Nibiru was a class M world that the USS Enterprise was assigned to observe in 2259 of the Nero-created alternate reality (TOS comic: "Countdown to Darkness, Number Four"; TOS video game: Star Trek; TOS movie & novelization: Star Trek Into Darkness; Star Trek (App)).
It was determined that a volcano on the world was going to erupt and cause the extinction of the planet's species, notably the Nibirans. The Enterprise crew enacted a Prime Directive-defying plan to save the people by stopping the eruption with a cold fusion device. In doing so, the natives became aware of the ship and crew. This eventually led to Admiral Christopher Pike chewing out Captain James T. Kirk and Spock, while regaining control of the vessel with Kirk becoming his first officer. (TOS movie & novelization: Star Trek Into Darkness)