Planet transiting an alien star, artwork. In 2009, NASA launched its Kepler Mission to discover Earth-like planets orbiting other stars. A 0.95 metre diameter telescope called a photometer will scan 100,000 stars in the Milky Way for evidence of Earth-sized terrestrial planets that transit the star's path (as shown here). The mission will orbit the Sun for at least 3.5 years, and is named in honour of German astronomer Johannes Kepler.