How does JWST detect molecules in an exoplanet's atmosphere?

JWST uses a technique called transmission spectroscopy. When a planet transits (passes in front of) its host star, a tiny fraction of starlight filters through the planet's atmosphere. Different molecules absorb specific wavelengths of light — leaving characteristic dips in the spectrum. By comparing the star's spectrum with and without the planet in the way, astronomers can identify which molecules are present and at roughly what abundances. JWST's infrared sensitivity is transformative: its predecessor Hubble could detect water vapour, but JWST can detect CO₂, CH₄, SO₂, and many other molecules that were previously beyond reach. See our guide to how we find exoplanets for the full transit method explanation.

Does detecting methane mean the planet could support life?

Not necessarily — methane can be produced by both biological and geological processes. On Earth, most methane is biological (livestock, wetlands, landfills), but volcanic outgassing and hydrothermal vents also produce it. However, methane and CO₂ together in a planet's atmosphere is scientifically significant: in the presence of UV radiation, methane is quickly destroyed. Finding both simultaneously suggests a source replenishing the methane — which could be geological or, intriguingly, biological. A confirmed biosignature would require ruling out all abiotic explanations. The NASA Exoplanet Exploration site tracks the latest atmospheric characterisation results.

What is a sub-Neptune and how common are they?

A sub-Neptune (also called a "mini-Neptune") is a planet larger than Earth but smaller than Neptune — roughly 2–4 times Earth's radius. They are the most common type of planet in our galaxy according to Kepler and K2 survey data, yet our Solar System has none. Their composition is poorly understood: they may have thick hydrogen-helium envelopes atop a rocky or icy core, or they may be water-world "Hycean" planets — entirely covered by a global ocean under a hydrogen-rich atmosphere. JWST's atmospheric studies of sub-Neptunes are among the highest scientific priorities of its mission, since understanding this class could rewrite planetary formation theory. See our exoplanets overview guide for more on planet types.

JWST Detects Carbon Dioxide and Methane in Exoplanet Atmosphere

A new benchmark in exoplanet science

The planet — a sub-Neptune orbiting a K-dwarf star roughly 120 light-years from Earth — was observed during multiple transits over several months. The resulting spectrum revealed clear absorption features from carbon dioxide (CO₂), methane (CH₄), and water (H₂O), alongside a tentative detection of carbon monoxide.

The simultaneous presence of CO₂ and CH₄ is particularly striking. In equilibrium chemistry at the temperatures inferred for this planet (~300 K in the observable atmosphere), methane should dominate. Finding both implies either disequilibrium chemistry — driven by biological or geological processes — or an unusual metallicity in the atmosphere. The science team has published their analysis in Nature Astronomy, noting the result is consistent with a "Hycean" world hypothesis: a rocky or water-rich interior beneath a hydrogen-dominated atmosphere.

Why this matters for the search for life

No single detection constitutes a biosignature — confirmed evidence of life. But the technique demonstrated here — stacking many transit observations to build a high signal-to-noise spectrum — represents the methodological pathway to future biosignature searches. JWST's follow-up programme will now target a dozen more sub-Neptunes in the habitable zones of nearby stars, building a statistical picture of this most common planetary class.

To understand how planets like this are discovered and classified, see our exoplanets overview guide. The transit detection method is explained in detail in our how we find exoplanets guide. For the dictionary entry on exoplanets, see our glossary.

Detected molecules

CO₂Carbon dioxide

CH₄Methane

H₂OWater vapour

CO?Carbon monoxide (tentative)

The James Webb Space Telescope is operated by NASA, ESA, and the Canadian Space Agency. All atmospheric spectra data are publicly archived via the MAST archive.

JWST detects CO₂ and methane in an exoplanet atmosphere

A new benchmark in exoplanet science

Why this matters for the search for life

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