The Transmission Spectrum of the Exo-Venus TRAPPIST-1c Measured with JWST NIRSpec PRISM

The TRAPPIST-1 system, located a mere 12 parsecs from Earth, hosts seven transiting Earth-sized planets. With its Jupiter-sized ultra-cool M-dwarf host star, the system presents an optimal opportunity for atmospheric studies of temperate rocky planets. The planets in the system receive between 0.1 and 4 times Earth’s solar irradiation, offering a unique laboratory to examine how varying stellar factors, such as insolation and frequent flares, affect the atmospheric retention capabilities of Earth-sized planets in the harsh stellar environments of M-dwarfs. We present the 0.5 to 5.5 micron transmission spectrum of the exo-Venus, TRAPPIST-1c, observed by a multi-transit campaign with JWST/NIRSpec PRISM. In line with early JWST results for the TRAPPIST-1 system, we find that stellar contamination, in the form of spots/faculae, and flares, poses a challenge in achieving the precision needed to probe the planetary atmosphere. We discuss how we can characterize these stellar heterogeneities, what they teach us about ultra-cool dwarfs like TRAPPIST-1, and address how we have mitigated the corresponding contamination on the transmission spectrum. While secondary eclipse observations with JWST/MIRI suggest that substantial CO2-dominated atmospheres are unlikely for TRAPPIST-1c, other atmospheric states remain consistent with the data. In light of these results, we present our insights from the NIRSpec PRISM program, shedding light on whether the data suggest the presence or absence of an atmosphere.