TRAPPIST-1’s two closest planets are bare rocks — a 400°C temperature gap proves it

For a decade, TRAPPIST-1 was the most promising address in the search for life beyond Earth — a red dwarf star 39 light-years away ringed by seven rocky worlds, three of them in the habitable zone. Webb has now delivered the first complete climate portraits of the two innermost planets, and the picture is unambiguous: blistering heat on one side, permanent deep freeze on the other, with nothing circulating warmth between them.

The two inner planets of the TRAPPIST-1 system experience temperature differences of more than 400 degrees Celsius between their permanently sunlit and permanently dark hemispheres. That gap is only possible on worlds with no atmosphere capable of transporting heat, making TRAPPIST-1b and TRAPPIST-1c confirmed airless rocks. The dayside of TRAPPIST-1b exceeds 200 degrees Celsius; its nightside plunges below minus 200. TRAPPIST-1c is cooler overall but shows the same brutal divide.

These findings represent the first complete thermal phase curves ever measured for rocky exoplanets — meaning scientists tracked the full temperature cycle of an Earth-sized world from dawn through noon to midnight, for the first time in history. The team, led by astronomer Emeline Bolmont at the University of Geneva, used Webb’s MIRI instrument to monitor infrared light from both planets across their entire orbits around the star.

The key mechanism behind the temperature collapse is tidal locking. Because TRAPPIST-1 is a red dwarf much smaller and cooler than our Sun, its inner planets orbit extraordinarily close — TRAPPIST-1b completes a full year in just 1.5 Earth days. At that proximity, the star’s gravity has permanently frozen each planet’s rotation, keeping one face in eternal daylight and the other in eternal night. On a planet with any substantial atmosphere, winds would carry heat from the day side to the night side, reducing the temperature contrast. The 400-degree gap observed here makes clear that no such redistribution is happening. If these planets ever had atmospheres, they were stripped away by the intense ultraviolet radiation and energetic particle bombardment characteristic of red dwarf stars.

The method the team used is elegant in its indirection. Because Webb cannot photograph TRAPPIST-1b directly — it is far too small and close to its star — the researchers instead measured how the combined infrared brightness of the star-plus-planet system changed as the planet moved through its orbit. As the day side rotated in and out of view, the temperature signature of the planet rose and fell in predictable ways. The resulting thermal map is as unambiguous as any direct measurement.

Caution is warranted before extrapolating to the whole system. TRAPPIST-1b and 1c are the two planets closest to the star, exposed to the most intense stellar radiation. The team’s own theoretical models suggest the outer planets — including the three in the habitable zone — could still retain atmospheres despite conditions on the inner two. The analogy offered by the researchers is instructive: Mercury has no atmosphere, while Venus and Earth, farther from the Sun, have held onto theirs. The fate of TRAPPIST-1b and 1c does not automatically predict the fate of TRAPPIST-1e, f, or g. Webb observations of those outer worlds are ongoing, and results remain preliminary.

The confirmed results were published in the journal Nature Astronomy. Webb observations of TRAPPIST-1e — the planet situated squarely in the system’s habitable zone — are currently in progress, with fifteen additional transits planned as part of the DREAMS program run by the Space Telescope Science Institute.