By Orbital Team
When an exoplanet passes in front of its host star from our point of view, a small fraction of the star's light grazes through the planet's atmosphere on its way to our telescopes. That sliver of starlight carries a chemical fingerprint of everything it passed through on the way.
Different molecules absorb light at specific, well known wavelengths. Water vapor, carbon dioxide, methane, and sodium each leave a distinct pattern of dips in the spectrum, and by comparing the star's light during transit against its light when the planet is not in front of it, astronomers isolate exactly which wavelengths the atmosphere absorbed.
This technique, transmission spectroscopy, does not require resolving the planet as a separate point of light from its star, which is what makes it possible for planets far too small and dim to ever be directly imaged with current instruments.
The technique has real limits. It works best for planets that transit frequently and orbit bright, nearby stars, and it is far more sensitive to puffy, hydrogen rich atmospheres around gas giants than to the thin atmospheres expected around small rocky worlds, which produce a much fainter signal.
Even so, transmission spectroscopy has already detected water vapor, carbon dioxide, and even tentative sulfur based signatures possibly linked to biological activity on planets dozens to hundreds of light years away, using nothing more than the shadow of starlight passing through an alien sky.