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". It works by the occultation method. A photometer continually monitors the brightness of over 145,000 main sequence stars in a fixed field of view. This data is transmitted to Earth, then analyzed. The aim is to find periodic dimming caused by extrasolar planets which cross in front of their host star. As of January 2013, there are a total of 2,740 candidates.