transmission spectroscopy

When a planet crosses in front of its star, a thin ring of starlight grazes through the planet's atmosphere on its way to us. Gases in that air absorb particular colors of light, taking little bites out of the spectrum — and different gases bite at different colors, like chemical fingerprints. By comparing the star's light during a transit to its light just before and after, astronomers can read which colors went missing and so figure out what the planet's air is made of, all without ever flying there. This is transmission spectroscopy: reading an exoplanet's atmosphere in the starlight that filters through it.

The effect is fantastically subtle. The atmosphere is a sliver around the planet's edge, so it changes the star's brightness by only a tiny extra amount, on the order of a hundredth of a percent — and that amount varies with color depending on what gases are present. A spectrum showing a dip at the right wavelengths can reveal water vapor, carbon dioxide, methane, sodium, or other molecules in the planet's atmosphere. The planet also looks very slightly larger at colors its atmosphere absorbs strongly, because that light is blocked higher up, giving a wavelength-by-wavelength measurement of the air.

Transmission spectroscopy is the main way we are beginning to characterize exoplanet atmospheres, and it is the technique with which JWST has detected gases like carbon dioxide on distant worlds. It is also the most realistic near-term route to searching for possible biosignatures in the air of small planets. The honest caveats are heavy: the signals are minuscule and easily mimicked by clouds, hazes, stellar activity, or instrument effects, so claims must be made cautiously, and reaching the air of a small, temperate Earth-like world is right at the edge of what is currently achievable.