Five Webb transits narrow the atmosphere around TRAPPIST-1 f

Webb's NIRISS instrument observed five transits of the habitable-zone, Earth-sized planet TRAPPIST-1 f. At least one stellar flare occurred during every visit, yet the team found no evidence that unocculted spots or faculae produced a transit light source effect in the combined data. The individual spectra still carried slopes attributed to stellar variability from flares or smaller events, demonstrating that a quiet-looking correction problem can survive even when the usual contamination signature is absent.

Using the visits least affected by flares, the analysis rules out hydrogen- and helium-dominated atmospheres with surface pressures above roughly 20 millibars at 95 percent confidence. Constraints on high-mean-molecular-mass atmospheres depend more strongly on the reduction pipeline and treatment of residual slopes. This is atmospheric reconnaissance, not an atmosphere detection, a biosignature, or evidence that the surface is habitable.

Why it matters

TRAPPIST-1 f is one of the best-known nearby temperate worlds. Establishing which atmospheres are absent—and how stellar activity limits the remaining search—is necessary before future spectra can support stronger claims.

What the source establishes

Worth Revisiting · Peer-reviewed Astronomical Journal paper published October 1. Five transits and the hydrogen-helium exclusion are measured analyses; dense-atmosphere limits remain reduction-sensitive.