Hundreds of years prior to 2266, a meteorite collided with the planet and penetrated to the core, bringing about rapid chemical changes. As a result, the planet entered a state of rapid changes and upheavals on the surface, and the populace was converted to energy.
This state of affairs continued until 2266, when the USS Enterprise visited the planet, searching for the planet's natural tiantianium deposits. Spock, while under the influence of the Metamorphans, constructed a device which reversed the meteorite's effects on the planet. (TOS comic: "The Peril of Planet Quick Change")