... Kepler mission discovers tiny planet system . The smallest planet, Kepler-37b, is slightly larger than our Moon, measuring about one-third the size of Earth. ... Kepler-37c, the second planet, is slightly smaller than Venus, measuring almost three-quarters the size of Earth. Kepler-37d, the third planet, is twice the size of Earth. ... NASAт??s Kepler mission scientists have discovered a new planetary system that is home to the smallest planet yet found around a star similar to our Sun. ...