Lowest mass exoplanet

The lowest mass extrasolar planet known is PSR B1257+12 b, also known as Draugr. It is the innermost planet orbiting the pulsar PSR B1257+12, and has just 2% of the mass of the Earth, or roughly twice the mass of Earth's moon. The planet was first discovered in 1994, and the measurement of its mass was made by Maciej Konacki and Aleksander Wolszczan in 2003, using the Arecibo radio telescope in Puerto Rico.

Coincidentally, the PSR B1257+12 system was also the first system discovered with extrasolar planets. This discovery was made in 1990 by Aleksander Wolszczan and Dale Frail also using the Arecibo radio telescope.

Recorded as "0.020±0.002" times the mass of the Earth in the paper: https://ui.adsabs.harvard.edu/abs/2003ApJ...591L.147K/abstract