Atmospheric Reconnaissance of TRAPPIST-1 f with JWST NIRISS SOSS: No Evidence for the Transit Light Source Effect
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.
Versions
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BibTeX
@misc{lim2026atmospheric,
title = {{Atmospheric Reconnaissance of TRAPPIST-1 f with JWST NIRISS SOSS: No Evidence for the Transit Light Source Effect}},
author = {Olivia Lim and René Doyon and Ryan J. MacDonald and Étienne Artigau and Michael Radica and Mykhaylo Plotnykov and Alexandrine L'Heureux and Caroline Piaulet-Ghorayeb and Louis-Philippe Coulombe and David Lafrenière and Thomas J. Fauchez and Martin Turbet and Diana Valencia and Loïc Albert and Laura Flagg and Björn Benneke and Neil J. Cook and Pierre-Alexis Roy and Lisa Kaltenegger and Charles Cadieux and Nicolas B. Cowan and Michaël Gillon and Romain Allart and Lisa Dang and Doug Johnstone and Stefan Pelletier and Jared Splinter and Jake Taylor},
howpublished = {arXiv},
year = {2026},
month = aug,
doi = {10.48550/arxiv.2608.17207},
eprint = {2608.17207},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2608.17207},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Atmospheric Reconnaissance of TRAPPIST-1 f with JWST NIRISS SOSS: No Evidence for the Transit Light Source Effect
AU - Olivia Lim
AU - René Doyon
AU - Ryan J. MacDonald
AU - Étienne Artigau
AU - Michael Radica
AU - Mykhaylo Plotnykov
AU - Alexandrine L'Heureux
AU - Caroline Piaulet-Ghorayeb
AU - Louis-Philippe Coulombe
AU - David Lafrenière
AU - Thomas J. Fauchez
AU - Martin Turbet
AU - Diana Valencia
AU - Loïc Albert
AU - Laura Flagg
AU - Björn Benneke
AU - Neil J. Cook
AU - Pierre-Alexis Roy
AU - Lisa Kaltenegger
AU - Charles Cadieux
AU - Nicolas B. Cowan
AU - Michaël Gillon
AU - Romain Allart
AU - Lisa Dang
AU - Doug Johnstone
AU - Stefan Pelletier
AU - Jared Splinter
AU - Jake Taylor
PY - 2026
DA - 2026/08/31
PB - arXiv
DO - 10.48550/arxiv.2608.17207
UR - https://arxiv.org/abs/2608.17207
AB - 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.
ER -