight from ESA’s tropical spaceport in French Guiana—its final Earthly port of call before sailing beyond the reach of human hands.

This sprawling launch facility is carved into the fringe of the northeastern Amazonian rainforest. Portions of the spaceport are so remote that it’s not unusual to spot jaguars slinking across empty roads. Inside the tall, cavernous buildings where rockets are assembled, the intoxicating melodies of tropical birds are often louder than the clanging contraptions that ready humankind’s machines for their journeys to the stars.

In the case of JWST, the vibrant landscapes surrounding the complex serve as a reminder of the space telescope’s mission: to help scientists understand how we got here—how, from the tangle of molecules, stars, galaxies, black holes, and planets that populate the universe, the ingredients necessary for life emerged and combined to make this place called Earth. Are the conditions that favored this thriving, noisy biosphere common among the millions, or perhaps billions, of rocky planets populating the galaxy?

To search for answers, JWST will observe hundreds, maybe thousands of exoplanets before its mission is done. It will stare at hellish lava worlds with molten surfaces that whip around their stars in mere hours. It will study worlds that somehow survived the violent deaths of their stars and now orbit their leftover stellar corpses. It will gaze through the atmospheres of gas giant planets, search for newborn worlds among the dusty disks that cling to infant stars, and squint at a handful of small, rocky worlds that just might be similar to Earth.

Of the thousands of exoplanets we’ve seen in our galaxy, only a handful resemble the planets we see in our own solar system. The rest are undeniably alien.

“One of the greatest discoveries that we’ve made within the field of exoplanets is that the diversity of planets within the galaxy is just so much greater than the diversity of planets in our own solar system,” says Natasha Batalha of NASA’s Ames Research Center. “We want to understand the process that led Earth to having a habitable environment. Is the fact that we have liquid water oceans and oxygen–is that unique, or is that fairly common within the galaxy?”

But first, JWST must survive its fiery trip into space and a nail-biting series of events that have astronomers around the world giddy with anticipation.

“I’m a little bit nervous,” says ESA’s Peter Jensen, a senior consultant on the mission and former project manager, as he gazes at the shimmering clamshell through the cleanroom windows. After launching, the telescope faces a long sequence of do-or-die maneuvers that must go exactly as planned, including unfolding its golden mirror and deploying a huge, crucial sunshield. "I'm feeling very strong about launch,” Jensen says. “But I have a mechanical engineering background, so I feel less strong about all the deployables."

The Milky Way’s many planets

Like many of humankind’s most ambitious science and engineering projects, JWST’s journey to the launch pad has been mired in technical delays and ballooning budgets. More recently, a controversy erupted over the telescope’s name. But if everything goes to plan, this flagship telescope, considered the successor to the venerable Hubble Space Telescope, should deliver a cornucopia of scientific delights.

JWST will see the universe primarily in infrared light. Once its hulking golden eye opens, the observatory’s exquisite sensitivity will allow the telescope to spy the faint, faraway signatures of the first stars and galaxies that populated the universe.

That sensitivity will also help it directly observe alien worlds—though nobody could have planned to use the telescope for this purpose when it was designed. When JWST was conceived in 1989, planets orbiting other stars had not yet been discovered. Planet formation theories suggested such worlds should exist, but the first exoplanets weren’t announced until 1992.

“In the beginning, when the observatory was born, I don’t think we even spoke about exoplanets,” Jensen says. “Exoplanets came later, when we started discovering there were actually exoplanets.”

Recently, exoplanet-hunters such as NASA’s Kepler space telescope have revealed that worlds orbiting other stars in the Milky Way are as common as grains of sand on Earth. That abundance affords the JWST team a remarkable opportunity to study some of the most intriguing worlds in detail.

By blocking the light of planets’ parent stars with an instrument called a coronograph, JWST can directly image some of these faint, faraway worlds in orbit—even those that might still be forming. The telescope can peer through alien atmospheres and determine what the gassy shrouds are made of and how they’ve evolved over a planet’s lifetime. And it can read molecular signatures in the day-side glow of planets.

“How did these planets get to where they are today? How did they evolve? And what scenario could potentially lead to biosignatures—to life?” asks NASA’s Knicole Colón, who studies alien worlds and is the deputy project scientist for JWST’s exoplanet mission. “We need to study everything to answer these questions,” she says. “Because in our solar system we have all these planets, and only one definitively has life on it.”

