TRAPPIST‑1b/c: No Atmospheres

Astronomers used the James Webb Space Telescope to track heat across TRAPPIST-1b and TRAPPIST-1c and found both worlds most likely lack thick atmospheres. (nature.com) The team published the result in *Nature Astronomy* on April 3, 2026, after measuring the planets’ thermal phase curves at 15 microns, a way of watching how a planet’s infrared glow changes through its orbit. (nature.com) A thermal phase curve works like a day-night heat map from 40 light-years away: if air moves heat around, the nightside stays warmer and the hottest point can shift away from noon. On TRAPPIST-1b, Webb saw a dayside brightness temperature of 490 ± 17 kelvin, no significant nightside emission, and no phase offset. (nature.com) TRAPPIST-1c looked cooler on its dayside, at 369 ± 23 kelvin, but its nightside also appeared cold. The paper says models with surface pressures above about 1 bar are strongly disfavored for both planets. (nature.com) TRAPPIST-1 is a red dwarf system about 40 light-years away with seven Earth-sized rocky planets, first unveiled as a seven-planet system in 2017. Because the star is small and cool, its habitable zone sits much closer in than the Sun’s. (nature.com) (science.nasa.gov) That has made the system a test case for a basic exoplanet question: can rocky planets around active red dwarfs hold onto air after long exposure to flares, radiation, and stellar wind? The new measurements push the answer toward “not always,” at least for the two innermost planets. (nature.com) (science.nasa.gov) The finding also extends a pattern Webb has been building inside the same system. In 2025, NASA and the European Space Agency said Webb data ruled out an Earth-like atmosphere on TRAPPIST-1d, the third planet out. (science.nasa.gov) (esa.int) Webb has not closed the book on the whole family. In September 2025, the Space Telescope Science Institute said early Webb observations of TRAPPIST-1e, a planet in the star’s habitable zone, were not enough to confirm an atmosphere but did narrow the possibilities, with more observations underway. (stsci.edu) For TRAPPIST-1c, the new paper leaves a small opening for a thin, oxygen-rich atmosphere, but not the kind of dense blanket that would smooth out day-night temperatures. For TRAPPIST-1b, the data fit a dark, airless rock. (nature.com) So the nearest famous lineup of Earth-sized planets now looks less like seven versions of Earth and more like a mixed set of worlds shaped by distance, starlight, and atmospheric loss. Webb’s next passes through the outer TRAPPIST-1 planets will decide how far that pattern extends. (nature.com) (stsci.edu)