NASA's Kepler space telescope has spotted a rocky exoplanet tinier than Mercury, the smallest planet in our solar system. Named Kepler-37b, the record-breaking planet is nearly half the size of the previous titleholder for smallest known exoplanet and is only about 10 percent larger than Earth's moon. The object orbits a star that is slightly smaller and cooler than our own sun and is located 215 light-years away.