Planet
The smallest known planet is PSR B1257+12A, one of the first extrasolar planets discovered, which was found in 1992 in orbit around a pulsar. Its mass is roughly half that of the planet Mercury. The smallest known planet orbiting a main-sequence star other than the Sun is Kepler-37b, with a mass (and radius) slightly higher than that of the Moon.
Kepler-124b
Kepler-124b is an extrasolar planet discovered in 2014. It is located from Earth, orbiting the unclassified star Kepler-124 in the constellation Cygnus. Within The Kepler-124 system (KOI-241) there are three known planets, Kepler-124b being both the smallest and closest to its parent star.