Slightly larger than the Moon, but smaller than Mercury, Kepler-37b was, at the time of its discovery in 2012, the smallest exoplanet ever found. 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.
Located 210 light years away in the constellation Lyra, it's so close to its host star that it takes a mere 13 days to orbit. This proximity gives this rocky planet an average temperature of about 425 C (~800 F.) Due to this high temperature and its lack of an atmosphere, Kepler-37b is unable to sustain life. Just one of over 100 planets already discovered by NASA's Kepler Mission, detecting this tiny world proved challenging. This discovery shows the precision and high quality of the data coming from Kepler. It also is an indication that small planets are probably common in the galaxy.
Launched in 2009, the Kepler spacecraft measures the light output of 150,000 stars simultaneously. The data from each star is then analyzed in order to look for periodic drops in the light curve being emitted. 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 an exoplanet candidate.