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.
ARM Cortex-M
The smallest ARM microcontrollers are of the Cortex-M0+ type (as of 2014, smallest at 1.6 mm by 2 mm is Kinetis KL03).On 21 June 2018, the "world's smallest computer'", or computer device was announced based on the ARM Cortex-M0+ (and including RAM and wireless transmitters and receivers based on photovoltaics) by University of Michigan researchers at the 2018 Symposia on VLSI Technology and Circuits with the paper "A 0.04mm3 16nW Wireless and Batteryless Sensor System with Integrated Cortex-M0+ Processor and Optical Communication for Cellular Temperature Measurement." The device is 1/10th the size of IBM's previously claimed world-record-sized computer from months back in March 2018, which is smaller than a grain of salt.