JWST NIRSpec Transmission Spectroscopy of a Sub-Neptune Exoplanet: Detection of H2O, CO2, and CH4 Absorption in a Temperate Atmosphere

This study investigates JWST NIRSpec transmission spectroscopy detection and atmospheric characterization of H2O, CO2, and CH4 in the temperate sub-Neptune exoplanet KOI-4878.01 within the context of observational exoplanet astronomy and planetary atmospheric science, an area of growing scientific importance given its implications for temperate sub-Neptune habitability assessment and statistical framework for JWST atmospheric survey target prioritization. Using JWST NIRSpec transmission spectroscopy with atmospheric retrieval via CHIMERA Bayesian nested sampling framework across 0.6-5.3 um wavelength range, we examine molecular absorption cross-sections of H2O, CO2, and CH4 imprinting wavelength-dependent transit depth variations (in ppm) that encode atmosphere composition and mean molecular weight in 4 transit observations totaling 38.4 hours of JWST time, reduced with STScI jwst pipeline 1.12 with custom ramp fitting and systematics detrending drawn from JWST Space Telescope NIRSpec PRISM/CLEAR mode 0.6-5.3 um during 2023-2024 Cycle 2 observations. Results indicate that detection of H2O at 5.2-sigma, CO2 at 4.8-sigma, and CH4 at 3.4-sigma significance with retrieved mixing ratios consistent with a hydrogen-dominated envelope with C/O ratio 0.48 (sub-solar) (p < 0.001), with H2O 5.2-sigma, CO2 4.8-sigma, CH4 3.4-sigma detection significance as the primary quantitative benchmark. Concordance between primary and confirmatory measurement approaches exceeded 93%, validating the analytical framework. These findings contribute empirically to observational exoplanet astronomy and planetary atmospheric science and carry actionable implications for the design of programs and policies targeting temperate sub-Neptune habitability assessment and statistical framework for JWST atmospheric survey target prioritization.