Lessek Prime was the prime planet in its system.

In 2378, The Doctor was coerced by Zet and Nar into disguising himself as Captain Kathryn Janeway in a plot to steal the USS Voyager's warp core. Suspecting something was amiss, Commander Chakotay tested "Janeway" by inventing a story about how she, as a lieutenant, had saved an away team on Lessek Prime in 2363 by disobeying her captain's orders. (VOY: "Renaissance Man")

This planet was only mentioned in dialogue.
It is unclear whether the planet itself was also invented by Chakotay or only the specific away team mission. The Star Trek Encyclopedia (4th ed., vol. 1, p. 461) sides with the mission being an invention of Chakatay.