Earth is the third planet from the sun and the only one that can support life. It contains the human species and its splinter species called mutants.
The mutant Magneto hatched a plan to take over the human race. He planned to send the Scorpio Comet towards Earth. It would send up a cloud of dust and debris, which would block out the sun for years. Magneto would then rule the survivors. From Asteroid M Magneto diverted the comet using the Mutant Power Circuit.
The X-Men attacked the asteroid with the hopes of again diverting the comet. They were successful and the Earth was saved.