This artist's visualization shows exoplanet Kepler-47c - a featureless ice giant covered by layers of methane and water vapor. An exoplanet is any identified planet outside of our own Solar System. As of June 2013, there have been 866 exoplanets identified, located around 671 stars, nearly all within the Milky Way Galaxy. Kepler-47c is the first exoplanet discovered that is part of a multi-planet system orbiting two stars.
Kepler-47c is about the same size as Neptune, making it an ice giant. It lies inside the habitable zone of its host star - the region in which liquid water could exist on a planet's surface. Liquid water is a key ingredient to life, so planets found within this zone are more likely to habitable worlds. However, Kepler-47c's large size makes it incapable of sustaining life. This planet takes 303 days to orbit around its host stars. This binary star system consists of a Sun-like star and a smaller, cooler, M-type dwarf which is about a third of the size its sister star. With its two known planets, the Kepler-47 system proves that complete planetary systems can indeed exist around binary stars.
Launched in 2009, NASA's Kepler mission measures the light output of 150,000 stars simultaneously. The data from each star are then analyzed in order to look for periodic drops in the light curve. These drops in brightness could indicate the presence of an orbiting planet passing in front of its host star, blocking some of its light. This is called a transit. Three or more transits of equal periods are needed to catalog an object as a planet candidate.