The shift is in what we ask of TRAPPIST-1. For years the question was whether its planets exist. Now it is what, if anything, surrounds them.
The system sits 12 parsecs away, about 39 light years, in the constellation Aquarius. Its star is a small, cool one: the discovery team puts its mass at 0.089 solar masses and its radius at 0.121 solar radii, only slightly larger than Jupiter. Its age is given as 7.6 billion years, give or take 2.2 billion, and its effective temperature as 2511 K. At least seven planets circle it.
Why this star suits the job
All seven transit, meaning they pass in front of the star, and that is how they were found: regular, repeated shadows. The team behind the discovery, working from the TRAPPIST telescope and later the Spitzer space telescope, the Very Large Telescope, UKIRT, the Liverpool Telescope and the William Herschel Telescope, used the timing to get orbital periods and sizes, and the exact transit times to estimate masses. Orbits run from 1.51087637 days for planet b to 18.767953 days for planet h. The planets are consistent with a rocky composition, with sizes and masses comparable to Earth and Venus.
The small star is the real gift. The team says transit signals from Earth-sized planets are 80 times more pronounced than they would be around a Sun-like star, and the atmospheric signal is boosted by the same factor. Confirming a planet is nearly 50 times faster, because a temperate world there circles in about a week rather than a year. As the team puts it, the TRAPPIST-1 worlds are the most optimal currently at our disposal for studying the climates of terrestrial planets beyond the Solar System.
They are also probably tidally locked, with a permanent dayside and a permanent nightside. How that affects climate, the team writes, remains mostly unknown.
What Webb has to say so far
The newest result comes from planet e. In an open-access paper in The Astrophysical Journal Letters, published in September 2025, astronomers used the James Webb Space Telescope to examine its atmosphere through transmission spectroscopy: starlight filtered through a planet’s air changes at different wavelengths, and each molecule leaves its own spectral fingerprint.
The answer was a careful one. The data rule out a hydrogen-dominated atmosphere, and carbon dioxide-rich atmospheres like those of Mars and Venus look unlikely. A warm, nitrogen-rich atmosphere similar to Saturn’s moon Titan remains possible. The evidence, though, is too weak to say whether any atmosphere is there at all. The data were consistent enough to leave open a surface ocean.
Stellar activity is the obstacle. “Stellar activity strongly interferes with the planetary interpretation of the data because we can only observe a potential atmosphere through starlight,” said Ana Glidden, a postdoc at MIT and first author of the paper. Her team’s workaround treats any signal that changes from visit to visit as most likely the star’s, and anything steady across visits as most likely the planet’s.
Sara Seager, an MIT professor of planetary science and a co-author, called TRAPPIST-1e “one of our most compelling habitable-zone planets”. Further observations are already in the works to narrow the possibilities.
