Through the Looking Glass

Like sunlight through a stained-glass window, starlight can shine through the atmosphere of a distant planet, allowing scientists to determine the atmosphere’s composition. The technique works only when, from our perspective on Earth, the faraway planet crosses in front of its star. Because atoms and molecules absorb light in various colors, missing colors in the starlight reveal which molecules are present in the planetary atmosphere. Finding water vapor, ozone and methane in exoplanet atmospheres might one day reveal the presence of life.