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Forty light-years from Earth, in the quiet darkness of the Aquarius constellation, a small red dwarf star named TRAPPIST-1 hosts a remarkable planetary family. Seven rocky worlds circle this dim, cool sun, and for years, astronomers have dreamed that some of them might be like our own—lush, blue, and alive.
One of the most intriguing members of this system is TRAPPIST-1 d. Similar in size to Earth and sitting right at the edge of the “Goldilocks zone” where liquid water could exist, this world once sparked hope that it might be a true Earth cousin. But a new study using NASA’s James Webb Space Telescope (JWST) has rewritten that hope into a mystery.
“Ultimately, we want to know if something like the environment we enjoy on Earth can exist elsewhere, and under what conditions,” says Caroline Piaulet-Ghorayeb of the University of Chicago and the Trottier Institute for Research on Exoplanets. “At this point, we can rule out TRAPPIST-1 d from a list of potential Earth twins or cousins.”
A Planet That Orbits in Four Days.
TRAPPIST-1’s planets huddle close to their star, thanks to its dim glow and low temperature. For TRAPPIST-1 d, “close” means just 2% of the Earth-Sun distance. A year here lasts only four Earth days, and the star looms far larger in the sky than our Sun ever could.
In 2017, this system made headlines when NASA’s retired Spitzer Space Telescope confirmed it as the record-holder for most Earth-sized planets around a single star. TRAPPIST-1 d was a prime candidate for habitability—until Webb’s powerful NIRSpec instrument took a closer look.
The Atmosphere That Wasn’t There.
When Webb’s instruments searched for molecules like water vapor, methane, and carbon dioxide—all staples of an Earth-like atmosphere—none appeared. Instead, astronomers were left with three possible scenarios:
- A thin, Mars-like atmosphere that’s nearly invisible to detection.
- A Venus-like shroud of thick, high-altitude clouds hiding the planet’s true nature.
- A barren rock stripped bare of air.
Whichever is true, TRAPPIST-1 d faces a harsh reality: life is tough around a red dwarf star. TRAPPIST-1 is known for its violent flares, blasts of radiation that can peel atmospheres from planets like layers of paint.
Why Astronomers Haven’t Given Up.
Despite this, scientists are far from discouraged. Red dwarfs are the most common type of star in the Milky Way. If planets can hold onto their atmospheres here, even under constant stellar assault, they might survive almost anywhere in the galaxy.
“We’re really just getting started,” says Björn Benneke of IREx. “Webb is giving us our first chance to define the line between planets that can hold onto an atmosphere and those that cannot.”
Now, the hunt moves outward—to TRAPPIST-1’s more distant planets e, f, g, and h. These worlds orbit farther from the star’s fury and might have better odds of keeping their air, though their colder climates will make detection even trickier.
A Step Closer to the Big Question.
For NASA, the stakes go beyond just TRAPPIST-1.
“As NASA leads the way in searching for life outside our solar system, one of the most important avenues is understanding which planets retain their atmospheres, and why,” says Shawn Domagal-Goldman of NASA Headquarters.
Every JWST observation builds the foundation for future missions like the Habitable Worlds Observatory, which could finally answer the question humanity has been asking for centuries: Are we alone?
For now, TRAPPIST-1 d may not be the Earth twin we hoped for—but it is one more stepping stone toward finding the worlds that are.