Micromius

Micromius was a planet that was located in the Alpha or Beta Quadrant.

In the 24th century, there was an unspecified medical disaster on this planet. The disaster led Doctor Beverly Crusher to pursue theories combining the use of cybernetics and regeneration. (TNG: "11001001")

This planet was only mentioned in dialogue.
The identification of Micromius as a planet came from StarTrek.com.[1](X)