Transmission spectroscopy is the primary technique used by jwst and its precursors to characterize exoplanet atmospheres. During a planetary transit, starlight passes through the thin limb of the planet’s atmosphere; absorption features imprinted on the stellar spectrum reveal atmospheric composition at specific pressure levels along the terminator.
Transmission spectra are 1D averages of intrinsically 3D atmospheric structure — a fundamental methodological limit emphasized by sara-seager et al. (src-jwst-biosignature-prospects-2025). The 1D-versus-3D mismatch contributes to the “parallel interpretations” problem in biosignature retrieval, in which the same spectrum admits multiple consistent atmospheric models. JWST can access only a handful of habitable-zone atmospheres before hitting its noise floor.
Beyond biosignature retrieval, transmission-spectrum metallicity, C/O ratio, and mean molecular weight constrain planetary formation pathway. The jwst toi-1130b retrieval (Barat et al., 2026) is a methodological reference: a high-mean-molecular-weight-atmosphere (μ = 5.5 amu) plus high metallicity supports ex-situ formation beyond the water-ice-line for that warm mini-neptune (src-jwst-toi-1130b-atmosphere-2026-05).
Contrasts with direct-imaging, where the planet is resolved from its host star and its emitted or reflected spectrum is measured directly (target of habitable-worlds-observatory and LIFE).
Related
- jwst, biosignatures, false-positive-biosignatures
- k2-18b, hycean-worlds, toi-1130b, mini-neptune
- high-mean-molecular-weight-atmosphere, water-ice-line
- direct-imaging, habitable-worlds-observatory
- radial-velocity — another exoplanet detection/characterization method
- toi-5205-b — 2026 GEMS JWST transmission spectrum; introduces a starspot-correction methodology for M-dwarf hosts (src-sciencedaily-forbidden-exoplanet-atmosphere-2026-04).