How infrared spectroscopy works
Molecules absorb light at specific wavelengths determined by their chemical bonds — stretching and bending vibrations correspond to infrared frequencies. Each molecule has a unique absorption pattern, like a fingerprint. Water absorbs strongly at ~2.7 μm; carbon dioxide at 4.3 μm; methane at 3.3 μm.
JWST operates primarily in the near- and mid-infrared (0.6–28 μm), a spectral range that Earth's atmosphere largely blocks — which is why a space telescope is essential. Its mirror (6.5 m diameter, vs Hubble's 2.4 m) and detectors cooled to −233°C give it sensitivity orders of magnitude better than any previous instrument.
For how electrons and atomic orbitals underlie spectroscopy, see our atomic structure guide. For how molecular bonds produce those infrared signals, see our chemical bonding guide.
Exoplanet atmospheres
JWST's first science results, published in 2022–2023, transformed exoplanet atmospheric chemistry:
- WASP-39b (hot Saturn, 700 light-years away) — the first unambiguous detection of CO₂ in an exoplanet atmosphere. SO₂ was also detected, produced by photochemical reactions driven by the host star's ultraviolet light.
- K2-18b (sub-Neptune in the habitable zone) — CH₄, CO₂ and a tentative detection of dimethyl sulfide (DMS), a molecule produced almost exclusively by marine biology on Earth. The DMS signal requires confirmation but generated significant scientific attention.
- TRAPPIST-1 planets — ongoing observations are searching for atmospheres around this system's Earth-sized planets in the habitable zone.
Molecules in interstellar space
JWST has also detected complex organic molecules in star-forming regions. In the Orion Bar — a region where intense stellar radiation meets a molecular cloud — researchers identified methyl cation (CH₃⁺) in 2023: the first detection of this molecule in space, confirming theoretical models of ion–molecule chemistry that may have seeded early solar systems with organic precursors.
Earlier observations confirmed polycyclic aromatic hydrocarbons (PAHs) including pyrene in the same region. PAHs are among the most abundant carbon-bearing molecules in the universe and are thought to be related to the organic molecules found in meteorites.
This interstellar organic chemistry connects directly to questions about the origin of life on Earth — and the possibility of life elsewhere. For how natural selection acts on chemical variation in extreme environments, see our natural selection guide and the deep-sea discovery article.