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 vulnerable Kitti's hog-nosed bat ("Craseonycteris thonglongyai"), also known as the bumblebee bat, from Thailand and Myanmar is the smallest mammal, at in length and in weight.