Transmission spectroscopy

Reading a planet's atmosphere in the starlight that passes through it during a transit.

During a transit a thin ring of starlight passes through the planet's atmosphere on its way to us, and the molecules there absorb at their own wavelengths. Subtract the star's spectrum from the in-transit spectrum and what remains is the atmosphere's fingerprint. The signal is minuscule — parts in ten thousand — which is precisely what Webb was built for. Water, carbon dioxide, methane and sulphur dioxide have all been identified this way. The method only reads the terminator — the ring of atmosphere at the day-night boundary — and only the upper part of it, above whatever cloud deck the planet has. A high haze layer flattens the whole spectrum into a featureless line, which is not a null result so much as a closed door, and it happens often enough to be the field's main frustration.