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.
Gliese 1132
As of June 12, 2018, there are two confirmed planets and one candidate planet orbiting GJ 1132. Gliese 1132 b is the innermost planet of the Gliese 1132 system, as well as the smallest. It is very similar in size and mass to Earth, with a radius of 1.13 and a mass of 1.66 . It is slightly denser than Earth with 30% more surface gravity, meaning it has a rocky composition. Despite its physical similarities to Earth, it is considered too hot to be habitable, getting 19 times more sunlight due to its 1.6 day orbital period. In April 2017, it was revealed that the planet has an atmosphere, potentially rich in water vapor and methane.