A game changer for detecting life on distant worlds
JWST detected carbon dioxide in WASP-39b's atmosphere for the first time, revealing thick clouds of sulfur and silicates that interact with starlight to produce sulfur dioxide. Chemical ratios in the atmosphere suggest the gas giant formed from colliding planetesimals and migrated closer to its star after formation, offering insights into planetary origins.
- WASP-39b orbits 700 light-years away in the constellation Virgo
- First detection of carbon dioxide in an exoplanet atmosphere
- Planet orbits 4.3 million miles from its star, eight times closer than Mercury to the sun
- Surface temperature reaches 1,650 degrees Fahrenheit (900 degrees Celsius)
- Chemical analysis suggests the planet formed farther from its star and migrated inward
The James Webb Space Telescope has revealed unprecedented atmospheric details of exoplanet WASP-39b, 700 light-years away, detecting carbon dioxide and sulfur compounds while testing methods for future alien life detection.
Seven hundred light-years from Earth, a planet the size of Saturn orbits so close to its star that its surface temperature reaches 1,650 degrees Fahrenheit. This world, WASP-39b, has become the most thoroughly studied exoplanet we have ever known—not because it might harbor life, but because the James Webb Space Telescope has turned it into a laboratory for understanding how planets form and, more ambitiously, how we might one day detect life on worlds we cannot see.
In late August, Webb detected carbon dioxide in WASP-39b's atmosphere, a first for any exoplanet. The discovery made headlines. But the real story unfolded in the months that followed. Three of Webb's four instruments—the NIRCam camera and two spectroscopes called NIRISS and NIRSpec—trained on the distant gas giant, splitting its starlight into chemical fingerprints. What emerged was a portrait of stunning complexity. The planet is wrapped in thick clouds laced with sulfur and silicates. When starlight strikes these compounds, they undergo a photochemical reaction that produces sulfur dioxide, much as ultraviolet radiation creates ozone in Earth's upper atmosphere. The sheer intensity of radiation battering WASP-39b makes it an ideal natural laboratory for studying such reactions.
But the observations revealed something deeper still. By measuring the ratios of carbon to oxygen, potassium to oxygen, and sulfur to hydrogen in the atmosphere, astronomers could read the planet's history. The data suggested that WASP-39b formed not where it orbits today, but much farther from its star—a conclusion drawn from the abundance of oxygen relative to carbon in its clouds. The planet appears to have assembled itself from collisions of smaller planetesimals, then migrated inward over time. Laura Kreidberg, director of the Max Planck Institute for Astronomy in Germany and part of the observing team, called the telescope's performance "nearly flawless—even better than we hoped."
The implications extend beyond understanding this single world. The same atmospheric analysis techniques used on WASP-39b could, in principle, be applied to planets in the habitable zones of distant stars. If such a world showed more oxygen in its atmosphere than models predicted, that excess could signal the presence of life. The observations on WASP-39b served as a proof of concept, a dry run for methods that might one day answer one of humanity's oldest questions. Natalia Batalha, an astronomy professor at UC Santa Cruz who coordinated the observing program, described the data as "a game changer."
Of course, WASP-39b itself is no candidate for life. Orbiting only 4.3 million miles from its parent star—eight times closer than Mercury orbits the sun—the planet is a hellscape of heat and radiation. But it has proven something more valuable: that Webb can see deeply enough into distant atmospheres to read their chemistry, their history, and perhaps one day, the signatures of life itself. Five studies based on these observations are under review or in press with the journal Nature, and they represent only the beginning of what this telescope will reveal about the worlds beyond our sun.
Notable Quotes
These early observations are a harbinger of more amazing science to come with JWST. We put the telescope through its paces to test the performance, and it was nearly flawless—even better than we hoped.— Laura Kreidberg, Max Planck Institute for Astronomy
Data like these are a game changer.— Natalia Batalha, UC Santa Cruz
The Hearth Conversation Another angle on the story
Why does it matter that we can see the atmosphere of a planet that's too hot to have life?
Because the techniques we're testing on WASP-39b are the same ones we'll use on planets that might actually be habitable. We're learning the language of atmospheric chemistry now, while we have a bright, nearby example to study.
So this planet is like a practice run?
Exactly. It's a loud, clear signal. The planet is so close to its star and so hot that its atmosphere is easier to read. We're building confidence in our methods before we turn them toward fainter, more distant worlds.
The carbon dioxide detection was announced months ago. What changed?
That was the headline. But the real work was what came after—measuring dozens of chemical compounds, understanding their ratios, and using those ratios to reconstruct the planet's past. That takes time and multiple instruments working together.
And what did that reconstruction tell us?
That this planet didn't form where it orbits now. It assembled itself farther out, in a cooler region, then migrated inward. That's a story written in the chemistry of its atmosphere.
Can we know that for certain?
Not with absolute certainty, but the chemical ratios are consistent with that history. It's the kind of inference astronomers have made about planets in our own solar system, but now we can do it for worlds we can barely see.
What happens next?
We keep watching. There are thousands of exoplanets out there. Webb will look at many of them. And eventually, we'll find one that shows signs we can't explain by geology alone.