Or... Idk, they didn't flee the planet?
You are making pretty extreme assumptions. I mean, there are places on earth now that are affected by Nuclear radiation where you need a suit, and there are species on earth now that are going extinct, yet we have no real need to leave the entire planet, nor is there any proof that it was destroyed.
I don't see the point in these massive assumptions that we as a species are able to blow up a planet in the near future.