There Is a Planet With Clouds Made of Sand
The James Webb Space Telescope’s new observations of exoplanet atmospheres are quietly thrilling.
Now that the world’s most powerful space telescope is finally up and running, we’re in for a constant stream of stunning images of the universe. Just a ton of galaxies everywhere, more detailed than you’ve ever seen them, and too many stars to count—all of it sparkling with an intensity that humankind hasn’t captured before.
Not every interesting image from the James Webb Space Telescope is going to be a pretty picture, though. Some will be charts, like the one that NASA unveiled last week, all lines and dots and squiggles. That chart doesn’t have the special oomph of Webb’s view of galaxy clusters, or the wow factor of the Carina Nebula snapshot. But the unassuming chart, and the many others like it that Webb will produce over the next 20 years, is quietly thrilling in its own way. These images will stretch our imagination about what the worlds that exist far beyond our solar system are like.
You do have to know what you’re looking at, first. The chart reveals detailed information about the atmosphere of a giant, gaseous exoplanet with Jupiterlike qualities. WASP-96b resides about 1,150 light-years from us, tracing a loop around its own sun in just 3.5 Earth days. Its temperatures are higher than 1,000 degrees Fahrenheit (about 538 degrees Celsius). By measuring starlight that passed through the exoplanet’s atmosphere, Webb was able to detect the unmistakable signature of water, and even found evidence of clouds. On a planet as scorching as this one, clouds are really something else. It’s so hot that rock can condense in the air the way that water does here on Earth. Which means that on WASP-96b, the clouds are made of sand.
Sand! When Nikole Lewis, an astrophysicist at Cornell, told me this, I nearly fell out of my chair. “There are lots of things that will condense, other than water, given the right pressure and temperature conditions,” Lewis explained, as if she hadn’t just upended my idea of something as seemingly simple as clouds. I looked out my window, at the clouds of this planet, fluffy white poofs hovering in a blue sky, and Earth suddenly felt like exactly what it was, one of many, many planets in the universe, each with its own story and potentially its own kind of atmosphere. This is what Webb is poised to do: transform exoplanets in our mind from unknowable cosmic objects to very real places with alien skies.
Astronomers so far have discovered more than 5,000 of these exoplanets using other space telescopes and ground-based observatories. They’re quite skilled at discerning, upon discovery, an exoplanet’s orbit, mass, density, and other fundamental properties. And they’re getting better at teasing out information about their atmospheres. The Webb mission will send that particular effort into overdrive, revealing hidden details in targets that astronomers have already studied, uncovering the unknowns of worlds that they couldn’t reach before, and detecting, perhaps, the molecules that we know to be associated with the presence of life.
Webb observes exoplanet atmosphere this way: The space telescope aims its gaze at a star system and waits, basking in the incoming light. When a planet comes into view, moving—from our perspective—across the face of the star, the world blocks a little bit of starlight. But some light makes it through, and it filters through the planet’s atmosphere on its way out. The light arrives at Webb carrying the chemical signatures of any gases in those cloudtops. The telescope’s detectors break apart the light, like a glass prism splitting sunlight into rainbow colors. Astronomers pore over those data, picking out signs of familiar molecules and compounds, and then display them all on a plot known as a spectrum.
In the WASP-96b spectrum, the peaks indicate the presence of water vapor. To spot the evidence of clouds and haze in there, it helps to be a trained astronomer. “When there are clouds and hazes in the atmosphere, they will actually cause the water vapor—those signatures, those big humps you’re seeing—to be muted, so they’ll actually be a little bit lower than we expected,” Lewis explained. “That’s because the light is also passing through those clouds and hazes, and that’s subduing the strength of that water-vapor feature.”
When NASA released the snapshot of WASP-96b’s atmosphere, Lewis and her colleagues raced to see how the findings compared with other observations of the exoplanet, particularly by the Hubble Space Telescope. Webb had spent only a few hours observing the exoplanet and its star, and yet the error bars on the new data were far smaller than previous, more time-consuming efforts, Lewis said. And Hubble hadn’t detected any signs of clouds at all. The exoplanet spectrum looked as magical to exoplanet scientists as that galaxy-strewn deep field was to galaxy researchers. Just as Webb can produce galaxy-strewn deep fields in a fraction of the time it took Hubble, Lewis said, “we can do the same thing with exoplanets, where we only need to observe the planet, say, once or twice, as opposed to 10 times,” in order to find the little features they’re most interested in.
Astronomers want to use Webb to spot compounds more intriguing than water, such as oxygen, methane, and carbon dioxide—or, better yet, more than one in the same atmosphere. “If you, like, took some water and some methane and put them in a box and left them at room temperature on Earth, they’d actually combine into carbon dioxide, and you wouldn’t expect there to be any methane left over,” Megan Mansfield, an astronomer at the University of Arizona who will use Webb to study exoplanets, told me. “The only reason we have methane in the Earth’s atmosphere is because it’s constantly produced by life.” Finding some particular combination of gases that shouldn’t appear together, not unless some form of life was producing at least one of them—that’s the dream.
As intriguing as such a detection would be, Webb won’t show us definitive proof of alien life. The space telescope can only reveal the presence of something intriguing, leaving astronomers to work out the exact cause. Astronomers are still locked in a heated debate over the origins of a gas that might be present in Venus’s atmosphere, and that planet is right next door. This kind of work will be even more difficult when scientists are dealing with planets many light-years away. “I don’t think we’ll necessarily be able to say there’s definitely life on a planet, but I think it is possible that we would find some really interesting planets that we’d want to follow up on,” Mansfield said—potentially with a whole new space telescope. When astronomers first started talking about building a telescope like Webb more than 30 years ago, exoplanets had not yet been discovered. It was only later, as exoplanet science began to blossom into a real field, that the necessary technical capabilities were added.
But until the next great space telescope comes along, Webb will familiarize us with the range of alien atmospheres in the cosmos. Nearly one-quarter of the telescope’s first year of observations will be devoted to studying exoplanets. Astronomers can search for other kinds of atmospheric gases, and other, weirder clouds. They can even use the data to infer what might lay beneath the clouds, and make predictions about alien surfaces. They can form theories about how and when these planets formed, telling us better stories about the worlds beyond us, including small, rocky planets like Earth, orbiting at just the right, cozy distance from their star—potentially habitable worlds.
When NASA says “habitable,” it doesn’t mean that we could live there, or even visit. Even our closest planetary neighbors, located 4.2 light-years away in the star system known as Proxima Centauri, would take tiny robotic spacecraft, equipped with little more than some cameras and curiosity, several decades to reach. The Webb telescope can help us become more familiar with all kinds of alien worlds, but we can only marvel from afar, looking to the data—at the little peaks and valleys on a plot—to guide our daydreams about what these places might be like. Knowing the mass of an exoplanet is no doubt scientifically useful, but it doesn’t exactly jolt the mind. Clouds of sand, though? That’s pretty out-there.