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

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Abstract
In just over 3 yr 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 nondetection 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 μ m −1 down to 15 ppm μ m −1 , which we attribute to stellar variability, that is, flares and/or smaller-scale events. The visits least affected by flares rule out H 2 /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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Funding Information
  • FRQ ∣ Nature et technologies (303926)
  • Natural Sciences and Engineering Research Council of Canada (22JWG01-3)
  • Canadian Space Agency (22EXPJWST)
  • FRQ ∣ Nature et technologies (349961)
  • Belgian Federal Science Policy Office (B2/212/PI/PORTAL)
  • Belgian Federal Science Policy Office (B2/212/B1/PORTAL)

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