General Zod and his gang of sexy rebels (get ready to see some Faora cosplay, am I right?) inject Earth with a giant shot of World Engine goop. Much though the whole idea of pumping Earth full of mass like it's on steroids is farfetched, we loved it! Terraforming entire new worlds is a huge part of scifi, and has allowed fans to imaginatively travel to exotic locations across the universe. There are some tentative ideas about how to terraform our neighboring planets in real science too, especially Mars because it is comparable to Earth in size, axial tilt, and solar day (the Venetian day, by comparison, lasts about 116 Earth days... yikes). Many suggestions about how to develop an artificial atmosphere and magnetosphere on Mars have already surfaced.
But the gravitational problem looms large: how could the human body adapt to a planet with only one third the g-force we're used to? The easier answer would be to adapt ourselves to the environment instead of the other way around (cyborgs FTW!), but it's nice to see a popular superhero movie confront this problem head on. The Kryptonian answer? Juice any old planet up until it means your specs. It's a hamfisted way to go about things, and it didn't seem to work all that well for the extended, interplanetary Kryptonian civilization, but we'll take what we can get when it comes to actually addressing the gravity problem in terraforming. We just hope that Zod's engines didn't alter the mass of the Earth too much before Superman stopped them, because it's already hard enough getting out of bed in the morning.