Mysteroid

Mysteroid is the former home world of the Mysterians. The planet is first mentioned in the 1957 film Mysterians.

Mysterians
Thousands of years before the dawn of civilisation on Earth, Mysteroid was a thriving world home to an advanced civilisation that was close to exploring the Solar System. However, a nuclear war broke out on the planet, eventually wiping out all but a small number of survivors. Despite the fact that their race had survived, these survivors were contaminated with Strontium-90, meaning that each new generation suffered severe genetic ilnesses and mutations. Realising they needed to colonise a world that wasn't ravaged by radioactivity, they set their sights on Earth, and journeyed to the moon. From there, they would plan for many years, before ultimately setting their invasion of Planet Earth into motion.