This month marks the third anniversary of the discovery of a remarkable system of seven planets known as TRAPPIST-1.

These seven rocky, Earth-size worlds orbit an ultra-cool star 39 light-years from Earth. Three of those planets are in the habitable zone, meaning they are at the right orbital distance to be warm enough for liquid water to exist on their surfaces. After its 2021 launch, NASA’s James Webb Space Telescope will observe those worlds with the goal of making the first detailed near-infrared study of the atmosphere of a habitable-zone planet.

To find signs of an atmosphere, astronomers will use a technique called transmission spectroscopy. They observe the  while the planet is crossing the face of the star, known as a transit. The light of the star filters through the planet’s atmosphere, which absorbs some of the starlight and leaves telltale fingerprints in the star’s spectrum.



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