What the giant James Webb telescope will see that Hubble can't

Astronomers like Christine Chen are thrilled about the looming launch of the James Webb Space Telescope.

The $10 billion instrument, half the size of a 737 airplane and replete with gold-coated mirrors, will orbit 1 million miles from Earth and peer into places humanity hasn't seen before. This includes some of the first stars ever born, the most distant galaxies, and curious planets in the cosmos.

"It's really cool," marveled Chen, an astronomer at the Space Telescope Science Institute, an organization that will run the James Webb Space Telescope, or JWST.

The instrument's deeply-anticipated launch is currently set for Dec. 18, 2021, though in recent months the telescope has often been in the news for reasons unrelated to its scientific endeavors. NASA named the remarkable instrument after James Webb, NASA's leader in the 1960s who oversaw the agency during a time when the federal government persecuted and fired LGBTQ employees from NASA and other departments. It was a shameful time in American history called the "Lavender Scare." For now, NASA said it will keep the JWST label after finding no evidence about Webb that "warrants changing the name."

JWST, originally dubbed the "Next Generation Space Telescope" in the 1990s, will join the legendary Hubble Space Telescope in capturing clear views of the universe from space. Hubble is a scientific treasure. Over the three decades it's orbited 340 miles above Earth, Hubble has provided unprecedented, brilliant views of the cosmos, galaxies, and planets. Yet JWST is not a replacement for the aging Hubble. JWST is a successor, with different, and advanced, abilities.

Here's what JWST, and ultimately you, will see that Hubble can't.

Seeing into the deep, deep past

Telescopes with bigger mirrors can see fainter objects. That's because larger mirrors capture more light. Imagine particles of light as ping pong balls, and a telescope's mirrors as a bucket. Hubble's mirror is around eight feet in diameter, while JWST's mirror is much larger, at over 21 feet in diameter.

"You're going to collect more ping pong balls," explained Jean Creighton, an astronomer and the director of the Manfred Olson Planetarium at the University of Wisconsin–Milwaukee.

Capturing more light is critical for observing the earliest stars and galaxies that formed in the universe, over 13 billion years ago. The universe is incessantly expanding, meaning it's constantly grown or stretched since its violent inception (the "Big Bang"), so the light from these ancient parts of the cosmos is very, very, very far away (billions of light-years). The most far-off light left stars billions of years ago, so observing this light is like peering into the deep, deep past.

"We're looking back in time," said Chen. (Even when we look at our own star — with protection — we're also peering into the past; it takes over eight minutes for sunlight to reach Earth.)

 "We're looking back in time." 

And we'll see unprecedented things.

"We're going to see the very first stars and galaxies that ever formed," said Creighton. "We have not been able to do this with Hubble."

Hubble can see faint light that's about 1 billion years old. If all goes as planned, JWST will see light that's nearly 13.7 billion years old, when the earliest stars and planets started to form.

Lifting the veil

Hubble largely views light that humans can see (aka "visible light"). But there are many types of light that our eyes can't see. JWST is specialized to observe one of these, called "infrared," which allows astronomers to see vastly more stars and planets.

How so?

The universe is filled with thick, smoke-like clouds of dust and gas. "That obscures things," explained Jason Steffen, an assistant professor of physics at the University of Nevada, Las Vegas, who researches planets outside our solar system (aka exoplanets).

 "It lifts the veil." 

But infrared light can slip through thick clouds of dust. Infrared has longer wavelengths than visible light, so the light waves don't get scattered as much (and obscured) by particles in the universe. Longer wavelengths, whose peaks and valleys are spread far apart, are less likely to collide with particles in space.

The image comparison below, taken by Hubble, shows how infrared light slips through cosmic dust. That's the Carina Nebula, a dazzling cloud of dust and gas. Hubble's infrared view reveals the many stars hidden behind the dust.

"It lifts the veil," said Creighton.

What's more, viewing extremely distant galaxies and stars is made much easier (or at times possible) by seeing them in infrared light. As described earlier, the universe is constantly expanding, and the light traveling through the cosmos stretches, too. "If you're looking at a distant galaxy, that light has been stretched out," explained Steffen. The light's wavelengths become longer.

That's a problem. "This can make distant objects very dim (or invisible) at visible wavelengths of light, because that light reaches us as infrared light," writes NASA. But JWST's infrared views make the invisible visible.


There are few things in the universe more enthralling than exoplanets. Why, some of these known planets, like "super-Earths" some two to 10 times the size of our planet, might contain habitable, rocky environments, or even water. JWST will spend a significant amount of time viewing exoplanets in other star systems. (To JWST, these planets will appear as dots, not grandiose, colorful planets.)

Already, NASA has confirmed finding over 4,500 exoplanets in the universe. But with JWST's ability to peer through clouds of dust, the astronomer Chen expects to find considerably more.

Crucially, JWST won't simply sleuth out the existence of exoplanets. It will analyze their atmospheres. The telescope carries an instrument called a spectrometer that can reveal what particles are composed of, based on how light reacts with them. (Light passing through water vapor or oxygen, for example, behaves in certain, well-known ways.)

Even a relatively unexciting spectrograph image can be considerably more useful to astronomers than a brilliant picture. "There's a whole lot of information," explained Creighton."It gives much more information than the pretty picture does."

Of particular interest to scientists are the rocky, seven known planets orbiting the star TRAPPIST-1, some 40 light-years (235 trillion miles) from Earth. Perhaps JWST will discover a rocky planet that has hints of potential life in its atmosphere, like the oxygen algae and plants breathe into our air.

Before any of this cosmic science begins, however, JWST has a great, if not daunting, journey ahead.

After a nail-biting launch (the robotic cargo is unusually precious), JWST must make the 1 million-mile journey through space. Its tightly packed hexagonal mirrors must unfold properly, and its tennis-court-sized sunshield must unfurl as planned. And unlike with Hubble, astronauts can't launch into space and fix any potential problems with the extremely distant telescope.

The launch looms large in the astronomical world. "Everyone's crossing their fingers," said Steffen.

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