A planet created by the Genesis device.
The Genesis Planet is a world that was created by the Genesis Device, a technology developed by Dr. Carol Marcus and her team to transform lifeless planets into worlds that could support life. It was formed when the device was detonated inside the Mutara Nebula, and it was an unstable place.
The device could reorganise matter at the molecular level, and it was intended to be a way of creating new homes for people. It also raised concerns, because it could be used as a weapon, and many feared its power. The planet that it created was fertile at first but proved to be doomed, because the process was not complete.
The Genesis Planet features in Star Trek III: The Search for Spock, where Kirk and his crew travel to the planet in search of Spock's body. They find that Spock has been regenerated by the planet's energies, though he has aged rapidly and lost his mind. Kirk's son David Marcus, who had worked on the project, is killed by Klingons, and the planet breaks up shortly afterwards.