Dissecting planetary atmospheres 

About six months after arriving in space, JWST will begin drinking in starlight and studying faraway worlds. As planets pass between their stars and the telescope, for a brief moment their atmospheres will appear in silhouette, illuminated by the starlight shining through from behind. In that starlight, Webb will read the signatures of the gases the light passed through.

A grab-bag of extreme systems and planetary oddballs are among Webb’s early transiting targets, including WD 1856b, a giant planet that orbits a stellar corpse, and HD 80606b, a planet whose orbit looks more like a comet’s elongated, lopsided loop around its star than a planet’s more circular path.

Those extreme systems could contain clues about how different kinds of planets form and survive, Colón says. And by surveying the gases clinging to a multitude of worlds, JWST will help scientists understand how a planet’s size, temperature, and evolution are linked to its climate and other characteristics. “How do all of these atmospheres compare, from Earth-size to Jupiter-size?” Colón asks.

Many of the telescope’s early observations will target hot Jupiters, a class of planets we don’t see in our solar system. Whipping around their stars in mere hours or days, these giant worlds are among the most common types of planets discovered so far by astronomers.

Natalie Batalha of the University of California, Santa Cruz, is shepherding one of JWST’s first exoplanet observing programs. She and her team will observe three hot Jupiters: WASP-39b, WASP-18b, and NGTS-10b. The researchers will put all four of the telescope’s instruments through their paces, and they will look for the ratio of carbon and oxygen in the planets’ atmospheres, which contains information about where the planets formed. The team will also compare what Webb sees with earlier observations from Hubble.

“We wanted to have a benchmark against which we could compare the data,” Batalha says. “And we’ll see what we knew from Hubble, but there will also be surprises and something completely new.”

Another intriguing type of planet that we don’t see in the solar system is the super-Earth, or the mini-Neptune—worlds that are bigger than Earth and smaller than Neptune, like Gliese 486b, GJ 1132b, and K2-18b.

“Those worlds have been just out of reach of Hubble, and they’re such an important class of planet. They’re among the most common planets we know of, and we really don’t know how they form,” says Laura Kreidberg of the Max Planck Institute for Astronomy. “We still are arguing—are these things super-Earths? Are they mini-Neptunes? Are they something else?”

Finding out whether these planets have atmospheres, and what those atmospheres are made of, is crucial for determining whether they might be habitable. Natasha Batalha, Natalie’s daughter, is co-leading one of the mission’s largest early exoplanet observing programs, which will observe about a dozen of these intermediate-size planets.

Whether such worlds could be habitable hinges on where and how they formed and how they evolved. Some could be the stripped cores of former Neptunes, Natalie Batalha says, which dims the odds for habitability. Others sit right on the border between large rocky worlds and small gas worlds–a transition that scientists don’t fully understand.

Other early targets include hot rocky planets that are snuggled up to their stars, making them more like roasted lumps of charcoal than overheated puffballs. On these worlds, which include 55 Cancri e and K2-141b, clouds made of rock and minerals might rain lava, creating a landscape that defies imagination. Kreidberg wants to find out what a lava world called LHS 3844b’s surface is made of and see if there’s an unexpected, teeny tiny atmosphere clinging to it.

“The rocky planets—I think that’s where JWST will really make its mark,” Kreidberg says. “Not so much in biosignatures, but in these most basic questions. Under what conditions would you expect an atmosphere to survive? How hot can you make a planet before an atmosphere escapes? If the planets do have atmospheres, what are their basic building blocks?”

JWST will also target each of the seven rocky, roughly Earth-size worlds in the TRAPPIST-1 system. These planets orbit a small, nearby star about the size of Jupiter, and three of them are temperate. TRAPPIST-1e, among the early targets, offers perhaps the best chance of detecting Earth-like conditions, while TRAPPIST-1c, another early target, is closer to its star and probably Venus-like in temperature.

“The goal of that program is very simple,” says Kreidberg, who is leading the TRAPPIST-1c observation. “It’s just to figure out if the planet has an atmosphere or not. We have no clue.”

