The centuries-old quest for other worlds like our Earth has been rejuvenated by the intense excitement and popular interest surrounding the discovery of hundreds of planets orbiting other stars.
There is now clear evidence for substantial numbers of three types of exoplanets; gas giants, hot-super-Earths in close orbits, and ice giants. The challenge now is to find terrestrial planets (i.e., those one half to twice the size of the Earth), especially those in the habitable zone of their stars where liquid water and possibly life might exist.
The Kepler Mission, a space telescope launched by NASA in partnership with several organizations including the Smithsonian Astrophysical Observatory, is designed to survey a portion 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. The spacecraft is named after Johannes Kepler, a German astronomer who worked out the laws of planetary motion and died in 1630.
Since March of 2009, Kepler has been continuously staring at more than 100,000 stars in a patch of sky in the constellation Cygnus. Kepler uses the transit method to find planets.
When a planet crosses between its star and an observer, the event is called a transit. Transits by terrestrial planets produce a small dip in a star's brightness of about 1/10,000, lasting for several hours. This dip is periodic if it is caused by a planet. In addition, all transits produced by the same planet must be of the same change in brightness and last the same amount of time, thus providing a highly repeatable signal and robust detection method.
Once detected, the planet's orbital size can be calculated from the period (how long it takes the planet to orbit once around the star) and the mass of the star using Kepler's Third Law of planetary motion. The size of the planet is found from the depth of the transit (how much the brightness of the star drops) and the size of the star. From the orbital size and the temperature of the star, the planet's characteristic temperature can be calculated. Knowing the temperature of a planet is key to whether or not the planet is habitable (not necessarily inhabited). Only planets with moderate temperatures are habitable for life similar to that found on Earth.
As of 2012, Kepler has identified several thousand planet candidates of varying sizes, though not all will be confirmed to be actual planets.
Adapted from the mission summary on the Kepler homepage, kepler.nasa.gov.