KRONOS I The 1–2.8 μm JWST Transmission Spectrum of the 23 Myr V1298 Tau c

While recent JWST observations of mature super-Earths and sub-Neptunes have frequently revealed featureless transmission spectra, their inflated progenitors offer a unique window into understanding their primordial compositions. As part of the Keys to Revealing the Origin and Nature of Sub-Neptune Systems (KRONOS) JWST program, we present the Near Infrared Imager and Slitless Spectrograph Single Object Slitless Spectroscopy transmission spectrum of V1298 Tau c, an ∼23 Myr super-Earth progenitor orbiting a young solar analog. We detect H₂O in V1298 Tau c’s atmosphere with a (Formula presented) log₁₀ volume mixing ratio of (Formula presented) −1.83^+0.68_−0.77, but no additional molecules from these data alone. We find consistent results for the planetary atmospheric properties in both retrievals with and without informed priors on stellar heterogeneities based on the observed stellar spectrum. We infer an atmospheric metallicity [O/H] of (Formula presented) 14.8^+56.0_−12.28× the solar value. This metallicity is similar to literature measurements for other young planets, including its massive outer companion V1298 Tau b. In contrast, this measured metallicity is systematically lower than the metallicities of mature planets of similar mass and temperature. Altogether, these results provide tentative but growing evidence that the exoplanet mass–metallicity relation evolves with planetary age.