Taking pictures of planets

Not all of JWST’s planetary targets transit their parent stars. Some trace orbits hundreds of times farther from their star than Earth is from the sun, circling in systems that we can see from the top-down (or bottom-up).

A portion of the early exoplanet observations will directly image planets in these kinds of systems by blocking out the host star, although the planets will just look like small pricks of light in the images. Most of these observations will target large planets–gas giants and maybe ice giants–that are very far from their stars and therefore easier to spot when the star’s light is erased. Such observations will let scientists learn more about the structures of atmospheres, the types and amounts of clouds that might exist, and the relative amounts of molecules such as methane and carbon monoxide that can point to a planet’s birthplace.

“Planet formation is an incredibly messy process. There’s just debris and dust everywhere,” says Sasha Hinkley of the University of Exeter, who is leading one of the early direct imaging programs. When astronomers use JWST to measure the composition of a planet, they will need to sort out which material is intrinsic to the planet and which was picked up as it formed and evolved. “Nature and nurture,” Hinkley says. “That’s what we’re trying to disentangle.”

Direct imaging will also help scientists hunt for planets around stars such as Alpha Centauri A, one of the nearest to the sun. For years scientists have wondered whether worlds orbit our nearest sunlike neighbor, although early evidence for one such world is awaiting confirmation. Other systems that JWST will photograph include HR 8799, where at least four large planets revolve around their host star; Beta Pictoris, which hosts at least two large planets; and 51 Eridani, which hosts one of the coolest and lightest planets discovered by direct imaging to date.

The long search for life

While JWST will transform our understanding of the worlds that populate our galaxy, the telescope is unlikely to spot signs of life—unless scientists are extremely lucky. That type of observation is just on the edge of the telescope’s capabilities, and it would require an oversized chunk of observing time.

“Finding life is going to be hard, and I’m not super confident that we will detect biosignatures, but I think we will be able to say something about the atmospheres of these planets around small stars,” says Kevin Stevenson of the Johns Hopkins Applied Physics Laboratory, who will observe five rocky planets with JWST.

Detecting distant alien biosignatures means looking for combinations of gases or chemical elements that scientists wouldn’t expect geologic processes alone to produce. And while astrobiologists have some ideas about what we could look for—methane, ozone, and other metabolic byproducts—it’s not clear what form alien life’s signatures might take.

Among the early planetary targets are a handful of rocky, Earth-size worlds—but they orbit small, reddish stars that are prone to tempestuous outbursts of radiation that might sterilize their surfaces. Whether those worlds have atmospheres at all is the first thing JWST will attempt to determine.

“There are still so many unanswered questions about whether or not a planet that is around such a small star could even have an atmosphere,” Natasha Batalha says. “Forget biosignatures—can we even sustain atmospheres?”

JWST can, however, lay the framework for future attempts to detect biospheres.

“What we’ve seen so far is that every exoplanet is a snowflake, in the sense that it is unique, and it just seems like we can’t make heads or tails of the population as a whole,” Stevenson says. “I think Webb will give us that full picture, and it will open the door for us to be able to understand these planets.”

Ultimately, the telescope will help scientists learn whether studying atmospheres in silhouette—as planets transit their stars—is likely to be a successful technique for detecting signs of life.

“We may see that we can’t pull out the signals that we’re looking for,” Stevenson says. “There may be a fundamental limit to what we can learn about potentially habitable planets using the transit technique. And that’s fine, because once we understand that limit, we can move on to other techniques.”

Although JWST isn’t off the ground yet, scientists are already designing its successor—a large space telescope with the specific mission of detecting biosignatures on Earth-like alien worlds. Such an instrument won’t launch for decades, making JWST’s exoplanetary observations even more crucial in the interim.

“The public wants a life detection, but all of this is necessary for understanding where the most likely abodes of life are going to be,” Natalie Batalha says. “You have to understand the physical processes that drive diversity in order to understand where habitable environments reside.”

For a few days more, all this powerful potential is perched on top of a rocket, surrounded by dense tropical forests stretching all the way to a warm, coffee-colored sea. Maybe someday, we’ll learn whether riotous jungles cover alien landscapes, whether otherworldly insects swarm in faraway atmospheres, and whether extraterrestrial rivers and seas also cradle life.

The first step is getting to know our exoplanetary neighbors, and JWST will help us see them in more detail than ever before.