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.
Smallest organisms
The smallest land snail is "Acmella nana". Discovered in Borneo, Malaysia, and described in November 2015, it measures only 0.7 mm. The previous record was that of "Angustopila dominikae" from China, which was reported in September 2015. This snail measures 0.86 mm.