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.
John Johnson (astronomer)
In 2012, Johnson's team discovered three small rocky exoplanets in a red dwarf star system observed with the Kepler space telescope. The system was renamed Kepler-42 and the outermost planet was found to be nearly as small as Mars, making it the smallest known exoplanet at the time. A subsequent study used the host star's similarity to Barnard's Star and observations from the Keck Observatory to more precisely measure the properties of the system, including the sizes of the three planets.