Kepler

The Kepler Telescope was a specialized planet hunter that functioned like a sensitive light meter in space. While Hubble looks at everything, Kepler stared fixedly at one patch of sky, looking for tiny dips in stellar brightness. If we imagine our Solar System, Kepler looked for "eclipses" that occur when a distant planet passes in front of its star—much like a mosquito flying across a car headlight hundreds of miles away. Thanks to it, we know there are more planets than stars in our galaxy and that Earth-like worlds are common in deep space. The mission ended in 2018 after it ran out of fuel.