Before it's destruction Krypton was a technologically advanced isolationist planet orbiting the red super-giant Rao hundreds of thousands of light years from Earth. After several wars, invasions, and the destruction of the moon Wegthor the Kryptonians had retreated to Mega-City Archologies leaving much of the world uninhabited. When the planet's radioactive core became unstable the ruling council banned those few scientists who knew of the situation from announcing it because they saw no way to save even half of the several billion Kryptonians on the planet and didn't want to cause a panic.
In the city of Argo scientists Zor-El and Alura Zor-El had reverse engineered technology left from the theft of Kandor, once the capital city of Krypton, by the space villain Brainiac and used it to cause the entire city to launch into space. Zor-El's brother Jor-El and his wife Lara Jor-El refused to leave the planet feeling they could save the it if they could convince the ruling council to make the coming disaster public. Instead they were confined to their home in Kryptonopolis and cut off from all public networks.
Jor-El and his colleague Non in their research found the coming planetary destruction was accelerating and while Jor-El and Lara built and tested rockets, something prohibited by the ruling council for decades, Non spoke out and tried to rally support and public awareness. Non was soon arrested, lobotomized, and sentenced to the Phantom Zone.
On the eve of the Kryptonian Apocolypse Jor-El and Lara having no choice but to accept their failure placed their infant son in a suspension creche rocket and fired it into space. The explosion of Krypton's radioactive core caused much of the planet to be converted into kryptonite.