An alien world just two-thirds the size of Earth - one of the smallest on record - detected by NASA's Spitzer Space Telescope is seen in this NASA artist's illustration released by NASA on July 18, 2012. The exoplanet candidate, known as UCF-1.01, orbits a star called GJ 436, which is located a mere 33 light-years away. UCF-1.01 might be the nearest world to our solar system that is smaller than our home planet. Evidence for UCF-1.01 turned up when astronomers were studying a known, Neptune-sized exoplanet, called GJ 436b, seen in the background in this image. The identification of nearby small planets may lead to their characterization using future instruments. In this way, worlds like UCF-1.01 might serve as stepping stones to one day finding a habitable, Earth-like exoplanet. [Photo/NASA]