Within our own solar system we can observe planetary transits of Mercury and Venus when they pass in front of the Sun as viewed from Earth. Kepler will observe the same phenomena in order to detect Earth-size planets, but from very far away. In fact, the distant suns that Kepler will observe are so far away that the stars are just points of light. Detecting an Earth-size planet transiting a distant star is like observing a gnat flying in front of a car’s headlight seen from many miles away.
Credit: NASA Kepler Mission/Dana Berry