How do you go looking for life on a planet 40 light-years from Earth? For astronomers, the solution starts with light itself and the James Webb Space Telescope is now providing the most nuanced hints so far.
TRAPPIST-1e, a rocky exoplanet that orbits a faint red dwarf star, was a top target for a long time in the quest for planets that could host life. It lies firmly in its star’s “Goldilocks zone,” where temperatures might permit liquid water to flow on the surface. Early findings from Webb indicate it might also have a nitrogen atmosphere a makeup surprisingly close to that of Earth and of Saturn’s moon, Titan. “If there’s one of these planets that could potentially sustain liquid water on the surface, it’s probably that one,” commented Nikole Lewis, a Cornell University exoplanet researcher and co-author on two new papers in The Astrophysical Journal Letters.
The studies were based on a method known as transmission spectroscopy. As TRAPPIST-1e moved in front of its star a transit Webb’s Near-Infrared Spectrograph recorded the starlight passing through the potential atmosphere of the planet. Molecules absorb at characteristic wavelengths, creating distinctive “fingerprints” in the spectrum. By matching these patterns with pre-existing molecular signatures, scientists can deduce atmospheric composition. “Webb’s infrared instruments are giving us more detail than we’ve ever had access to before,” said Néstor Espinoza of the Space Telescope Science Institute.
Initial data have already ruled out a hydrogen-dominated primary atmosphere, the kind a planet might retain from its formation. The red dwarf TRAPPIST-1 is highly active, with frequent flares that likely stripped away such a lightweight envelope long ago. Rather, the indications are of a denser secondary atmosphere perhaps nitrogen-dense, and possibly including gases such as methane or carbon dioxide that would impart a stabilizing greenhouse effect. As MIT’s Sara Seager explained, the results “sharpen our focus on the scenarios still in play,” eliminating Venus-style and Mars-style atmospheres while leaving the potential for an ocean-carrying world open.
The difficulty is that TRAPPIST-1’s stellar activity complicates the measurement as well. Temperature variations and spots on stars can simulate or hide planetary signals. Ryan MacDonald of St Andrews University clarified that in 2023, “we quickly realized that the system’s red dwarf star was contaminating our data in ways that made the search for an atmosphere extremely challenging.” To mitigate this, scientists are cross-referencing observations of TRAPPIST-1e with those of TRAPPIST-1b, a known bare rock. Any absorption characteristics that are special to TRAPPIST-1e’s transits would then be ascribable to its atmosphere and not to the star.
Webb has thus far observed four transits of TRAPPIST-1e. Fifteen more are scheduled through 2025, a pace enabled because the planet’s close orbit takes only slightly more than six Earth days. That quick cycle is one reason red dwarf systems are preferred for atmospheric analysis: their compact size means a planet eclipses a greater portion of starlight, and their shallow habitable zones facilitate regular, repeatable observations.
The stakes are high. If confirmed, TRAPPIST-1e would be the first rocky, habitable-zone exoplanet known to host an atmosphere a milestone in exoplanet science. Such a discovery would also refine astrobiology’s criteria for habitability, underscoring the interplay between orbital distance, stellar radiation, and atmospheric chemistry. As Ana Glidden of MIT put it, “We’re in a new age of exploration that’s very exciting to be a part of.”
For the time being, the planet is a mystery: it might be a temperate planet with oceans beneath a sky of nitrogen, or a cold rock floating in space. The next round of Webb observations might determine which of these visions is reality.
