Locarno is a new planet formed when a Genesis Device detonated around the rebel leader Nick Locarno, and Starfleet named it after him.
Locarno is a planet that did not exist until the climax of Old Friends, New Planets. The disgraced former cadet Nick Locarno, leader of a rebel fleet formed from mutinous lower decks crews, threatened to use a Genesis Device unless his movement was recognized.
After Mariner sets the device to activate and Freeman transports her to safety, Locarno fails to stop it. The device converts him and the surrounding matter into a new world, which Starfleet names Locarno in his memory.