James Webb Can Read the Air on Alien Planets

James Webb can do something that sounds impossible: read the chemistry of the air on a planet trillions of miles away.

When a distant planet passes in front of its star, a sliver of starlight filters through the planet's atmosphere on its way to us, carrying the fingerprints of whatever gases it passed through.

Different molecules absorb different colors of light, each leaving a unique dark line in the spectrum. James Webb can read those lines from hundreds of light-years away. On a scorching gas giant called WASP-39b, 700 light-years off, it found the clear signature of carbon dioxide, the first time that gas was ever detected on a planet beyond our solar system. It has since found water and other chemistry on other worlds.

This is how we may one day find life. The same trick that spotted carbon dioxide could reveal oxygen or methane in a planet's air, gases that on Earth are produced in abundance only by living things.

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