Astronomers are working their way through the TRAPPIST-1 system with the NASA/ESA/CSA James Webb Space Telescope, demonstrating its unprecedented ability to capture detailed information about exoplanet atmospheres and learning to work with that data. The first results are now in from Webb’s observations of TRAPPIST-1e. While the initial four observations by Webb are not enough to confirm an atmosphere, the scientists are using the data to narrow possibilities for the planet, including possibilities such as a global surface ocean or a methane-enriched environment similar to Saturn’s moon Titan. Meanwhile, additional, innovative Webb observations are underway that will eventually show which type of world TRAPPIST-1e turns out to be.
The Earth-size exoplanet TRAPPIST-1e, depicted at the lower right, is silhouetted as it passes in front of its flaring host star in this artist’s concept of the TRAPPIST-1 system. Image credit: NASA / ESA / CSA / STScI / Joseph Olmsted, STScI.
TRAPPIST-1 is an ultracool dwarf star in the constellation Aquarius, 38.8 light-years away.
The star is barely larger than Jupiter and has just 8% of our Sun’s mass. It is rapidly spinning and generates energetic flares of UV radiation.
TRAPPIST-1 is host to seven transiting planets, named TRAPPIST-1b, c, d, e, f, g and h.
All these planets are similar in size to Earth and Venus, or slightly smaller, and have very short orbital periods: 1.51, 2.42, 4.04, 6.06, 9.21, 12.35 and 20 days, respectively.
They are likely all tidally locked, meaning the same face of the planet is always pointed at the star, as the same side of the Moon is always pointed at Earth. This creates a perpetual night side and perpetual day side on each TRAPPIST-1 planet.
Of the seven planets in the system, TRAPPIST-1e is of particular interest because it orbits the star at a distance where water on the surface is theoretically possible, but only if the planet has an atmosphere.
Dr. Néstor Espinoza from the Space Telescope Science Institute and colleagues aimed Webb’s NIRSpec (Near-Infrared Spectrograph) instrument at the system as TRAPPIST-1e transited, or passed in front of, its star.
Starlight passing through the planet’s atmosphere, if there is one, will be partially absorbed, and the corresponding dips in the light spectrum that reaches Webb will tell the astronomers what chemicals are found there.
With each additional transit, the atmospheric contents become clearer as more data are collected.
Though multiple possibilities remain open for TRAPPIST-1e because only four transits have been analyzed so far, the researchers feel confident that the planet does not still have its primary, or original, atmosphere.
TRAPPIST-1 is a very active star, with frequent flares, so it is not surprising to the researchers that any hydrogen-helium atmosphere with which the planet may have formed would have been stripped off by stellar radiation.
However many planets, including Earth, build up a heavier secondary atmosphere after losing their primary atmosphere.
It is possible that TRAPPIST-1e was never able to do this and does not have a secondary atmosphere.
“We developed novel approaches to working with Webb’s data to determine TRAPPIST-1e’s potential atmospheres and surface environments,” the scientists said.
It is unlikely that the atmosphere of TRAPPIST-1e is dominated by carbon dioxide, analogous to the thick atmosphere of Venus and the thin atmosphere of Mars.
However, the astronomers are careful to note that there are no direct parallels with our Solar System.
“TRAPPIST-1 is a very different star from our Sun, and so the planetary system around it is also very different, which challenges both our observational and theoretical assumptions,” said Cornell University’s Dr. Nikole Lewis.
“If there is liquid water on TRAPPIST-1e, it would be accompanied by a greenhouse effect, in which various gases, particularly carbon dioxide, keep the atmosphere stable and the planet warm.”
“A little greenhouse effect goes a long way, and the measurements do not rule out adequate carbon dioxide to sustain some water on the surface.”
According to the team’s analysis, the water could take the form of a global ocean, or cover a smaller area of the planet where the star is at perpetual noon, surrounded by ice.
This would be possible because, due to the TRAPPIST-1 planets’ sizes and close orbits to their star, it is thought that they all are tidally locked, with one side always facing the star and one side always in darkness.
“We are really still in the early stages of learning what kind of amazing science we can do with Webb,” said Dr. Ana Glidden, an astronomer with the Kavli Institute for Astrophysics and Space Research at MIT.
“It’s incredible to measure the details of starlight around Earth-sized planets 40 light-years away and learn what it might be like there, if life could be possible there.”
“We’re in a new age of exploration that’s very exciting to be a part of.”
The new results from Webb are analyzed in two new papers published in the Astrophysical Journal Letters.
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Néstor Espinoza et al. 2025. JWST-TST DREAMS: NIRSpec/PRISM Transmission Spectroscopy of the Habitable Zone Planet TRAPPIST-1e. ApJL 990, L52; doi: 10.3847/2041-8213/adf42e
Ana Glidden et al. 2025. JWST-TST DREAMS: Secondary Atmosphere Constraints for the Habitable Zone Planet TRAPPIST-1e. ApJL 990, L53; doi: 10.3847/2041-8213/adf62e
