NASA recently started searching regions of the Milky Way Galaxy relatively close to us for Earth-like planets. It will do so via the Kepler spacecraft which will monitor the brightness of over 100,000 individual stars simultaneously as it trails the Earth in its orbit around the Sun.
To be detected, a planet must travel between the star it revolves around and Kepler. Then the spacecraft should see a change in the star’s brightness of around 100 ppm. (This assumes the planet’s orbital path lines up edgewise with Kepler. The probability of such an orbit is 0.5% for a planet in an orbit similar to Earth’s, according to NASA.)
The change in a star’s brightness should reoccur the next time the planet circles, which should take about a year. This data will let scientists calculate the size of it, along with the mass and size of its star. NASA calculates that if Earth-like planets are common, Kepler should find hundreds of them in its 3.5-year mission, which could stretch to six years.