An extrasolar planet, or exoplanet, is a planet outside the Solar System.
One of the ways to detect an exoplanet is to carefully observe the
brightness of the star over time. As the planet passes between the
star and the earth the brightness of the star decreases slightly.
Exoplanet Transit Simulator
The Kepler Mission
The Kepler Spacecraft was launch in March 2009. The Kepler Mission is specifically designed to survey a
small 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.
Kepler uses the exoplanet transit method of detecting extrasolar planets.