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Atmospheric Reconnaissance of TRAPPIST-1 b with JWST/NIRISS: Evidence for Strong Stellar Contamination in the Transmission Spectra Published: 9/13/2023 6:57:58 PM Updated: 9/13/2023 6:57:58 PM

Paper abstract: TRAPPIST-1 is a nearby system of seven Earth-sized, temperate, rockyexoplanets transiting a Jupiter-sized M8.5V star, ideally suited for in-depthatmospheric studies. Each TRAPPIST-1 planet has been observed in transmissionboth from space and from the ground, confidently rejecting cloud-free,hydrogen-rich atmospheres. Secondary eclipse observations of TRAPPIST-1 b withJWST/MIRI are consistent with little to no atmosphere given the lack of heatredistribution. Here we present the first transmission spectra of TRAPPIST-1 bobtained with JWST/NIRISS over two visits. The two transmission spectra showmoderate to strong evidence of contamination from unocculted stellarheterogeneities, which dominates the signal in both visits. The transmissionspectrum of the first visit is consistent with unocculted starspots and thesecond visit exhibits signatures of unocculted faculae. Fitting the stellarcontamination and planetary atmosphere either sequentially or simultaneously,we confirm the absence of cloud-free hydrogen-rich atmospheres, but cannotassess the presence of secondary atmospheres. We find that the uncertaintiesassociated with the lack of stellar model fidelity are one order of magnitudeabove the observation precision of 89 ppm (combining the two visits). Withoutaffecting the conclusion regarding the atmosphere of TRAPPIST-1 b, thishighlights an important caveat for future explorations, which calls foradditional observations to characterize stellar heterogeneities empiricallyand/or theoretical works to improve model fidelity for such cool stars. Thisneed is all the more justified as stellar contamination can affect the searchfor atmospheres around the outer, cooler TRAPPIST-1 planets for whichtransmission spectroscopy is currently the most efficient technique.