How the Webb telescope will show us planets like never before

When the James Webb Space Telescope (JWST) begins operations over the summer, it will be training the largest, most powerful set of mirrors and instruments ever launched into space on some of the most distant and fascinating targets in the cosmo: The very first stars and galaxies to form in our universe, of course, but also exoplanets.

JWST is not really an exoplanet hunter, but with its 6.5-meter diameter primary mirror and infrared spectroscopy instruments, it’s perfectly suited to peer more closely at these distant worlds than ever before. Telling us what they are made of and, potentially, if there are signs of life in their atmospheres.

Cornell astrophysicist Nikole Lewis says she plans to devote some of her JWST observing time to a “deep field” exploration WASP-17b. It’s a “hot Jupiter” exoplanet about 1,000 light years from Earth. The telescope will spend “80 hours looking at a single planet in all directions using a broad range of instruments, which will allow us to start to understand what the different parts of the planet look like,” Lewis tells Inverse. Combining measurements of temperature, cloud structure, and atmospheric chemistry, “we’re going to really be able to paint a 3-D picture of what this particular hot Jupiter WASP-17b looks like,” she says.

And what will such an exoplanet look like? Paradoxically, it will look like not much at all and like nothing we’ve seen before. It’s a bit complicated, but the results may yet reframe our understanding of our place in the universe.

What will exoplanets look like to JWST?

“Full disclosure,” Lewis says, “we’re not going to get pretty pictures of exoplanets.” JWST is big and powerful and will see billions of years into the past, but resolving a distant exoplanet next to its star so that it looks like a Hubble or Voyager image of a planet in our Solar System is still far beyond its powers.

We’ll see exoplanets directly, Lewis says, the larger ones anyway, but they’ll appear as “just one bright dot.”

Don’t get disappointed. That dot is just the beginning. JWST will help build a more complex picture of distant exoplanets over time by mapping them in more detail than ever before and looking at neglected wavelengths.

“When we look at planets, we think of them as they look like in optical because of the light reflected off of them,” Lewis says. “But if you really want to tease into what makes them tick, you want to look at them in the infrared,” like if you want to know if there are organic compounds in their atmospheres.

The venerable Hubble Space Telescope has done amazing astronomy, but it sees primarily in the optical, UV, and near-infrared wavelengths. The now-retired Spitzer Space Telescope was tuned to the infrared, but it was retired in 2020, and while Lewis points out it conducted great exoplanet astronomy, it was never designed for such a mission.

There are also ground-based telescopes that can see in the infrared, but certain wavelengths are inaccessible to them due to the filtering effects of Earth’s atmosphere. All together, that means “We were able to find chemical fingerprints in the atmosphere,” of exoplanets, Lewis says, “But in almost all cases, we treat the atmosphere as being uniform, homogenous, we treat it as a one-dimensional object basically.”

Based in space and optimized for a wide swath of the infrared spectrum, Webb will provide data scientists can use to create truly multi-dimensional models of exoplanets. To understand how their atmospheres are structured and what makes up their composition.

“We’re going to be able to look at signatures from things like carbon dioxide, carbon monoxide, methane, all sorts of fun species,” Lewis says. “We can start to move away from that one-dimensional view of the planet and start to understand what it looks like in two in three dimensions.”

What will our Solar System look like to JWST?

While Webb’s capacity to study the most extremely distant objects in the universe rightfully garners a lot of attention and excitement, the space telescope will spend a lot of time peering deeply at objects closer to home as well.

Heidi Hammel, an interdisciplinary scientist involved with Webb since the early 2000s, will be using her observing time to look at just about everything visible in our Solar System outside the orbit of the Moon, from Mars, to asteroids, the outer planets, and even the strange frigid worlds of the Kuiper belt.

She may be most excited about viewing Uranus. The ringed and tilted ice giant planet has only been visited once by Voyager 2 in 1986, and it just so happens it orbits at just the right distance for an optimal field of view for Webb. We really will get some great photos of Uranus with Webb, though, of course, they’ll be in infrared.

In explaining what Uranus will look like through Webb, she refers to a collection of images of the gas giant taken by Hubble, the Keck Observatory, and the European Very Large Telescope (VLT). The blue and pinkish cloud tops are visible in the optical and near-infrared images taken by Hubble and Keck, but the mid-infrared images taken by the VLT appear like somewhat blurry, blunted Eyes of Sauron, or a lump of hot coal in the back of a furnace.

“Webb will have better image quality,” Hammel says. “We’ll be able to tighten up these images, and then they won’t look so mottled.”

Webb will allow Hammel and other planetary scientists to better understand how Uranus's upper and lower atmospheres interact. Webb’s spectrometer will enable them to map the planet's chemical composition like never before.

“Where is methane coming from? Where is ethane coming from?” Hammel says. “We’re going to be able to tease out this chemistry as a function of altitude, and figure out the linkages.

Why it matters— It’s not a coincidence that scientists looking at distant exoplanets and planets in our backyard are all interested in the spectra and chemical composition of their targets. Such observations don’t always provide immediately stunning visual images you can slap on a poster as you can with many Hubble images, but over time they can help scientists paint a broader, deeper conceptual picture of how all planets and solar systems work, including our own.

Scientists spend a lot of time trying to answer questions about how we got here, Lewis says.

How did our solar system form? How did Earth turn out to be the only habitable planet in the Solar System?

“But we’ve always had just a sample of eight things to compare to, right? And now we’re going to have a sample of 300 to 400 things,” she says. “That allows us to test our models of the physics and chemistry of what makes planets tick.”