As telescopes have become more powerful, we’ve been finding tons of “exoplanets”—planets orbiting faraway stars.
One such planet, known as exoplanet TRAPPIST-1 d, has intrigued astronomers looking for possibly habitable worlds beyond our solar system; it is similar in size to Earth, rocky, and resides in an area around its star where liquid water on its surface is theoretically possible.
But according to a new study using data from NASA’s James Webb Space Telescope, it does not have an Earth-like atmosphere.
“Ultimately, we want to know if something like the environment we enjoy on Earth can exist elsewhere, and under what conditions,” said Caroline Piaulet-Ghorayeb of the University of Chicago and Trottier Institute for Research on Exoplanets (IREx) at Université de Montréal, lead author of the study published Aug. 13 in The Astrophysical Journal. “While NASA’s James Webb Space Telescope is giving us the ability to explore this question in Earth-sized planets for the first time, at this point we can rule out TRAPPIST-1 d from a list of potential Earth twins or cousins.”
Planet TRAPPIST-1 d
The TRAPPIST-1 system is located 40 light-years away and was revealed as the record-holder for most Earth-sized rocky planets around a single star in 2017, thanks to data from NASA’s retired Spitzer Space Telescope and other observatories.
TRAPPIST-1 d, the third planet, is much closer to its star than Earth is to the sun—it completes an entire orbit around its star in only four Earth days—but it’s still the right temperature for liquid water, because its star is much dimmer and cooler than our sun.
The Webb Telescope was able to take a detailed look at TRAPPIST-1 d’s atmosphere using its near-infrared spectrograph instrument. It did not detect molecules that are common in Earth’s atmosphere, like water, methane or carbon dioxide.
However, Piaulet-Ghorayeb outlined several possibilities for the exoplanet that remain open for follow-up study.
“There are a few potential reasons why we don’t detect an atmosphere around TRAPPIST-1 d,” she explained. “It could have an extremely thin atmosphere that is difficult to detect, somewhat like Mars. Alternatively, it could have very thick, high-altitude clouds that are blocking our detection of specific atmospheric signatures—something more like Venus. Or, it could be a barren rock, with no atmosphere at all.”
The star
No matter what the case may be for TRAPPIST-1 d, it’s tough being a planet in orbit around a red dwarf star.
TRAPPIST-1, the host star of the system, is known to be volatile, often releasing flares of high-energy radiation with the potential to strip off the atmospheres of its small planets, especially those orbiting most closely.
Nevertheless, scientists are motivated to seek signs of atmospheres on the TRAPPIST-1 planets because red dwarf stars are the most common stars in our galaxy. If planets can hold on to an atmosphere here, under waves of harsh stellar radiation, they could, as the saying goes, make it anywhere.
“Webb’s sensitive infrared instruments are allowing us to delve into the atmospheres of these smaller, colder planets for the first time,” said Björn Benneke of IREx at Université de Montréal, a co-author of the study. “We’re really just getting started using Webb to look for atmospheres on Earth-sized planets, and to define the line between planets that can hold onto an atmosphere, and those that cannot.”
Sibling planets
TRAPPIST-1 has at least six other planets orbiting it. The two closest to the star have already been ruled out as habitable, but scientists are using Webb to observe the four that are further beyond.
These hold both potential and peril. On the one hand, Benneke said, planets e, f, g and h may have better chances of having atmospheres because they are further away from the energetic eruptions of their host star. However, their distance and colder environment will make atmospheric signatures more difficult to detect, even with Webb’s infrared instruments.
“All hope is not lost for atmospheres around the TRAPPIST-1 planets,” Piaulet-Ghorayeb said. “While we didn’t find a big, bold atmospheric signature for planet d, there is still potential for the outer planets to be holding onto a lot of water and other atmospheric components.”
The James Webb Space Telescope is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).
Citation: "Strict Limits on Potential Secondary Atmospheres on the Temperate Rocky Exo-Earth TRAPPIST-1 d." Piaulet-Ghorayeb et al, The Astrophysical Journal, Aug. 13, 2025.
Funding: Canadian Space Agency, Natural Sciences and Engineering Research Council, Trottier Family Foundation, Brinson Foundation, Fonds de recherche du Québec—Nature et technologies, iREx, SNSF, Sorbonne University, BELSPO BRAIN, InitiaSciences, TESS Guest Investigator Program, Swiss National Science Foundation, NRC Canada.