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arXiv · 2608.17207

Atmospheric Reconnaissance of TRAPPIST-1 f with JWST NIRISS SOSS: No Evidence for the Transit Light Source Effect

Olivia Lim·René Doyon·Ryan J. MacDonald·Étienne Artigau·Michael Radica·Mykhaylo Plotnykov·Alexandrine L'Heureux·Caroline Piaulet-Ghorayeb·Louis-Philippe Coulombe·David Lafrenière·Thomas J. Fauchez·Martin Turbet·Diana Valencia·Loïc Albert·Laura Flagg·Björn Benneke·Neil J. Cook·Pierre-Alexis Roy·Lisa Kaltenegger·Charles Cadieux·Nicolas B. Cowan·Michaël Gillon·Romain Allart·Lisa Dang·Doug Johnstone·Stefan Pelletier·Jared Splinter·Jake Taylor
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Abstract

In just over three years of operation, JWST has observed all seven planets of the TRAPPIST-1 system. The two innermost planets were found to have little to no atmosphere, barring the presence of high-altitude aerosols. Here we present the first JWST transit spectra of the habitable-zone exoplanet TRAPPIST-1 f, which were obtained with NIRISS SOSS over the course of five transits. At least one stellar flare occurred in each visit, but unlike observations of closer-in TRAPPIST-1 planets, no evidence for contamination of the transit spectra from unocculted stellar surface heterogeneities was found. This non-detection does not guarantee the absence of unocculted heterogeneities in all future transit observations of this planet, and it could be explained by the transit chord of TRAPPIST-1 f having properties similar to the average, out-of-transit, visible stellar hemisphere at the time of observation. The transit spectra exhibit slopes ranging from -365 ppm/um down to 15 ppm/um, which we attribute to stellar variability, that is, flares and/or smaller-scale events. The visits least affected by flares rule out H2/He-dominated atmospheres with surface pressures higher than about 20 mbar at 95% confidence. For high-mean-molecular-mass atmospheres, the exact upper limits on surface pressures depend on the reduction pipeline and on the treatment of the residual slopes in the transit spectra.

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Olivia Lim, René Doyon, Ryan J. MacDonald, Étienne Artigau, Michael Radica, Mykhaylo Plotnykov, Alexandrine L'Heureux, Caroline Piaulet-Ghorayeb, Louis-Philippe Coulombe, David Lafrenière, Thomas J. Fauchez, Martin Turbet, Diana Valencia, Loïc Albert, Laura Flagg, Björn Benneke, Neil J. Cook, Pierre-Alexis Roy, Lisa Kaltenegger, Charles Cadieux, Nicolas B. Cowan, Michaël Gillon, Romain Allart, Lisa Dang, Doug Johnstone, Stefan Pelletier, Jared Splinter, Jake Taylor. 2026-08-31. Atmospheric Reconnaissance of TRAPPIST-1 f with JWST NIRISS SOSS: No Evidence for the Transit Light Source Effect. https://doi.org/10.3847/1538-3881%2Fae990a

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