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
It is the smallest discovered planet in the Kepler-124 system, and has the closest orbit of the three known planets. Kepler-124b orbits 96% closer to its star than Earth (approximately 3 Earth days), which in the Kepler-124 system is inside the inner limit of the star's habitable zone.