Designed to survey a portion of our region of the Milky Way to discover dozens of Earth-size extrasolar planets in or near the habitable zone and estimate how many of the billions of stars in our galaxy have such planets , Kepler 's sole instrument is a photometer that continually monitors the brightness of over 145,000 main sequence stars in a fixed field of view .
The Kepler mission 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 '' .