The Kepler observatory is “specifically designed to survey a portion of our region of the Milky Way galaxy to discover dozens of Earth-size planets in or near the habitable zone and determine how many of the billions of stars in our galaxy have such planets”.[7] A photometer continually monitors the brightness of over 145,000 main sequence stars in a fixed field of view.[8] This data is transmitted to Earth, then analyzed to detect periodic dimming caused by extrasolar planets that cross in front of their host star. It’s already found almost 3000 candidates.