Let's call them hypotheses, isn't that a better word? I mean, a scientific theory is something that's supported by big amounts of evidence and used to explain sets of facts, like evolution or the theory of relativity.
Anyways, I don't think you need to worry so much about it, life has survived for 4.5 billion years now, and mas extinctions happen so often that there's an extremely low chance that we will experience one.
If a comet or asteroid get's towards us, we might be able to blow it up into smaller pieces that will burn up in our atmosphere.
The solar wind itself can destroy power plants and electric wires, so our electronic infrastructure can be heavily damaged by it, but that doesn't mean we will all die.