The reason why Earth planets have not been discovered is because they are tiny. The parent star blots them out. Traditional methods of exoplanet detection all rely on indirect means of inferring the existence of orbiting bodies. These methods include:
1.astrometry - watching a star move slightly due to the gravitational influence of a nearby planet
2.Observing doppler shifts of the stars spectrum due to the star's movement
3.Observing the amount of light from a star change as an extrasolar planet transits the star, preventing a portion of the light from reaching the observer.
4.Pulsar timing
5.Gravitational microlensing
6.Observing radiation from Circumstellar disks in the infrared.
The Kepler Mission, is a NASA mission which is able to detect extrasolar planetsThe NASA Kepler Mission uses the transit method to scan a hundred thousand stars in the constellation Cygnus for planets. Kepler will be sensitive enough to detect planets even smaller than Earth. By scanning a hundred thousand stars simultaneously, it will not only be able to detect Earth-sized planets, it will be able to collect statistics on the numbers of such planets around sunlike stars